Cable Tray Manual

CABLE TRAY MANUALBased on the2005 National Electrical Code

2 0 0 5

CT05MAN

Table of ContentsPage No.

Introduction ...................................................................................................................... 2

Why Cable Tray?Safety .................................................................................................................... 3Dependability ........................................................................................................... 4Space Savings .......................................................................................................... 4Cost Savings ......................................................................................................... 5-8

An In-depth Look at the 2005 NEC, Section 392Types of Cable Trays (NEC 392.1 Scope)............................................................ 9-11EMI/RFI Cable Tray ......................................................................................... 10-11Cable Tray Materials ............................................................................................... 12Types of Cables Allowed in Cable Tray [392.3 (A)] ..................................................... 12

MI - Mineral Insulated Metal Sheathed Cable [Article 332] ............................... 12MC - Metal Clad Cable [Article 330] .............................................................. 13TC - Power and Control Tray Cable [Article 336] ........................................... 13ITC - Instrumentation Tray Cable [Article 727] ............................................... 13PLTC - Power Limited Tray Cable [Sections 725.61 (C) and 725.71 (E)] .......... 14Other Types - Fire Alarm [Article 760],

Multipurpose and Communications Cable [Article 800] ................14Single Conductor & Type MV Cables [392.3 (B)] ....................................................... 14Cable Tray Use in Hazardous Locations [392.3 (D)] .............................................. 15-17Limitations on Cable Tray Use [392.4] ..................................................................... 18Cable Tray Loading [392.5 (A)]........................................................................... 18-20Fiberglass Cable Tray [392.3 (E) & 392.5 (F)] ............................................................ 20Discontinuous Cable Tray and Fittings [392.6 (A)] ................................................ 21-22Covers [392.6 (D)]................................................................................................... 23Barriers [392.6 (E) & (F)].......................................................................................... 24Spacing of Multiple Cable Trays [392.6 (I)] ................................................................ 25Supporting Conduit from Cable Tray [392.6 (J)] ........................................................ 25Use of Cable Tray as an Equipment Grounding Conductor [392.7 Grounding] ........ 26-29Fastening Cables [392.8 (B)] .................................................................................... 30Cable Installation [392.8] ................................................................................... 30-32Sizing Cable Tray

Multiconductor - 2000 volts or less [392.9] ............................................... 32-34Single conductor - 2000 volts or less [392.10]........................................... 34-36Type MC or MV - 2001 volts or greater [392.12] ........................................... 37

Ampacities of Cables in Cable Tray .................................................................... 36-38Cable Tray Wiring System Design and Installation Hints ....................................... 38-39Fireproofing Tray ................................................................................................... 40Expansion and Contraction ............................................................................... 41-42

Appendix Index & Appendix Sheets...................................................................... 44-55

Cable Tray Installation & Specification Checklists ........................................... 54-55

Footnotes ..................................................................................................................... 56

1Cable Tray Manual Cooper B-Line, Inc

INTRODUCTION

The B-Line Cable Tray Manual was produced by B-Line's technical staff. B-Line has recognizedthe need for a complete cable tray re f e rence source for electrical engineers and designers. Thefollowing pages address the 2005 National Electric Code requirements for cable tray systemsas well as design solutions from practical experience. The information has been organized for use asa reference guide for both those unfamiliar and those experienced with cable tray.

Nearly every aspect of cable tray design and installation has been explored for the use of thereader. If a topic has not been covered sufficiently to answer a specific question or if additionalinformation is desired, contact the engineering department at B-Line. We sincerely hope you willfind the B-Line Cable Tray Manual a helpful and informative addition to your technical library.

The information contained herein has been carefully checked for accuracy and is believed to becorrect and current. No warranty, either expressed or implied, is made as to either its applicabilityto, or its compatibility with, specific requirements, of this information, nor for damages consequentto its use. All design characteristics, specifications, tolerances and similar information are subject tochange without notice.

Cooper B-Line, Inc.509 West Monroe Street

Highland, IL 62249-0326Tel: (618) 654-2184Fax: (618) 654-5499

National Electrical Code and NEC are registered trademarks of theNational Fire Protection Association, Inc. Quincy, MA 02269.

2Cooper B-Line, Inc Cable Tray Manual

L a rge numbers of electrical engineers havelimited detail knowledge concerning wiring systems.T h e re is the tendency by engineers to avoidbecoming involved in the details of wiring systems,leaving the wiring system selection and design todesigners or contractors. Certain decisions must bemade for any wiring system installation, and thesedecisions should be made in the design andconstruction activities' chain where maximumimpact is achieved at the lowest possible cost.Deferring design decisions to construction canresult in increased costs and wiring systemsincompatible with the owner's future requirements.Early in the project's design life, the costs andfeatures of various applicable wiring systems shouldbe objectively evaluated in detail. Unfortunately,such evaluations are often not made because of thetime and money involved. It is important to realizethat these initial evaluations are important and willsave time and money in the long run. Theevaluation should include the safety, dependability,space and cost requirements of the project. Manyindustrial and commercial electrical wiring systemshave excessive initial capital costs, unnecessarypower outages and require excessive maintenance.M o re o v e r, the wiring system may not have thef e a t u res to easily accommodate system changesand expansions, or provide the maximum degree ofsafety for the personnel and the facilities.

Cable tray wiring systems are the preferred wiringsystem when they are evaluated against equivalentconduit wiring systems in terms of safety,dependability, space and cost. To properly evaluatea cable tray wiring system vs. a conduit wiringsystem, an engineer must be knowledgeable of boththeir installation and the system features. Theadvantages of cable tray installations are listedbelow and explained in the following paragraphs.

Safety Features Dependability Space Savings Cost Savings Design Cost Savings Material Cost Savings Installation Cost & Time Savings Maintenance Savings

CABLE TRAY SAFETY FEATURES

A properly engineered and installed cable traywiring system provides some highly desirable safetyfeatures that are not obtainable with a conduit wiringsystem.

Tray cables do not provide a significant path forthe transmission of corrosive, explosive, or toxicgases while conduits do. There have been explosionsin industrial facilities in which the conduit systemsw e re a link in the chain of events that set up theconditions for the explosions. These explosionswould not have occurred with a cable tray wiringsystem since the explosive gas would not have beenpiped into a critical area. This can occur eventhough there are seals in the conduits. There doeshave to be some type of an equipment failure ora b n o rmal condition for the gas to get into theconduit, however this does occur. Conduit sealsprevent explosions from traveling down the conduit(pressure piling) but they do not seat tight enough toprevent moisture or gas migration until an explosionor a sudden pre s s u re increase seats them. TheOctober 6, 1979 Electrical Substation Explosion atthe Cove Point, Maryland Columbia LiquefiedNatural Gas Facility is a very good example of whereexplosive gas traveled though a two hundred footlong conduit with a seal in it. The substation wasdemolished, the foreman was killed and an operatorwas badly burned. This explosion wouldnt haveo c c u r red if a cable tray wiring system had beeninstalled instead of a conduit wiring system. A NewJersey chemical plant had the instrumentation andelectrical equipment in one of its control ro o m sdestroyed in a similar type incident.

In addition to explosive gases, corrosive gasesand toxic gases from chemical plant equipmentfailures can travel through the conduits to equipmentor control rooms where the plant personnel and thesensitive equipment will be exposed to the gases.

In facilities where cable tray may be used as theequipment grounding conductor in accordance withN E C Sections 392.3(C) & 392.7, the gro u n d i n gequipment system components lend themselves tovisual inspection as well as electrical continuity checks.

WHY CABLE TRAY?

BECAUSE A CABLE TRAY WIRING SYSTEM PROVIDESSAFE AND DEPENDABLE WAYS TO SAVE NOW AND LATER


CABLE TRAY DEPENDABILITY

A properly designed and installed cable traysystem with the appropriate cable types will providea wiring system of outstanding dependability for thec o n t ro l, communicat ion, data handl ing,instrumentation, and power systems. Thedependability of cable tray wiring systems has beenp roven by a 40 year track re c o rd of excellentperformance.

Cable tray wiring systems have an outstandingre c o rd for dependable service in industry. It is themost common industrial wiring system in Euro p e .In continuous process systems, an electrical systemf a i l u re can cost millions of dollars and pre s e n tserious process safety problems for the facility, itspersonnel and the people in the surro u n d i n gcommunities. A properly designed and installedcable tray system with the appropriate cable typeswi ll provide a wiring system of outstandingdependability for process plants.

Television broadcast origination facilities andstudios make use of cable tray to support and routethe large volumes of cable needed for theiroperations with a high degree of dependability. Itwould be impossible to have the wiring systemflexibility they need with a conduit wiring system.

L a rge retail and warehouse installations usecable tray to support their data communicationcable systems. Such systems must be dependable sothat there are no outages of their continuousinventory control systems.

Cable tray wiring systems have been widelyused to support cabling in both commercial andindustrial computer rooms overhead and beneaththe floor to provide orderly paths to house andsupport the cabling. These types of installationsneed a high degree of dependability which can beobtained using cable tray wiring systems.

CABLE TRAY SPACE SAVINGS

When compared to a conduit wiring system, anequivalent cable tray wiring system installationrequires substantially less space.

I n c reasing the size of a structure or a supportsystem to handle a high space volume conduitwiring system is unnecessary when this problem canbe avoided by the selection of a cable tray wiringsystem.

Facilities with high density wiring systemsdevoted to control, instrumentation, data handlingand branch circuit wiring have the choice ofselecting cable tray or conduit wiring systems. Aconduit wiring system is often a poor choicebecause large conduit banks re q u i re significantspace, competing with other systems andequipment. Choosing a cable tray wiring systemgreatly reduces this problem.

Financial institutions with large computerinstallations have high density wiring systems underfloors or in overhead plenum areas that are besthandled by cable tray wiring systems.

Airport facilities have extensive cable traywiring systems to handle the ever expanding needsof the airline industry.

Cable tray is used in many facilities because ofthe ever present need of routing more and morecables in less space at lower costs.

L a rge health care facilities have high densitywiring systems that are ideal candidates for cabletray.


CABLE TRAY WIRING SYSTEM COST SAVINGS

Usually, the initial capital cost is the major factorin selecting a project's wiring system when anevaluation is made comparing cable tray wiringsystems and conduit wiring systems. Such anevaluation often covers just the conductors, material,and installation labor costs. The results of theseinitial cost evaluations usually show that the installedcable tray wiring system will cost 10 to 60 percentless than an equivalent conduit wiring system. Theamount of cost savings depends on the complexityand size of the installation.

T h e re are other savings in addition to the initialinstallation cost savings for cable tray wiring systemsover conduit wiring systems. They include re d u c e dengineering costs, reduced maintenance costs,reduced expansion costs, reduced production lossesdue to power outages, reduced enviro n m e n t a lp roblems due to continuity of power and re d u c e ddata handling system costs due to the continuity ofp o w e r. The magnitudes of many of these costssavings are difficult to determine until the conditionexists which makes them real instead of potentialcost savings.

DESIGN COST SAVINGS

Most projects are roughly defined at the start ofdesign. For projects that are not 100 perc e n tdefined before design start, the cost of and timeused in coping with continuous changes during theengineering and drafting design phases will besubstantially less for cable tray wiring systems thanfor conduit wiring systems. A small amount ofengineering is re q u i red to change the width of acable tray to gain additional wiring space capacity.Change is a complex problem when conduit banksare involved.

The final drawings for a cable tray wiringsystem may be completed and sent out for bid orconstruction more quickly than for a conduit wiringsystem. Cable tray simplifies the wiring systemdesign process and reduces the number of details.

Cable tray wiring systems are well suited forcomputer aided design drawings. A spread sheetbased wiring management program may be used tocontrol the cable fills in the cable tray. While such asystem may also be used for controlling conduit fill,l a rge numbers of individual conduits must be

m o n i t o red. For an equal capacity wiring system,only a few cable tray runs would have to bemonitored.

Dedicated cable tray installation zones alertother engineering disciplines to avoid designs thatwill produce equipment and material installationconflicts in these areas. As more circuits are added,the cable tray installation zone will increase only afew inches; the space re q u i red for the additionalconduits needed would be much greater.

The fact that a cable can easily enter andexit cable tray anywhere along its ro u t e,allows for some unique opportunities that pro v i d ehighly flexible designs.

Fewer supports have to be designed and lessc o o rdination is re q u i red between the designdisciplines for the cable tray supports compared toconduit supports.

MATERIAL COST SAVINGS

Excluding conductors, the cost of the cabletrays, supports, and miscellaneous materials willprovide a savings of up to 80% as compared to thecost of the conduits, supports, pull boxes, andmiscellaneous materials. An 18 inch wide cable trayhas an allowable fill area of 21 square inches. Itwould take 7 - 3 inch conduits to obtain thisallowable fill area (7 x 2.95 square inches = 20.65square inches).

The cost of 600 volt insulated multiconductorcables listed for use in cable tray is greater than thecost of 600 volt insulated individual conductors usedin conduit. The cost diff e rential depends on theinsulation systems, jacket materials and cableconstruction.

For some electrical loads, parallel conductorsare installed in conduit and the conductors must bederated, requiring larger conductors to make up forthe deration. If these circuits were installed in cabletray, the conductor sizes would not need to bei n c reased since the parallel conductor deratingfactors do not apply to three conductor or singleconductor cables in cable tray.

Typical 300 volt insulated multiconductorinstrumentation tray cables (ITC) and power limitedtray cables (PLTC) cost the same for both cable trayand conduit wiring systems. This applies forinstrumentation circuits, low level analog and digital


signal circuits, logic input/output (I/O) circuits, etc.There are other cable tray installations which requirea higher cost cable than the equivalent conduitinstallation. Such installations are limited to are a swhere low smoke emission and/or low flame spreadITC or PLTC cables must be used.

Conduit banks often require more frequent andhigher strength supports than cable trays. 3 inchand larger rigid metal conduits are the only sizesallowed to be supported on 20 foot spans [NationalElectrical Code (NEC) Table 344.30(B)(2)].

When a cable tray width is increased 6 inches,the cable tray cost increase is less than 10 percent.This substantially increases the cable trays wiringcapacity for a minimal additional cost. To obtainsuch an increase in capacity for a conduit wiringsystem would be very costly.

INSTALLATION COST AND TIME SAVINGS

Depending on the complexity and magnitude ofthe wiring system, the total cost savings for theinitial installation (labor, equipment and material)may be up to 75 percent for a cable tray wiringsystem over a conduit wiring system. When therea re banks of conduit to be installed that are morethan 100 feet long and consist of four or more 2inch conduits or 12 or more smaller conduits, thelabor cost savings obtained using cable tray wiringsystems are very significant.

Many more individual components are involvedin the installation of a conduit system and itsconductors compared to the installation of a cabletray system and its cables. This results in thehandling and installing of large amounts of conduititems vs. small amounts of cable tray items for thesame wiring capacity.

6

40000

35000

30000

25000

20000

15000

10000

5000

0

TotalInstalledCost ($)

COST - Cable Tray vs. Conduit(Equivalent Conductor Fill Areas)

Material Cost

Labor Cost @$60/hr per NECAlabor units.

Aluminum LadderCable Tray

Steel LadderCable Tray

Solid BottomCable Tray

EMT Rigid SteelConduit

Installation: 200 linear feet of cable supported with four 90 direction changes and all trapeze supports on 8 ft. spans.

1. Aluminum, 18" wide, ladder cable tray (9" rung spacing) with all hardware.2. Hot dip galvanized steel, 18" wide, ladder cable tray (9" rung spacing) with all hardware.3. Hot dip galvanized steel, 18" wide, solid bottom cable tray and all hardware.4. 7 parallel runs of 3" diameter EMT with concentric bends.5. 7 parallel runs of 3" diameter galvanized conduit with concentric bends.

Note: Above costs do not include cable and cable pulling costs. Cable costs differ per installation and cable/conductor pulling costs have been shown to be considerably less for cable tray than for conduit.

Cooper B-Line, Inc Cable Tray Manual

4

3

12

5

The higher the elevation of the wiring system,the more important the number of componentsre q u i red to complete the instal lation. Manyadditional man-hours will be re q u i red just movingthe components needed for the conduit system upto the work location.

Conduit wiring systems re q u i re pull boxes orsplice boxes when there is the equivalent of morethan 360 degrees of bends in a run. For larg econductors, pull or junction boxes may be requiredmore often to facilitate the conductors installation.Cable tray wiring systems do not require pull boxesor splice boxes.

Penetrating a masonry wall with cable trayrequires a smaller hole and limited repair work.

M o re supports are normally re q u i red for rigidsteel conduit due to the re q u i rements of N E CTable 344.30(B)(2).

Concentric conduit bends for direction changesin conduit banks are very labor intensive and difficultto make. However if they are not used, theinstallation will be unattractive. The time required tomake a concentric bend is increased by a factor of3-6 over that of a single shot bend. This timeconsuming practice is eliminated when cable traywiring systems are used.

Conductor pulling is more complicated and timeconsuming for conduit wiring systems than for cabletray wiring systems. Normally, single conductor wirepulls for conduit wiring systems require multiple reelsetups. For conduit wiring systems, it is necessary topull from termination equipment enclosure totermination equipment enclosure. Tray cables beinginstalled in cable trays do not have to be pulled intothe termination equipment enclosures. Tray cablemay be pulled from near the first term i n a t i o ne n c l o s u re along the cable tray route to near thesecond termination enclosure. Then, the tray cableis inserted into the equipment enclosures fortermination. For projects with significant numbers oflarge conductors terminating in switchgear, this maybe a very desirable feature that can save hours of anelectrician's time. Unnecessary power outages canbe eliminated since tray cable pulls may be madewithout de-energizing the equipment. For conduitinstallations, the equipment will have to be de-e n e rgized for rubber safety blanketing to beinstalled, otherwise the conductor pulls might have

to be made on a weekend or on a holiday atp remium labor costs to avoid shutting downp roduction or data processing operations duringnormal working hours.

Conductor insulation damage is common inconduits since jamming can occur when pulling theconductors. Jamming is the wedging of conductorsin a conduit when three conductors lay side by sidein a flat plane. This may occur when pulling aroundbends or when the conductors twist. Ninety-twopercent of all conductor failures are the result of theconductors insulation being damaged during theconductors insta l lation. Many commoncombinations of conductors and conduits fall intocritical jam ratio values. Critical jam ratio (J.R.=Conduit ID/Conductor OD) values range from 2.8to 3.2. The J. R. for 3 single conductorTHHN/THWN insulated 350 kcmil conductors in a21/2 inch conduit would be 3.0 (2.469 inches/0.816 inches). If conductor insulation damageoccurs, additional costs and time are re q u i red forreplacing the conductors. This cannot occur in acable tray wiring system.

Smaller electrician crews may be used to installthe equivalent wiring capacity in cable tray. Thisallows for manpower leveling, the peak and averagec rew would be almost the same number, and theelectrician experience level re q u i red is lower forcable tray installations.

Since the work is completed faster there is lesswork space conflict with the other constructiondisciplines. This is especially true if installations areelevated and if significant amounts of piping arebeing installed on the project.

MAINTENANCE SAVINGS

One of the most important features of cabletray is that tray cable can easily be installed inexisting trays if there is space available. Cable traywiring systems allow wiring additions ormodifications to be made quickly with minimumdisruption to operations. Any conceivable changethat is re q u i red in a wiring system can be done atlower cost and in less time for a cable tray wiringsystem than for a conduit wiring system.


M o i s t u re is a major cause of e lectricalequipment and material failures. Breathing due tot e m p e r a t u re cycling results in the conduitsaccumulating relatively large amounts of moisture .The conduits then pipe this moisture into theelectrical equipment enclosures which over a periodof time results in the deterioration of the equipmentinsulation systems and their eventual failure. Also,m o i s t u re may become a factor in the corro s i o nfailure of some of the critical electrical equipment'smetallic components. Conduit seals are not effectivein blocking the movement of moisture. The conduitsystems may be designed to reduce the moisturep roblems but not to completely eliminate it. Fewdesigners go into the design detail necessary toreduce the effects of moisture in the conduitsystems. Tray cables do not provide intern a lmoisture paths as do conduits.

In the event of external fires in industrialinstallations, the damage to the tray cable and cabletray is most often limited to the area of the flamecontact plus a few feet on either side of the flamecontact area. For such a fire enveloping a steelconduit bank, the steel conduit is a heat sink and theconductor insulation wi ll be damaged for aconsiderable distance inside the conduit.T h e rmoplastic insulation may be fused to the steelconduit and the conduit will need to be replaced formany feet. This occurred in an Ohio chemical plantand the rigid steel conduits had to be replaced for90 feet. Under such conditions, the repair cost forfire damage would normally be greater for a conduitwiring system than for cable tray and tray cable. Inthe Ohio chemical plant fire, there were banks ofconduits and runs of cable tray involved. The cabletray wiring systems were repaired in two days. Theconduit wiring systems were repaired in six days andrequired a great deal more manpower.

In the event of an external fire, the conduitbecomes a heat sink and an oven which decreasesthe time re q u i red for the conductor insulationsystems to fail. The heat decomposes the cablejackets and the conductor insulation material. Ifthese materials contain PVC as do most cables,hydrogen chloride vapors will come out the ends ofthe conduits in the control rooms. These fumes arevery corrosive to the electronic equipment. They arealso hazardous to personnel. A flame impingementon a cable tray system will not result in the fumesgoing into the control room as there is nocontainment path for them. They will be dispersedinto the atmosphere.

IN MOST CASES AN OBJECTIVEE VA L U ATION OF THE REQUIREMENTSFOR MOST HIGH DENSITY WIRINGSYSTEMS WILL SHOW THAT A CABLET R AY WIRING SYSTEM PROVIDES AWIRING SYSTEM SUPERIOR TO ACONDUIT WIRING SYSTEM.

Abandoned CablesEasily identified, marked, or removed - allpossible from an open Cable Tray System

For the 2002 National Electrical Code, severalproposals were submitted to the NFPA to revise the1999 NEC for Articles 300, 640, 645, 725, 760,770, 800, 820, and 830 to require all abandonedcables to be removed from plenum spaces.

The purpose of the proposals is to remove thecables as a source of excess combustibles fro mplenums and other confined spaces such as raisedfloors and drop ceilings. All of the Code MakingPanels agreed that this should be acceptable practiceexcept Code Making Panel 3, which overseesArticle 300.

Because Artic le 300 is exempt f rom thisre q u i rement only low-voltage and communicationcables are affected.

Each Article adopted a definition of abandonedcables and the rule for removal. The generalconsensus is that abandoned cable is cable that isnot terminated at equipment or connectors and isnot identified for future use with a tag. Please referto each individual NEC Article for specifics.

Having to tag, remove, or rearrange cables withinan enclosed raceway can be a time consuming andd i fficult job. Without being able to clearly see thecables and follow their exact routing throughout afacility, identifying abandoned cables would be verydifficult and expensive.

With the open accessibility of cable tray, thesechanges can be implemented with ease. Abandonedcables can be identified, marked, rearranged, orremoved with little or no difficulty.


392.1. Scope.

Of the types of cable trays listed in this section,ladder cable tray is the most widely used type ofcable tray due to several very desirable features.

The rungs provide a convenient anchor fortying down cables in vertical runs or where thepositions of the cables must be maintained inhorizontal runs.

Cables may exit or enter through the top or thebottom of the tray.

A ladder cable tray without covers provides forthe maximum free flow of air, dissipating heatproduced in current carrying conductors.

M o i s t u re cannot accumulate in ladder cabletrays and be piped into electrical equipment ashappens in conduit systems.

Ladder cable tray cannot pipe hazardous orexplosive gasses from one area to another ashappens with conduit systems.

In areas where there is the potential for dust toaccumulate, ladder cable trays should be installed.The dust buildup in ladder cable trays will be lessthan the dust buildup in ventilated trough or solidbottom cable trays.

Ladder cable trays are available in widths of 6, 9,12, 18, 24, 30, 36, and 42 inches with rungspacings of 6, 9, 12, or 18 inches. Wider rungspacings and wider cable tray widths decrease the

overall strength of the cable tray. Specifiers shouldbe aware that some cable tray manufacturers do notaccount for this load reduction in their publishedcable tray load charts. B-Line uses stronger rungs inwider cable trays to safely bear the loads published.

With one exception, the specifier selects the rungspacing that he or she feels is the most desirable forthe installation. The exception is that 9 inches is themaximum allowable rung spacing for a ladder cabletray supporting any 1/0 through 4/0 singleconductor cables [See Section 392.3(B)(1)(a)].

W h e re the ladder cable tray supports smalldiameter multiconductor control andinstrumentation cables; 6, 9, or 12 inch rungspacings should be specified. Quality Type TC,Type PLTC, or Type ITC smal l d iametermulticonductor control and instrumentation cableswill not be damaged due to the cable tray rungspacing selected, but the installation may not appearneat if there is significant drooping of the cablesbetween the rungs.

For ladder cable trays supporting large powercables, 9 inch or wider rung spacings should beselected. For many installations, the cable trays arerouted over the top of a motor control center (MCC)or switchgear enclosure. Cables exit out the bottomof the cable trays and into the top of the MCC orswitchgear enclosure. For these installations, thecable manufacturer's recommended minimumbending radii for the specific cables must not beviolated. If the rung spacing is too close, it may benecessary to remove some rungs in order tomaintain the proper cable bending radii. Thisconstruction site modification can usually be avoidedby selecting a cable tray with 12 or 18 inch rungspacing.

If you are still uncertain as to which rung spacingto specify, 9 inch rung spacing is the most commonand is used on 80% of the ladder cable tray sold.

9

Standard Aluminum Ladder

AN IN-DEPTH LOOK AT 2005 NECARTICLE 392 - CABLE TRAY

(The following code explanations are to be used with a copy of the 2005 NEC.)To obtain a copy of the NEC contact:

National Fire Protection Association1 Batterymarch Park P.O. Box 9101Quincy, Massachusetts 02269-9101

1-800-344-3555

Cable Tray Manual Cooper B-Line, Inc

The 1999 N E C added the word ventilated infront of trough to clear up some confusion that solidtrough is treated the same as ventilated trough. It isnot. Solid trough is recognized as solid bottom cabletray.

Ventilated trough cable tray is often used when thespecifier does not want to use ladder cable tray tosupport small diameter multiconductor control andinstrumentation cables. As no drooping of the smalldiameter cables is visible, ventilated trough cabletrays provide neat appearing installations. Smalldiameter cables may exit the ventilated trough cabletray through the bottom ventilation holes as well asout the top of the cable tray. For installations wherethe cables exit the bottom of the cable tray and thesystem is subject to some degree of vibration, it isadvisable to use B-Line Trough Drop-Out Bushings(Cat. No. 99-1124). These snap-in bushings provideadditional abrasion protection for the cable jackets.Just as for ladder cable tray, ventilated trough cabletray will not pipe moisture into electrical equipment.

S t a n d a rd widths for ventilated trough cable traysystems are 6, 9, 12, 18, 24, 30, and 36 inches.The standard bottom configuration for ventilatedtrough cable tray is a corrugated bottom with 2 7/8inch bearing surfaces - 6 inches on centers and 21/4 inch x 4 inch ventilation openings. Since acorrugated bottom cannot be bent horizontally, thes t a n d a rd bottom configuration for horizontal bendfittings consists of rungs spaced on 4 inch centers.This diff e rence in bottom construction may beobjectionable to some owners, so be sure you areaware of the owner's sensitivity to aesthetics for thecable tray installation.

Channel cable tray systems (B-Line 's cablechannel) are available in 3, 4, and 6 inch widthswith ventilated or solid bottoms. The NEC n o wrecognizes solid bottom cable channel. Priorto the 2002 Code, the N E C did not have anyspecific provisions for the use of solid cable channel.

Instead of large conduits, cable channel may beused very effectively to support cable drops from thecable tray run to the equipment or device beingserviced and is ideal for cable tray runs involving asmall number of cables. Cable channel may also beused to support push buttons, field mountedinstrumentation devices, etc. Small diameter cablesmay exit ventilated cable channel through thebottom ventilation holes, out the top or through theend. For installations where the cables exit throughthe ventilation openings and the cable channel orthe cables are subject to some degree of vibration, itis advisable to use B-Line Cable Channel Bushings(Cat. No. 99-1125). These snap-in plastic bushingsprovide additional abrasion protection for the cablejackets.

Some specifiers prefer solid bottom cable tray tosupport large numbers of small diameter control andmulticonductor instrumentation cables. Solid bottomsteel cable trays with solid covers and wrap aroundcover clamps can be used to provide EMI/RFIshielding protection for sensitive circuits.

Unlike ladder and ventilated trough cable trays,solid bottom cable trays can collect and re t a i nm o i s t u re. Where they are installed outdoors orindoors in humid locations and EMI/RFI shieldingp rotection is not re q u i red, it is recommended that1/4 inch weep holes be drilled in their bottoms atthe sides and in the middle every 3 feet to limitwater accumulation.

10

Vent. Channel Cable Tray(B-Line's Cable Channel)

Aluminum Solid Bottom Trough

Cooper B-Line, Inc Cable Tray Manual

Steel Ventilated Trough

The words "and other similar structures." wereincorporated in Section 392.1 for future types ofcable tray that might be developed, such as centersupported type cable tray. Al l the technicali n f o rmation developed by the 1973 N E CTechnical Subcommittee on Cable Tray for Article318 - Cable Trays was based on cable trays withside rails and this technical information is still thebasis for the 2005 NEC Article 392 - Cable Trays.

The standard lengths for cable trays are 10, 12,20 and 24 feet (consult B-Line for the availability ofn o n s t a n d a rd cable tray lengths). Selecting a cabletray length is based on several criteria. Some ofthese criteria include the required load that the cabletray must support, the distance between the cabletray supports, and ease of handling and installation.One industry standard that is stro n g l yrecommended is that only one cable traysplice be placed between support spans and,for long span trays, that they ideally be place at1/4-span. This automatically limits the length of trayyou choose, as the tray must be longer than or equalto the support span you have selected. Matching thetray length to your support span can help ensurethat your splice locations are controlled.

Cable trays can be organized into 4 categories:Short Span, Intermediate Span, Long Span, andExtra-Long Span.

Short Span trays, typically used for non-industrialindoor installations, are usually supported every 6 to8 feet, while Intermediate Span trays are typicallysupported every 10 to 12 feet. A 10 or 12 footcable tray is usually used for both of these types ofinstallations. To keep from allowing two splices tooccur between supports, a 12 foot tray should beused for any support span greater than 10 feet, upto 12 feet. Placing the cable tray splices at 1/4-spanis not crit ical in a short or intermediate spanapplication given that most trays have suff i c i e n t l ystrong splice plates.

In an indoor industrial installation 10 or 12 foottray sections may be easier to handle and install asyou may have piping or ducting to maneuveraround. However, using 20 foot instead of 12 footstraight sections may provide labor savings duringinstallation by reducing the number of splice joints.If this is done, the selected tray system should meetthe loading re q u i rements for the support span youa re using. If you are interested in supporting 100lbs/ft and you are buying 20 foot tray sections whilesupporting it every 12 feet, it isnt necessary tospecify a NEMA 20C tray (100 lbs/ft on a 20 footspan). A NEMA 20A tray (50 lbs/ft on a 20 footspan) will support over 130 lbs/ft when supportedon a 12 ft span with a safety factor of 1.5.Specifying a 20C tray is not an economical use ofp roduct. If you desire to use 20 foot sections ofcable tray, it makes more sense to increase yoursupport span up to 20 feet. This not only saveslabor by decreasing the number of splices, but alsoby decreasing the number of supports that must beinstalled.

Long Span trays are typically supported anywherefrom 14 to 20 foot intervals with 20 feet being themost popular. In long span situations, the placementof the splice locations at 1/4-span becomes muchm o re important. Matching the tray length to yoursupport span can help control your splice locations.

Extra-Long Span trays are supported on spansexceeding 20 feet. Some outdoor cable trayinstallations may have to span anywhere from 20 to30 feet to cross roads or to reduce the number ofexpensive outdoor supports. The distance betweensupports affects the tray strength exponentially;t h e re f o re the strength of the cable tray systemselected should be designed around the specificsupport span chosen for that run.

[See Sect ion 392.5(A) on page 18 for addit ionalinformation on cable tray strength and rigidity.]

B-Line has many cataloged fittings and accessoryitems for ladder, ventilated trough, ventilatedchannel, and solid bottom cable trays whicheliminate the need for the costly field fabrication ofsuch items. When properly selected and installed,these factory fabricated fittings and accessoriesimprove the appearance of the cable tray system inaddition to reducing labor costs.


Center Supported Cable Tray(B-Lines Cent-R-Rail System)

Cable Tray Materials

Metallic cable trays are readily available in aluminum,p regalvanized steel, hot-dip galvanized afterfabrication, and stainless steel. Aluminum cable trayshould be used for most installations unless specificc o r rosion problems prohibit its use. Aluminum's lightweight significantly reduces the cost of installationwhen compared to steel.

A fine print note is included in the 2005 NEC thatre f e rences the National Electrical Manufacture r sAssociation (NEMA) documents for furtheri n f o rmation on cable tray. These documents:ANSI/NEMA VE-1, Metal Cable Tray Systems;NEMA VE-2, Cable Tray Installation Guidelines; andNEMA FG-1, Non Metallic Cable Tray Systems, arean excellent industry re s o u rce in the application,selection, and installation of cable trays both metallicand non metallic. Contact Cooper B-Line for moreinformation concerning these helpful documents.

392.2. Definition. Cable Tray System.This section states that cable tray is a rigid

structural support system used to securely fasten orsupport cables and raceways. Cable trays are notraceways. Cable trays are mechanical supports justas strut systems are mechanical supports. N E CArticle 392 - Cable Trays is an article dedicated to atype of mechanical support. It is very important thatthe personnel involved with engineering andinstalling cable tray utilize it as a mechanical supportsystem and not attempt to utilize it as a racewaysystem. There are items in the NEC that apply toraceways and not to cable tray. There are also itemsin the N E C that apply to cable tray and not toraceways. These diff e rences will be covered at theappropriate locations in this manual.

392.3. Uses Permitted. Cable trayinstallations shall not be limited toindustrial establishments.

The text in Section 392.3 clearly states that cabletray may be used in non-industrial establishments.The use of cable tray should be based on soundengineering and economic decisions.

For clarity, the NEC now lists all types of circuitsto explicitly permit their use in cable trays. Thesec i rcuit types include: services, feeders, branchcircuits, communication circuits, control circuits, andsignaling circuits.

The 2002 N E C also added a new re q u i re m e n tthat where cables in tray are exposed to the directrays of the sun, they shall be identified as sunlightresistant for all occupancies, not just industrial.

392.3. Uses Permitted. (A) WiringMethods.

This section identif ies the 300 & 600 vol tmulticonductor cables that may be supported bycable tray. The "Uses Permitted" or "Uses NotPermitted" sections in the appropriate NEC cablearticles provide the details as to where that cabletype may be used. Where the cable type may beused, cable tray may be installed to support it exceptas per Section 392.4 which states that cable traysshall not be installed in hoistways or where subjectto severe physical damage. Where not subject tosevere physical damage, cable tray may be used inany hazardous (classified) area to support thea p p ropriate cable types in accordance with theinstallation requirements of the various Articles thatmake up NEC Chapter 5 or in any non-hazardous(unclassified) area.

It should be noted that Section 300.8 ofthe NEC states that cable trays containingelectric conductors cannot contain any otherservice that is not electrical. This includesany pipe or tube containing steam, water, air,gas or drainage.

For commercial and industrial cable tray wiringsystems: Type ITC, Type MC, Type TC, and TypePLTC multiconductor cables are the most commonlyused cables. Type MI and Optical-Fiber cables arespecial application cables that are desirable cablesfor use in some cable tray wiring systems. Thefollowing paragraphs provide information andcomments about these cable types.

Type MI Cable: Mineral- Insulated, MetalSheathed Cable (Article 332). This cable has aliquid and gas tight continuous copper sheath overits copper conductors and magnesium oxideinsulation. Developed in the late 1920's by theFrench Navy for submarine electrical wiring systems,p roperly installed MI cable is the safest electricalwiring system available. In Europe, Type MI cablehas had a long, successful history of being installed(with PVC jackets for corrosion protection) in cabletrays as industrial wiring systems. This cable may beinstalled in hazardous (classified) areas or in non-h a z a rdous (unclassified) areas. The single limitation


on the use of Type MI cable is that it may not beused where it is exposed to destructive corro s i v econditions unless protected by materials suitable forthe conditions. Type MI cable without overallnonmetallic coverings may be installed in ducts orplenums used for environmental air and in otherspace used for environmental air in accordance withSections 300.22(B) and (C). Cable tray may beinstalled as a support for Type MI cable in anylocation except where the cable is installed in ahoistway. Section 332-30 states that MI cable shallbe securely supported at intervals not exceeding 6feet (1.83 m). Type MI cable has a UL two hour fireresistive rat ing when properly instal led. Aninstallation re q u i rement for this rating is that thecable be securely supported every 3 feet. Steel orstainless steel cable trays should be used to supportType MI cable being used for critical circuit service.During severe fire conditions, steel or stainless steelcable tray will remain intact and provide supportlonger than aluminum or fiberglass reinforced plasticcable trays.

Type MC Cable: Metal-clad cable (Article 330).There are large amounts of Type MC cable installedin industrial plant cable tray systems. This cable isoften used for feeder and branch circuit service andp rovides excellent service when it is pro p e r l yinstalled. The metallic sheath may be interlockingmetal tape or it may be a smooth or corrugatedmetal tube. A nonmetallic jacket is often extrudedover the aluminum or steel sheath as a corro s i o np rotection measure. Regular MC cable, withoutnonmetallic sheath, may be supported by cable trayin any hazardous (classified) area except Class I andClass II, Division 1 areas. For Type MC cables toqualify for installation in Class I and Class II DivisionI areas (Section 501-4(A) (1) (c&d), they must have agas/vapor tight continuous corrugated aluminumsheath with a suitable plastic jacket over the sheath.They must also contain equipment gro u n d i n gconductors and listed termination fittings must beused where the cables enter equipment. Type MCCable employing an impervious metal sheathwithout overall nonmetallic coverings may beinstalled in ducts or plenums used for environmentalair in accordance with Section 300.22(B) and maybe installed in other space used for enviro n m e n t a lair in accordance with Section 300.22(C). Themaximum support spacing is 6 feet (1.83 m).

Type TC Cable: Power and control tray cable(Article 336). This cable type was added to the

1975 NEC (as an item associated with the revisionof Article 318-Cable Trays). Type TC cable is amulticonductor cable with a f lame re t a rd a n tnonmetallic sheath that is used for power, lighting,c o n t rol, and signal circuits. It is the most commoncable type installed in cable tray for 480 voltfeeders, 480 volt branch circuits, and contro lc i rcuits. Where Type TC cables comply with thecrush and impact re q u i rements of Type MC cableand is identified for such use, they are permitted asopen wiring between a cable tray and the utilizationequipment or device. In these instances where thecable exits the tray, the cable must be supported ands e c u red at intervals not exceeding 6 feet (SeeSection 336.10(6)). The service record of UL listedType TC cable where properly applied and installedhas been excellent.

For those installations where the NEC allows itsuse, a cost savings is realized by using Type TCcables instead of Type MC cables. Type TC cablemay be instal led in cable tray in hazard o u s(classified) industrial plant areas as permitted inArticles 392, 501, 502, 504 and 505 provided theconditions of maintenance and supervision assurethat only quali f ied persons wi ll service theinstallation [See Section 336.10(3)].

Where a cable tray wiring system containing TypeTC cables will be exposed to any significant amountof hot metal splatter from welding or the torc hcutting of metal during construction or maintenanceactivities, temporary metal or plywood covers shouldbe installed on the cable tray in the exposure areasto prevent cable jacket and conductor insulationdamage. It is desirable to use only quality Type TCcables that will pass the IEEE 383 and UL VerticalFlame Tests (70,000 BTU/hr). Type TC cableassemblies may contain optical fiber members as perthe UL 1277 standard.

Type ITC Cable: Instrumentation Tray Cable(Article 727). Although this was a new cable articlein the 1996 N E C, it is not a new type of cable.Thousands of miles of ITC cable have been installedin industrial situations since the early 1960s. Thisis a multiconductor cable that most often has anonmetallic jacket. The No. 22 through No. 12insulated conductors in the cables are 300 voltrated. A metallic shield or a metallized foil shieldwith a drain wire usually encloses the cablesconductors. These cables are used to transmit thelow energy level signals associated with the industrial


instrumentation and data handling systems. Theseare very critical circuits that impact on facility safetyand on product quality. Type ITC cable must besupported and secured at intervals not exceeding 6feet [See Section 727.4].

Type ITC Cable may be installed in cable trays inh a z a rdous (classified) areas as permitted in Articles392, 501, 502, 504 and 505. It states in Article727 that Type ITC cables that comply with thecrush and impact re q u i rements of Type MC cableand are identified for such use, are permitted asopen wiring in lengths not to exceed 50 ft. betweena cable tray and the utilization equipment or device.W h e re a cable tray wiring system containing Ty p eITC cables will be exposed to any significant amountof hot metal splatter from welding or the torc hcutting of metal during construction or maintenanceactivities, temporary metal or plywood covers shouldbe installed on the cable tray to prevent cable jacketor conductor insulation damage. It is desirable to useonly quality Type ITC cables that will pass the IEEE383 and UL Vertical Flame Tests (70,000BTU/hr).

Type PLTC Cable: P o w e r-Limited Tray Cable(Sections 725-61(C), and 725-71(E)). This is amulticonductor cable with a f lame re t a rd a n tnonmetallic sheath. The No. 22 through No. 12insulated conductors in the cables are 300 voltrated. A metallic shield or a metallized foil shieldwith drain wire usually encloses the cable 'sconductors. This cable type has high usage incommunication, data processing, fire pro t e c t i o n ,signaling, and industrial instrumentation wiringsystems.

There are versions of this cable with insulation andjacket systems made of materials with low smokeemission and low flame spread properties whichmake them desirable for use in plenums. InIndustrial Establishments where the conditions ofmaintenance and supervision ensure that onlyqualified persons service the installation and wherethe cable is not subject to physical damage Ty p eP LTC cable may be insta lled in cable traysh a z a rdous (classified) areas as permitted in Section501.4(B), 502.4(B) and 504.20. Type PLTC cablesthat comply with the crush and impact requirementsof Type MC cable and are identified for such use,a re permitted as open wiring in lengths not toexceed a total of 50 ft. between a cable tray and theutilization equipment or device. In this situation, thecable needs to be supported and secured at intervalsnot exceeding 6 ft. Where a cable tray wiring system

containing Type PLTC cables will be exposed to anysignificant amount of hot metal splatter fro mwelding or the torch cutting of metal duringconstruction or maintenance activities, temporarymetal or plywood covers should be installed on thecable tray to prevent cable jacket and conductorinsulation damage. It is desirable to use only qualityType PLTC cables that will pass the IEEE 383 andUL Vertical Flame Tests (70,000 BTU/hr). Ty p eP LTC cable assemblies may contain optical fibermembers as per the UL 1277 standard.

Optical Fiber Cables (Article 770). The additionof optical fiber cables in the Section 392.3(A) cablelist for the 1996 NEC was not a technical change.Optical f iber cables have been allowed to besupported in cable trays as per Section 770.6.Optical fibers may also be present in Type TCcables as per UL Standard 1277.

For the 1999 N E C code, Article 760 - FireAlarm Cables and Articles 800 - Multipurpose andCommunications Cables were added to the list ofcables permitted to be installed in cable traysystems.

For the 1993 NEC, the general statement in the1990 NEC which allowed all types of raceways tobe supported by cable trays was replaced byindividual statements for each of the ten specificraceway types that may now be supported by cabletray. The chances of any such installations beingmade are very low, since strut is a more convenientand economic choice than cable tray to supportraceway systems.

392.3. Uses Permitted. (B) In IndustrialEstablishments.

This section limits the installation of singleconductor cables and Type MV multiconductorcables in cable trays to quali fy ing industrialestablishments as defined in this section.

Per the 2002 NEC solid bottom cable trays arenow permitted to support single conductor cablesonly in industrial establishments where conditions ofmaintenance and supervision ensure that onlyqualified persons will service the installed cable traysystem. However, at this time, no fill rules for singleconductor cables in solid bottom cable tray havebeen established. [see Section 392.3(B)]


392.3. Uses Permitted. (B) In IndustrialEstablishments. (1) Single Conductor.

Section 392.3(B)(1) covers 600 volt and Type MVsingle conductor cables.

T h e re are several sections which cover therequirements for the use of single conductor cablesin cable tray even though they only comprise a smallp e rcentage of cable tray wiring systems. Suchinstallations are limited to qualifying industrialfacilities [See Section 392.3(B)]. Many of the facilityengineers prefer to use three conductor powercables. Normally, three conductor power cablesprovide more desirable electrical wiring systems thansingle conductor power cables in cable tray ( S e eSection 392.8. Cable installation - three conductor vs. singleconductor cables).

392.3(B)(1)(a)Single conductor cable shall be No. 1/0 or larger

and shall be of a type listed and marked on thes u rface for use in cable trays. Where Nos. 1/0t h rough 4/0 single conductor cables are used, themaximum allowable rung spacing for ladder cabletray is 9 inches.

392.3(B)(1)(b)Welding cables shall comply with Article 630, Part

IV which states that the cable tray must pro v i d esupport at intervals not to exceed 6 inches. Apermanent sign must be attached to the cable trayat intervals not to exceed 20 feet. The sign mustread CABLE T R AY F O R W E L D I N G C A B L E SONLY.

392.3(B)(1)(c)This section states that single conductors used as

equipment grounding conductors (EGCs) in cabletrays shall be No. 4 or larger insulated, covered orbare.

The use of a single conductor in a cable tray asthe EGC is an engineering design option. Section300.3(B) states that all conductors of the samec i rcuit and the EGC, if used, must be containedwithin the same cable tray.

The other options are to use multiconductorcables that each contain their own EGC or to usethe cable tray itself as the EGC in qualifyinginstallations [see Section 392.3(C)]

If an aluminum cable tray is installed in a moiste n v i ronment where the moisture may containmaterials that can serve as an electrolyte, a bare

copper EGC should not be used. Under suchconditions, electrolytic corrosion of the aluminummay occur. For such installations, it is desirable touse a low cost 600 volt insulated conductor andremove the insulation where connections toequipment or to equipment grounding conductorsa re made. (See Section 392.7. Grounding, for additionalinformation on single conductors used as the EGC for cabletray systems).

392.3. Uses Permitted. (B) In IndustrialEstablishment (2) Medium Voltage.

Single and multiconductor type MV cables must besunlight resistant if exposed to direct sunlight. Singleconductors shall be installed in accordance with392.3(B)(1)

392.3. Uses Permitted. (C) EquipmentGrounding Conductors.

Cable tray may be used as the EGC in anyinstallation where qualified persons will service theinstalled cable tray system. There is no restriction asto where the cable tray system is installed. Themetal in cable trays may be used as the EGC as perthe limitations of table 392.7(B)(2). See Section392.7. Grounding in this manual for additionalinformation on the use of cable trays as the EGC.

392.3. Uses Permitted. (D) Hazard o u s(Classified) Locations.

This section states that if cable tray wiring systemsa re installed in hazardous (classified) areas, thecables that they support must be suitable forinstallation in those hazardous (classified) areas. Thecable carries the installation restriction. T h einstallation restriction is not on the cable tray exceptthat the cable tray installations must comply withSection 392.4. The following is an explanation ofthe parts of the code which affect the use of cabletray in hazardous locations.

501.10. Wiring Methods - Listed Te rm i n a t i o nFittings. (A) Class I, Division 1 (Gases or Vapors).501.10(A)(1)(b) Type MI cable may be installed incable tray in this type of hazardous (classified) area.

501.10(A)(1)(c) allows Type MC-HL cables to beinstalled in Class I, Division I areas if they have agas/vapor tight continuous corrugated aluminumsheath with a suitable plastic jacket over the sheath.They must also contain equipment gro u n d i n gconductors sized as per Section 250.122 and listedt e rmination fittings must be used where the cablesenter equipment.


501.10(A)(1)(d) allows Type ITC-HL cable to beinstalled in Class I, Division I areas if they have agas/vapor tight continuous corrugated aluminumsheath with a suitable plastic jacket over the sheathand provided with termination fittings listed for theapplication.

501.10. Wiring Methods. (B) Class I, Division 2(Gases or Vapors). Types ITC, PLTC, MI, MC, MV,or TC cables may be installed in cable tray in thistype of hazardous (classified) area. Under theconditions specified in Section 501.15(E), Cableseals are required in Class 1, Division 2 areas. Cableseals should be used only when absolutely necessary.

501.15. Sealing and Drainage. (E) Cable Seals,Class 1, Division 2. (1) Cables will be required to besealed only where they enter c e r t a i n types ofenclosures used in Class 1, Division 2 areas. Factorysealed push buttons are an example of enclosure sthat do not require a cable seal at the entrance ofthe cable into the enclosure.

501.15. Sealing and Drainage. (E) Cable Seals,Class 1, Division 2. (2) Gas blocked cables areavailable from some cable manufacturers but theyhave not been widely used. For gas to pass throughthe jacketed multiconductor cable's core, a pressuredifferential must be maintained from one end of thecable to the other end or to the point where there isa break in the cable's jacket. The existence of such acondition is extremely rare and would re q u i re thatone end of the cable be in a pre s s u re vessel or apressurized enclosure and the other end be exposedto the atmosphere. The migration of any significantvolume of gas or vapor though the core of amulticonductor cable is very remote. This is one ofthe safety advantages that cable tray wiring systemshave over conduit wiring systems. There aredocumented cases of industrial explosions caused bythe migration of gases and vapors through conduitswhen they came in contact with an ignition source.T h e re are no known cases of cables in cable traywiring systems providing a path for gases or vaporsto an ignition source which produced an industrialexplosion.

501.15. Sealing and Drainage. (E) Cable Seals,Class 1, Division 2. (3)

Exception: Cables with an unbroken gas/vapor-tight continuous sheath shall be permitted to passt h rough a Class 1, Division 2 location withoutseals.

This is an extremely important exception statingthat cable seals are not required when a cable goesf rom an unclassified area through a classified are athen back to an unclassified area.

501.15. Sealing and Drainage. (E) Cable Seals,Class 1, Division 2. (4)

If you do not have a gas/vapor-tight continuoussheath, cable seals are required at the boundary ofthe Division 2 and unclassified location.

The sheaths mentioned above may be fabricatedof metal or a nonmetallic material.

502.10. Wiring Methods. (A) Class II, Division 1(Combustible Dusts). Type MI cable may beinstalled in cable tray in this type of hazard o u s(classified) area.

The Exception allows Type MC cables to beinstalled in Class II, Division 1 areas if they have agas/vapor tight continuous corrugated aluminumsheath with a suitable plastic jacket over the sheath.They must also contain equipment gro u n d i n gconductors sized as per Section 250.122 and listedt e rmination fittings must be used where the cablesenter equipment.

502.10. Wiring Methods. (B) Class II, Division 2(Combustible Dusts).

This section states:

Type ITC and PLTC cables may be installed inladder or ventilated cable trays following the samepractices as used in non-hazardous (unclassified)a reas. No spacing is re q u i red between the ITC orP LTC cables. This is logical as the ITC and PLT Ccable circuits are all low energy circuits which do notp roduce any significant heat or heat dissipationproblems.

Type MC, MI and TC [See Section 336.4(3)] c a b l e smay be installed in ladder, ventilated trough, orventilated cable channel, but they are not allowed to

be installed in solid bottom cable trays.

Required Spacing in Cable Trays for Type MC, MI & TCCables in Class II, Division 2 Hazardous (Classified) Areas


D1 D1 D2 D3D2 D1 D1 D1

Note 1. The cables are limited to a single layerwith spacing between cables equal to the diameterof the largest adjacent cable. This means that thecables must be tied down at frequent intervals inhorizontal as well as vertical cable trays to maintainthe cable spacing. A reasonable distance betweenties in the hor izontal cable tray would bea p p roximately 6 feet (See Sect ion 392.8 CableInstallation - Tying cables to cable trays).

Note 2. Spacing the cables a minimum of 1 inchf rom the side rails to prevent dust buildup isrecommended. This is not an NEC requirement buta recommended practice.

W h e re cable tray wiring systems with curre n tcarrying conductors are insta lled in a dustenvironment, ladder type cable trays should be usedsince there is less surface area for dust buildup thanin ventilated trough cable trays. The spacing of thecables in dust areas will prevent the cables fro mbeing totally covered with a solid dust layer. In dustyareas, the top surfaces of all equipment, raceways,supports, or cable jacket surfaces where dust layerscan accumulate will require cleanup housekeeping atcertain time intervals. Good housekeeping isrequired for personnel health, personnel safety andfacil ity safety. Excessive amounts of dust onraceways or cables will act as a thermal barrierwhich may not allow the power and lightinginsulated conductors in a raceway or cable to safelydissipate internal heat. This condition may result inthe accelerated aging of the conductor insulation. Acable tray system that is properly installed andmaintained will provide a safe dependable wiringsystem in dust environments.

Exception: Type MC cable listed for use in ClassII,Division I locations shall be permitted to beinstalled without the above spacing limitations. Thiswas a new exception for the 1999 NEC code.

For this type of wiring there is no danger of thecables being overheated when covered with dust.The current flow in these circuits is so low that thei n t e rnally generated heat is insufficient to heat thecables and cable spacing is not a necessity. Evenunder such conditions, layers of dust should not beallowed to accumulate to critical depths as they maybe ignited or explode as the result of pro b l e m scaused by other than the electrical system.

502.10(B)(3). Nonincendive Field WiringWiring in nonincendive circuits shall be perm i t t e d

using any of the wiring methods suitable for wiring

in ordinary locations.

503.10. Wiring Methods. (A) Class III, Division1 and (B) Class III, Division 2 (Ignitable Fibers orFlyings). Type MI or MC cables may be installed incable tray in these types of hazardous (classified)a reas. The installations should be made usingpractices that minimize the build-up of materials inthe trays. This can be done by using ladder cabletray with a minimum spacing between the cablesequal to the diameter of the largest adjacent cable.In some cases, a greater spacing between cablesthan that based on the cable diameters might bedesirable depending on the characteristics of thematerial that requires the area to be classified. Hereagain, i t must be emphasized that goodhousekeeping practices are required for all types ofwiring systems to insure the safety of the personneland the facility.

504.20. Wiring Methods. This section allowsintrinsically safe wiring systems to be installed incable trays in hazardous (classified) areas. Section504.30 specifies the installation re q u i rements forintrinsically safe wiring systems that are installed incable trays. Section 504.70 specifies the sealingrequirements for cables that may be part of a cabletray wiring system. Section 504.80(B) states thatcable trays containing intrinsically safe wiring mustbe identified with permanently affixed labels.

Cable trays are ideal for supporting bothintrinsically safe and nonintrinsically safe cablesystems as the cables may be easily spaced and tiedin position or a standard metallic barrier strip maybe insta l led between the intrinsical ly andnonintrinsically safe circuits.

505.15. Wiring Methods. This section wasadded to the 2002 NEC to explicitly permit cabletrays in hazardous areas classif ied by theinternational zone system, if the cables comply withthe cable requirements for zone locations.

392.3. Uses Permitted. (E) NonmetallicCable Tray.

T h e re are limited numbers of applications wherenonmetallic cable trays might be pre f e r red overmetallic cable trays for electrical safety re a s o n sand/or for some corrosive conditions. An exampleof an electrical safety application would be in anelectrolytic cell room. Here, the amperages are veryhigh and significant stray current paths are present.Under such conditions, there is the possibility for a


high amperage short circuit if a low re s i s t a n c emetallic path (metallic cable tray or metallic raceway)is present [See information under Section 392.5(F)Nonmetallic Cable Trays].

392.4. Uses Not Permitted. This is the only place in the NEC where all the

various types of cable tray have limitations on theirplace of use. No cable trays can be used inhoistways or where subject to severe physicaldamage. The designer must identify the zones ofinstallation where a cable tray might be subjected tos e v e re physical damage. Usually such areas arelimited and provisions can be made to protect thecable tray by relocating it to a more desirablelocation or as a last resort to provide pro t e c t i o nusing the appropriate structural members.

The second sentence of Section 392.4 states thatcable tray shall not be used in ducts, plenums, andother air-handling spaces except to support thewiring methods recognized for use in such spaces.This is not a restriction on cable tray as long as it isused as a support for the appropriate cable types.

Metallic cable trays may support cable typesa p p roved for installation in ducts, plenums, andother air-handling spaces as per Section 300.22(B)and the cable types approved for installation inOther Space Used for Environmental Air as perSection 300.22(C).

The second sentence of Section 300.22(C)(1) is asfollows:

Other types of cables and conductors shallbe installed in electrical metallic tubing,flexible metallic tubing, intermediate metalconduit, rigid metal conduit without anoverall nonmetallic covering, flexible metalconduit, or, where accessible, surface metalraceway or metal wireway with metal coversor solid bottom metal cable tray with solidmetal covers.

Reprinted with permission from NFPA 70-1999, the National Electrical Code,Copyright 1998, National Fire Protection Association, Quincy, MA 02269. This reprintedmaterial is not the complete and official position of the National Fire Protection Association,on the referenced subject which is represented only by the standard in its entirety.

This part of Section 300.22(C) is confusing. Thestatement as underlined in the above paragraphleads some to assume, for installations in OtherSpaces Used for Environmental Air, that the typesof insulated single conductors which are installed inraceway installations may also be installed in solidbottom metal cable trays with metal covers. This is

not so. Only the appropriate multiconductor cabletypes as per Section 392.3(A) may be installed insolid bottom cable trays.

Cable tray may be used to support data pro c e s swiring systems in air handling areas below raisedfloors as per Sections 300.22(D) and 800.52(D).

392.5. Construction Specifications. (A)Strength and Rigidity.

The designer must properly select a structurallysatisfactory cable tray for their installation. Thisselection is based on the cable tray's strength, thecable tray loading and the spacing of the supports.The ANSI/NEMA Metallic Cable Tray SystemsS t a n d a rd Publication VE-1 contains the cable trayselection information and it is duplicated in B-Line'sCable Tray Systems Catalog.

The NEMA Standard provides for a static loadsafety factor of 1.5. A number (Span in Feet - thedistance between supports) and letter (Load in lbs/ft)designation is used to properly identify the cabletray class on drawings, in specifications, in quotationrequisi t ions, and in purchase requisit ions toguarantee that the cable tray with the pro p e rcharacteristics will be received and installed. Thedesigner must specify the cable tray type, thematerial of construction, section lengths, minimumbend radius, width, rung spacing (for a ladder typecable tray), and the total loading per foot for thecables on a maximum support spacing (See page 52for cable tray specificat ions checkl ist). For manyinstallations, the cable trays must be selected so thatthey are capable of supporting specific concentratedloads, the weight of any equipment or materialsattached to the cable tray, ice and snow loading,

and for some installations the impact of windloading and/or earthquakes must be considered.

Most cable trays are utilized as continuous beamswith distributed and concentrated loads. Cable trayscan be subjected to static loads like cable loads anddynamic loads such as wind, snow, ice, and evenearthquakes. The total normal and abnormal loading


for the cable tray is determined by adding all theapplicable component loads. The cable load + theconcentrated static loads + ice load (if applicable) +snow load (if applicable) + wind load (if applicable) +any other logical special condition loads that mightexist. This total load is used in the selection of thecable tray.

The following is an explanation of thehistorical NEMA cable tray loadclassifications found in ANSI/NEMA VE-1.

T h e re used to be four cable tray support spancategories, 8, 12, 16, and 20 feet, which arecoupled with one of three load designations, "A" for50 lbs/ft, "B" for 75 lbs/ft, and "C" for 100 lbs/ft.For example, a NEMA class designation of 20Bidentifies a cable tray that is to be supported at amaximum of every 20 feet and can support a staticload of up to 75 lbs/linear foot.

The cable load per foot is easy to calculate usingthe cable manufacturer's literature. If the cable trayhas space available for future cable additions, acable tray has to be specified that is capable ofsupporting the final future load. Although thesehistorical load designations are stil l useful innarrowing down the choices of cable trays, NEMAhas recent ly changed the VE-1 document.ANSI/NEMA VE-1 now requires the marking on thecable trays to indicate the exact rated load on aparticular span. Trays are no longer limited to thefour spans and three loads listed above. Now, forexample, a tray may be rated for 150 lbs/ft on a 30ft. span. It is recommended when specifying cabletray, to specify the required load, support span andstraight section length to best match the installation.

Example of Cable Loading per foot:

10 - 3/C No. 4/0 (2.62 lbs/ft) Total = 26.20 lbs/ft

3 - 3/C No. 250 kcmil (3.18 lbs/ft) Total = 9.54 lbs/ft

4 - 3/C No. 500 kcmil (5.87 lbs/ft) Total = 23.48 lbs/ft

Total Weight of the Cables = 59.22 lbs/ft

These cables would fill a 30 inch wide cable trayand if a 36 inch wide cable tray were used therewould be space available for future cables (See pages47 thru 53 for information on calculating tray width.). Tocalculate the proper cable tray design load for the36" wide cable tray multiply 59.22 lbs/ft x 36inches/30 inches = 71.06 lbs/ft. If this cable tray is

installed indoors, a load symbol "B" cable tray wouldbe adequate. If there were additional loads on thecable tray or the cable tray were installed outdoors,it would be necessary to calculate all the additionalpotential loads. The potential load most oftenignored is installation loads. The stresses of pullinglarge cables through cable trays can produce 3 timesthe stress of the cables' static load. If the installationload is not evaluated the cable tray may be damagedduring installation. A 16C or 20C NEMA Classshould be specified if large cables are to be pulled.

Even though walking on cable tray is notrecommended by cable tray manufacturers andOSHA regulations, many designers will want tospecify a cable tray which can support a 200 lb.concentrated load "just in case". A concentratedstatic load applied at the midspan of a cable tray isone of the most stressful conditions a cable tray willexperience. To convert a static concentrated load atmidspan to an equivalent distributed load take twicethe concentrated load and divide it by the supportspan [(2 x 200 lbs.)/Span]. The strength of the rungis also a very important consideration whenspecifying a concentrated load. The rung must beable to withstand the load for any tray width, as wellas additional stresses from cable installation.Excessive rung deflection can weaken the entirecable tray system. B-Line uses heavier rungs ontheir wider industrial trays as a standard. Most cabletray manufacturer's rungs are not heavy enough towithstand concentrated loads at 36" tray widths.

For outdoor installations a cable tray might besubject to ice, snow, and wind loading. Section 25of the National Electrical Safety Code (published bythe Institute of Electrical and Electronic Engineers)contains a weather loading map of the United Statesto determine whether the installation is in a light,medium, or heavy weather load district. NESC Table250-1 indicates potential ice thicknesses in eachloading district as follows: 0.50 inches for a heavyloading district, 0.25 inches for a medium loadingdistrict, and no ice for a light loading district. Tocalculate the ice load use 57 pounds per cubic footfor the density of glaze ice. Since tray cables arec i rcular and the cable tray has an irregular surf a c ethe resulting ice load on a cable tray can be 1.5 to2.0 times greater than the glaze ice load on a flatsurface.

Snow load is significant for a cable tray that iscompletely full of cables or a cable tray that hascovers. The density of snow varies greatly due to its


m o i s t u re content, however the minimum densitythat should be used for snow is 5 pounds per cubicfoot. The engineer will have to contact the weatherservice to determine the potential snow falls for theinstallation area or consult the local building codefor a recommended design load.

Usually cable trays are installed within structure ssuch that the structure and equipment shelter thecable trays from the direct impact of high winds. Ifwind loading is a potential problem, a structuralengineer and/or the potentia l cable traym a n u f a c t u rer should review the installation foradequacy. To determine the wind speed for properdesign consult the Basic Wind Speed Map of theUnited States in the NESC (Figure 250-2).

For those installations located in earthquake areas,design engineers can obtain behavioral data forB-Line cable trays under horizontal, vertical andlongitudinal loading conditions. Testing done fornuclear power plants in the 1970's indicates thatcable trays act like large trusses when loadedlaterally and are actually stronger than when loadedvertically. Cable tray supports may still need to beseismically braced and designers should consult theB-Line Seismic Restraints Catalog for detaileddesign information.

The midspan deflection multipliers for all B-Linecable trays are listed in the Cable Tray Systemscatalog. Simply pick your support span and multiplyyour actual load by the deflection multiplier shownfor that span. The calculated deflections are forsimple beam installations at your specified loadcapaci ty. If a deflection re q u i rement wil l bespecified, extra care needs to be taken to ensurethat it does not conflict with the load re q u i re m e n tand provides the aesthetics necessary. Keep in mindthat continuous beam applications are morecommon and will decrease the deflection valuesshown by up to 50%. Also, aluminum cable trayswill deflect 3 times more than steel cable trays of thesame NEMA class.

To complete the design, the standard straightsection length and minimum bend radius must bechosen. When selecting the recommended length ofstraight sections, be sure that the standard length isgreater than or equal to the maximum support span.Choose a fitting radius which will not only meet orexceed the minimum bend radius of the cables butwill facilitate cable installation.

[See page 11 for more information on selecting theappropriate cable tray length]

392.5. Construction Specifications. (B)Smooth Edges.

This is a quality statement for cable tray systemsand their construction. B-Line cable tray is designedand manufactured to the highest standards toprovide easy, safe installation of both the cable trayand cables.

392.5. Construction Specifications. (C)Corrosion Protection.

Cable tray shall be protected from corrosion perSection 300.6, which lists some minimum criteriafor diff e rent corrosive environments. The B-LineCable Tray Catalog contains a corrosion chart forcable tray materials. Cable trays may be obtained ina wide range of materials including aluminum,p regalvanized steel, hot dipped galvanized steel(after fabrication), Type 304 or 316 stainless steel,polyvinyl chloride (PVC) or epoxy coated aluminumor steel and also nonmetallic (fiber re i n f o rc e dplastic). Check with a metallurgist to determ i n ewhich metals and coatings are compatible with aparticular corrosive environment. B-Line hascorrosion information available and may be able torecommend a suitable material. Remember that nomaterial is totally impervious to corrosion. Stainlesssteel can deteriorate when attacked by certainchemicals and nonmetal l ic cable t rays candeteriorate when attacked by certain solvents.

392.5. Construction Specifications. (D) SideRails.

The technical information in Article 392 wasoriginally developed for cable trays with rigid siderails by the 1973 N E C Technical Subcommitteeon Cable Tray. Equivalent Structural Members wasadded later to incorporate new styles of cable traysuch as center rail type tray and mesh or wirebasket tray.

392.5. Construction Specifications. (E)Fittings.

This section has been misinterpreted to mean thatcable tray fittings must be used for all changes indirection and elevation [See Section 392.6(A) Completesystem for further explanation). When two cable trayruns cross at diff e rent elevations, lacing a cablebetween the rungs of one tray and dropping into theother is a common practice which changes thedirection of the cable while providing adequate cable


support. Although the use of cable tray fittings is notmandatory, it is often desirable to use them whenpossib le to improve the appearance of theinstallation.

392.5. Construction Specifications. (F)Nonmetallic Cable Tray.

This type of cable tray is usual ly made ofFiberglass Reinforced Plastic (FRP). Applications forFRP cable tray systems include some corro s i v ea t m o s p h e res and where non-conductive material isre q u i red. B-Line fiberglass cable tray systems arem a n u f a c t u red from glass fiber re i n f o rced plasticshapes that meet ASTM flammability and self-extinguishing requirements. A surface veil is appliedduring pultrusion to ensure a resin rich surface andincrease ultraviolet resistance, however, for extendede x p o s u re to direct sunlight, additional measure s ,such as painting the tray, are sometimes employed

to insure the longevity of the product. Ambientt e m p e r a t u re is also a design consideration whenFRP cable tray is used. An ambient temperature of100F wi ll decrease the loading capacity ofpolyester resin fiberglass cable tray by 10%.

392.6. Installation. (A) Complete System. This section states that cable tray systems can

have mechanically discontinuous segments, and thatthe mechanically discontinuous segment cannot beg reater than 6 feet. A bonding jumper sized perSection 250.102 is necessary to connect across anydiscontinuous segment. The bonding of the systemshould be in compliance with Section 250.96.


Nomenclature1. Ladder Type Cable Tray 10. 30 Vertical Inside Bend, Ladder Type Tray2. Ventilated Trough Type Cable Tray 11. Vertical Bend Segment (VBS)3. Splice Plate 12. Vertical Tee Down, Ventilated Trough Type Tray4. 90 Horizontal Bend, Ladder Type Tray 13. Left Hand Reducer, Ladder Type Tray5. 45 Horizontal Bend, Ladder Type Tray 14. Frame Type Box Connector6. Horizontal Tee, Ladder Type Tray 15. Barrier Strip Straight Section7. Horizontal Cross, Ladder Type Tray 16. Solid Flanged Tray Cover8. 90 Vertical Outside Bend, Ladder Type Tray 17. Cable Channel Straight Section, Ventilated9. 45 Vertical Outside Bend, Ventilated Type Tray 18. Cable Channel, 90 Vertical Outside Bend

1

2

3

4

6

7

8

10

11 12 13

14

15

16

17

18

Typical CableTray Layout

5

9

Cable Tray Elevation Change Without Fittings

BondingJumper

T h e re are some designers, engineers, andinspectors that do not think that cable tray is amechanical support system just as strut is amechanical support system. Cable tray is not araceway in the N E C but some designers, engineers,and inspectors attempt to apply the re q u i rements forraceway wiring systems to cable tray wiring systemseven when they are not applicable. Cable tray wiringsystems have been used by American industry forover 35 years with outstanding safety and continuityof service re c o rds. The safety service re c o rd of cabletray wiring systems in industrial facilities has beensignificantly better than those of conduit wiringsystems. There have been industrial fires andexplosions that have occurred as a direct result of thewiring system being a conduit wiring system. In thesecases, cable tray wiring systems would not havep rovided the fires and explosions that the conduitsystems did by providing as explosion gas flow pathto the ignition source even though the conduitsystems contained seals.

The most significant part of this section is that themetallic cable tray system must have electricalcontinuity over its entire length and that the supportfor the cables must be maintained. Thesere q u i rements can be adequately met even thought h e re will be installation conditions where the cabletray is mechanically discontinuous, such as at af i rewall penetration, at an expansion gap in a longstraight cable tray run, where there is a change inelevation of a few feet between two horizontal cabletray sections of the same run, or where the cablesd rop from an overhead cable tray to enterequipment. In all these cases, adequate bondingjumpers must be used to bridge the mechanicald i s c o n t i n u i t y .

C o n t rol Cable Entering Pushbutton andPower Cable Entering Motor Te rminal Boxf rom 6 Inch Channel Cable Tray System(Bottom entries provide drip loops to prevent moistureflow into enclosures.)

Cables Exiting 480 Volt OutdoorSwitchgear and Entering Cable Tray System(Cable fittings with clamping glands are required to preventmoisture flow into equipment due to the cable's overheadentry into the switchgear enclosure).

Cables Entering and Exiting Motor ControlCenters from Cable Tray Systems.

392.6. Installation. (B) Completed BeforeInstallation.

This means that the final cable tray system mustbe in place before the cables are installed. It doesnot mean that the cable tray must be 100%mechanically continuous. The electrical bonding ofthe metallic cable tray system must be completeb e f o re any of the circuits in the cable tray systemare energized whether the cable tray system is beingutilized as the equipment grounding conductor inqualifying installations or if the bonding is beingdone to satisfy the requirements of Section 250.96.


392.6. Installation. (C) Supports.

The intent of this section is to ensure that theconductor insulation and cable jackets will not bedamaged due to stress caused by improper support.Multiconductor 600 volt Type TC cables and 300volt Type PLTC cables exhibit a high degree ofdamage resistance when exposed to mechanicalabuse at normal temperatures.

During an inspection of industrial installations bythe 1973 NEC Technical Subcommittee on CableTray, a test setup was constructed of an 18 inchwide Class 20C aluminum cable tray supportedt h ree feet above ground level containing severalsizes of multiconductor cables. This installation wascontinuously struck in the same area with eightpound sledge hammers until the cable tray wass e v e rely distorted, the cables however, exhibitedonly cosmetic damage. When these cables weretested electrically, they checked out as new traycable. Since that time, significant impro v e m e n t shave been made in cable jacket and conductorinsulation materials so that the cables available todaya re of better quality than the 1973 test cables.Although tray cables are capable of taking a gre a tdeal of abuse without any problems, cable trayinstallations must be designed by taking appropriatemeasures to ensure that the tray cables will not besubjected to mechanical damage.

392.6. Installation. (D) Covers.

Cable tray covers provide protection for cableswhere cable trays are subject to mechanical damage.The most serious hazard to cable in cable trays iswhen the cables are exposed to significant amountsof hot metal spat ter during construct ion ormaintenance from torch cutting of metal andwelding activities. For these exposure areas, thecable tray should be temporarily covered withplywood sheets. If such exposure is to be a frequentoccurrence, cable tray covers should be inst

Documents

Cable Tray Manual