AC Load Centers

7/31/2019 AC Load Centers

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Types Of Load Centers

Main Breaker There are two types of load centers, main breaker and main lugonly. Main breaker load centers are suitable for use in serviceentrance applications. The incoming supply cables of a mainbreaker type load center are connected to the main breakerwhich in turn feeds power to the load center and its branchcircuits. The main breaker disconnects power from the loadcenter and provides overcurrent protection.


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Main Lug Only The following illustration shows the interior of a main lug onlyload center. A main lug type load center does not have a maincircuit breaker. The incoming supply cables are connecteddirectly to the main lugs and bus bars. Primary overloadprotection for the load center is not provided as an integral partof the load center. Main lug units are primarily used as adistribution panel where there is a main breaker upstream in the

system. There are situations where a backfed circuit breaker anda retainer clip (required by NEC) are used to convert a mainlug load center to a main breaker load center.

Main lug load centers are sometimes referred to as add-on,secondary or downstream panels. These panels are addedwhen all circuit slots in the main breaker load center are full orwhen a remote panel is desired. For example, a main breakerload center might supply power to a main lug load centerlocated in an area of the home used as a workshop. Main lugload centers are also fed from metering equipment when usedin apartment installations.

NEC and National Electrical Code are registered trademarks of the NationalFire Protection Association.


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Feed-Thru Lugs There are two ways to sub feed power to a secondary loadcenter. One way is to use a branch circuit breaker or a plug-onlug kit in the existing load center. Another way is to use a loadcenter with feed-thru lugs. In the following illustration utilitypower is supplied to a main breaker load center. Power is takenfrom the feed-thru lugs and supplied to another load centerdownstream.


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Branch Circuit Breakers It is important to note the difference betw een a main breakerand branch circuit breakers. The main breaker of a main breakertype load center shuts off power to the entire load center and allcircuits supplied by that load center. A main lug only load centerdoes not have such a breaker. A main breaker load center andmain lug only load center both have branch circuit breakers.Branch circuit breakers provide protection for each branch circuit

conductor in the distribution system. Typical branch circuits:

15 and 20 ampere, 120 volt supply power for lightingcircuits and electrical outlets

20 ampere, 120 volt supply power for some kitchen andbath electrical outlets

Appliances requiring larger amounts of power, such asclothes dryers, ranges, furnaces and air conditioners useseparate branch circuit breakers rated for the appropriate

voltage and current


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Power Supply Systems

Two-Wire System* Homes built prior to 1936, especially in rural areas, used a two-wire supply system. This system provided 120 volts between ahot conductor and a grounded conductor. A tw o-wire system isusually inadequate for todays residential electrical demands.


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Three-Wire System The most common supply system used in residentialapplications today is a three-wire supply system. There are 120volts betw een any phase and neutral and 240 volts betweenphases.

The following illustration shows a three-wire, single-phase 120/240 volt system. 120 volts can be obtained by connecting a loadbetween phase A and neutral, or phase B and neutral. 240 voltscan be obtained by connecting a load between phase A andphase B.


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Load centers can also be used in commercial applications.Electric power is brought into a building at one location andthen is distributed through the building by means of separatecircuits. Two distribution systems used in commercialapplications that are suitable for Siemens load centers arethree-wire, three-phase, 240 volts; and four-wire, three-phase,208Y/120 volts.

3-Wire, 3-Phase, 240 Vol t The following illustration shows a three-wire, three-phase, 240volt system. There are 240 volts available between any phase.

4-Wire, 3-Phase, 208Y/120V The following illustration shows a four-wire, three-phase, 120/208 volt system. There are 208 volts betw een any two phasesand 120 volts available between any phase and neutral.


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Individual Overcurrent Protection

The National Electrical Code requires load centers to beindividually protected against overcurrent. Main overcurrentprotection may be an integral part of a load center or locatedremote f rom the load center. NEC Article 384-16a states thateach lighting and appliance branch-circuit panelboard shall beindividually protected on the supply side by not more than twomain circuit breakers or tw o sets of fuses having a combinedrating not greater than that of the panelboard.

Individual Protection The following illustration shows two possible ways individual

load center overcurrent can be accomplished. If a main circuitbreaker is located as an integral part of the load center, it is amain breaker load center. If a main circuit breaker or main fusibleis located remotely, then a main lug load center could be used.In this example the main breaker and load center are both ratedfor 200 amps.


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Except ion to NEC 384-16 There is an exception to NECArticle 384-16a. Exception No.1: Individual protection for a lighting and appliance panelboardshall not be required if the panelboard feeder has overcurrentprotection not greater than the rating of the panelboard.Thefollowing illustration shows two load centers protected by asingle 200 amp circuit breaker. Note that the circuit breakerprovides overcurrent protection not greater than the rating of

the load centers.

NEC and National Electrical Code are registered trademarks of the NationalFire Protection Association. Reprinted with permission from NFPA 70-1999,

the National Electrical Code, Copyright 1998, National Fire ProtectionAssociation, Quincy, MA 02269.


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Review 31. The two types of load centers are main ____________

and main ____________ only.

2. Main lug only load centers are sometimes referred to as____________ - ____________ , ____________ , or____________ panels.

3. ____________ - ____________ lugs is one method usedto supply power from one load center to another.

4. Homes built prior to 1936 used a ____________ -____________ system.

5. The voltage between any phase and neutral is____________ volts on a three-wire, single-phase, 240volt system.

6. The voltage between phases on a three-wire, three-phase, 240 volt system is ____________ volts.

7. The NEC article that covers individual overcurrentprotection for load centers is ____________ .


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Service Entrance/Equipment Load Centers

Load centers are frequently used as service equipment for a

building. This is the equipment located near where the powersupply enters the building. The incoming power supply isconnected to this equipment which provides a means to controland cut off the supply. The National Electrical Code discussesservice entrance equipment in Article 230. Load centers usedas service equipment must be listed and labeled as such. AllSiemens EQ main breaker load centers are factory labeled assuitable for use as service equipment.



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Maximum Num ber of Service-entrance conductors must have a readily accessibleDisconnects for Service means of being disconnected f rom the power supply. NECEnt rance Equipment Article 230-71a specifies that for each set of service entrance

conductors no more than six switches or circuit breakers shallbe used to disconnect and isolate the service from all otherequipment. There are two ways load centers can be configuredto meet this requirement. In one example, a main breaker load

center is used. A single main circuit breaker w ill disconnectpower to all equipment being supplied by the service. Inanother example, a main lug only load center is equipped withup to six circuit breakers (not lighting and appliance) todisconnect power to all equipment being supplied by theservice. In any case, the circuit breaker must be clearly labeledfor the load it supplies.


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Load Center Grounding

Grounding is an important aspect of any electrical equipmentand must be considered carefully. Article 250 of the NECcovers mandatory grounding requirements. The NationalElectrical Code defines ground as a conducting connection,whether intentional or accidental, between an electrical circuitor equipment and the earth, or to some conducting body thatserves in place of the earth.

The following illustration, for example, shows the neutral (N)conductor of a secondary connected to ground.

There are tw o objectives to the intentional grounding ofelectrical equipment:

Keep voltage differentials between different parts of asystem at a minimum which reduces shock hazard.

Keep impedance of the ground path to a minimum. Thelower the impedance the greater the current is in the eventof a fault. The greater the current the faster an overcurrentdevice will open.



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Service Ent rance The circuit is grounded only at the service entrance, never atGrounding any downstream equipment. In the following illustration the

neutral is grounded at the service equipment by connecting agrounding electrode conductor f rom the neutral (groundedconductor) to a grounding electrode. The neutral and thepanelboard enclosure are bonded together at the serviceequipment so that the enclosure is also connected to ground

through the grounding electrode connector.

Bonding Bonding (previous paragraph) is the permanent joining ofmetallic parts to form an electrically conductive path that willensure electrical continuity. This is accomplished in Siemensload centers by attaching a metal strap between the neutral busand the enclosure.


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Using the Neutral w ith In the following illustration load #2 has become shorted to itsService Ent rance and metal enclosure. Fault current is returned to the source throughDownstream Panelboards the path indicated. With a properly coordinated system the

branch circuit breaker in the dow nstream load center w ill openremoving the load from the power source. For a discussion ofcircuit breaker coordination refer to the STEP 2000 course,Molded Case Circuit Breakers.


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Load Center Ratings

When selecting load centers and overcurrent protectiondevices, it is extremely important to know both the maximumcontinuous amperes and available fault current. NEC article110-9 states:

Equipment intended to interrupt current at fault levels shallhave an interrupting rating sufficient for the nominal circuitvoltage and the current that is available at the line terminals ofthe equipment.

Equipment intended to interrupt current at other than faultlevels shall have an interrupting rating at nominal circuit voltagesufficient for the current that must be interrupted.

Full Rating There are two ways to meet this requirement. The full ratingmethod is to select circuit protection devices with individualratings equal to or greater than the available fault current. Thismeans that, in the case of a building w ith 22,000 amperes offault current available at the service entrance, every circuitprotection device must have an interrupting rating of at least22,000 amperes. In the following example, the main circuitbreaker and each branch breaker is rated 22,000 AIC.

NEC and National Electrical Code are registered trademarks of the NationalFire Protection Association. Reprinted w ith permission from NFPA 70-1999, the

National Electrical Code, Copyright 1998, National Fire ProtectionAssociation, Quincy, MA 02269.


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Series Rated The series rated method is that the main upstream circuitprotection device must have an interrupting rating equal to orgreater than the available fault current of the system, butsubsequent downstream circuit protection devices connectedin series can be rated at lower values. For example, a buildingwith 22,000 amperes of available fault current might have thebreaker at the service entrance rated at 22,000 amperes and

additional downstream branch breakers rated at 10,000amperes.

In single family homes the available fault current normally doesnot exceed 10,000 amperes. In this situation a load center witha main and branch circuit breakers with an interrupting rating of10,000 Amperes w ill satisfy the requirement. In some instancesthe available fault current may exceed 10,000 amperes, butrarely 22,000 amperes. In these instances a load center with amain circuit breaker with an interrupting rating of 22,000amperes would be required. Using the full rating method thebranch breakers would also have to be 22,000 amperes. Usingthe series rated method the branch breakers could be 10,000amperes. In larger installations, such as apartments,condominiums and commercial facilities, the available faultcurrent will normally be greater than 10,000 amperes. In thesesituations the series ratings become more important.

Series-rated breaker combinations must be tested in series inorder to be UL recognized. The NEC requires the series

ratings to be marked on the load center (240-86). Selectedseries rated breakers are listed in the Speedfax catalog. YourSiemens sales engineer can provide more information onSiemens series-rated circuit breakers.


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Review 41. The point at which the incoming power supply enters a

building is known as the ____________ ____________ .

2. According to NEC Article 230-71a, the maximumnumber of circuit breakers that can be used todisconnect and isolate the service from all other

equipment is ____________ .

3. One objective of intentional grounding is to keep____________ differentials between different parts of asystem at a minimum which reduces shock hazard.

4. ____________ is the permanent joining of metallic partsto form an electrically conductive path.

5. The ____________ conductor is grounded only at theservice entrance equipment, never at any downstream

equipment.

6. Circuit breakers selection with individual ratings equalto or greater than the available fault current is referred toas the ____________ _____________ method.

7. Circuit breaker selection with the main upstream circuitbreaker equal to or greater than the available faultcurrent, and downstream circuit breakers rated at low ervalues is referred to as the ____________ ____________method.


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AC Load Centers