SCCR Codes and Standards

Including:

Basic Principles

NEC® 409 and UL 508A

How to Determine the SCCR of an Industrial Enclosure

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Agenda

Introduction and Basics

National Electric Code 2005

Underwriters Laboratories

Determining SCCR

How to Increase SCCR

Mersen Tools

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Short Circuit Current– An overcurrent usually defined as being in excess of ten times normal

continuous rating usually caused by insulation breakdown or wiring error

Short Circuit Current Rating (SCCR)– The prospective symmetrical fault current at a nominal voltage to

which an apparatus or system is able to be connected without sustaining damage exceeding the defined acceptance criteria

Available Fault Current– The maximum short circuit current that could flow in an unprotected

circuit

Definitions

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SCCR is now an important factor when considering UL 508A and the NEC. SCCR is important to you because:

– By knowing about SCCR and available fault currents you will create safer working conditions for the personnel who work on or around electrical equipment

– You will be able to ensure compliance with the NEC and UL

– You can simplify your inspection approval process

Why is SCCR important?QA

&

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Everybody

Industrial Facilities

Manufacturing Facilities

Contractors

Panel Builders

System Integrators

OEMs

Etc…

Who will be affected?QA

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NEC® Article 409 – Industrial Control Panels– Introduced in the 2005 NEC, “Industrial Control Panels”

require panels to be clearly marked with a short circuit current rating, SCCR

UL 508A – Industrial Control Panels– Listed panels will require the SCCR to be clearly

marked on the panel effective April 25, 2006

The changes are already in placeWhen will the changes be enforced?Q

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National Electric Code – 2011

The National Electric Code is a standard for the safe installation of electrical wiring and equipment

Part of the NFPA (National Fire Protection Association)

Not a U.S. Law but is mandated by local or state officials

Articles Related to SCCR– Article 230

– Article 409

– Article 670

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NEC Article 230 - Services

230.1 - Scope– This article covers service conductors and equipment for control

and protection of services and their installation requirements

230.82– Equipment Connected to the Supply Side of Service Disconnect– Only the following equipment shall be permitted to be connected to

the supply side of the service disconnecting means:• (3) – Meter disconnect switches nominally rated not in excess of 600

volts that have a short circuit current rating equal to or greater than the available short circuit current,…

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NEC Article 409 – Industrial Control Panels

409.1 – Scope– This article covers industrial control panels intended for general use

and operating at 600 volts or less

409.110 – Marking– An industrial control panel shall be marked with the following

information that is plainly visible after installation: • (4) Short circuit current rating of the industrial control panel based on

one of the following:– a. Short circuit current rating of a listed and labeled assembly

– b. Short circuit current rating established utilizing an approved method

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NEC Article 430 – Motors, Motor Circuits, and Controllers

430.1 – Scope– This article covers motors, motor branch circuit and feeder

conductors and their protection, motor overload protection, motor control circuits, motor controllers, and motor control centers

430.7(D) – Multimotor and Combination Load Equipment

430.8 – Markings– A controller shall be marked with the manufacturer’s name or

identification, the voltage, the current or horsepower rating, the short circuit current rating, and other necessary data to properly indicate the applications for which it is suitable

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NEC Article 670 – Industrial Machinery

670.1 – Scope– This article covers the definition of, the nameplate data for, and the

size and overcurrent protection of supply conductors to industrial machinery

670.3 – Machine Nameplate Data– (A) Permanent Nameplate. A permanent nameplate shall be

attached to the control equipment enclosure or machine and shall be plainly visible after installation. The nameplate shall include the following information: (1)…(5)

• (4) Short circuit current rating of the machine industrial control panel based on one of the following:

– a. Short circuit current rating of a listed and labeled machine control enclosure or assembly

– b. Short circuit current rating established utilizing and approved method

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Underwriters Laboratories

Independent Testing Company

Offer 3rd Party Certification

Consistent with NEC

Relative Articles– UL 98 Disconnect Switch

– UL 248 Low Voltage Fuses

– UL 508 Industrial Control Equipment

– UL 512 Fuse Blocks

– UL 1059 Terminal Blocks (PDB)

– UL 1449 Surge Protective Devices

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UL 508A – Industrial Control Panels

Guideline for construction and general design

Manufacturers that adhere to the requirements are eligible for UL508A Listing of their product

Four Parts of UL 508A– Part 1: General Use

– Part 2: Specific Use• Enclosures, Elevator Control, Industrial Machinery, Flame Control,

Crane Control, Marine Use, Service Equipment Use, AC and Refrigeration

– Part 3: Specific Component Requirements

– Part 4: Short Circuit Current Ratings

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UL 508A – Industrial Control Panels

Industrial control panel assemblies:– May include motor controllers, overload relays, fused disconnects,

circuit breakers, pushbuttons, switches, timers, indicators, wiring, terminals, enclosures, etc…

– Will be rated 600V or less

– Where ambient temperatures do not exceed 40°C

UL 508A does not include panels for Hazardous Locations “NRBX”, which are covered under UL 698A

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Four Step Process

How do you determine the SCCR of an industrial control panel?Q

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It’s as easy as 1…2…3

Step 1: Assign a SCCR to each component in the power circuit

Step 2: Assign a SCCR to each branch circuit

Step 3: Assign a SCCR to the feeder circuit

Step 4: Consider the current-limiting effects of the feeder over-current protection devices (OCPD)

Before we apply the standard:Power Electrical Symbols

Power Circuit vs. Control CircuitBranch Circuit vs. Feeder Circuit

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Power Electrical Symbols

Fuse

PushbuttonElectric MotorMotor StarterCircuit Breaker

RelayPower Transformer

Fused Disconnect Switch

NC

NO

NO NC

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Control Circuit vs. Power Circuit

Power Circuit Power Circuit

Control Circuit

Control Circuit

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Branch Circuit vs. Feeder Circuit

Branch Circuit #1 Branch Circuit #2

Feeder Circuit

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Applying the Standard

Let’s analyze an example circuit

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Example Circuit

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Step 1: Assign a SCCR to each component in the power circuit

SCCR of a component shall be established by one of the following:

1. SCCR marked on the component or on instructions provided with the component

2. SCCR can be determined by the voltage rating of the component and the assumed short circuit current from Table SB4.1

3. SCCR for a load controller, motor overload relay, or combination motor controller can be determined by type testing under the procedure specified by UL508 and as described in the manufacturers procedure

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QA

&All Power Circuit ComponentsWhat Components Do I Look For?

Including disconnect switches, branch circuit protective devices, branch circuit fuse holders, load controllers, motor overload relays, terminal blocks, and bus bars, shall have a short circuit current rating expressed in amperes or kilo amperes and volts

Exception No. 1: Power transformers, reactors, current transformers, dry-type capacitors, resistors, varistors, and voltmeters are not required to have a short circuit current rating

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QA

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In today’s industry short circuit current ratings are usually listed in kilo-amperes, or kA

On the product label or in the product manual

Where do I find the SCCR of my components?

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If the component SCCR is not listed on the product label or in the product manual use Table SB4.1

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Mersen Resources for Determining SCCR

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Control Circuit

10kA

200kA

5kA

200kA

200kA

65kA

Unknown

100kA

200kA

NA

200kA

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12A (5HP = 3.7kW) Motor Starter does not have a labeled SCCR and is also not listed in product manual

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SB4.1 5kA

Control Circuit

10kA

200kA

5kA

200kA

200kA

65kA

100kA

200kA

NA

200kA

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Step 2: Assign a SCCR to each Branch Circuit

Make a pass of each branch circuit– The lowest component SCCR value will be equal to the branch

SCCR

In this case we have three branch circuits to analyze

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Branch 1 Branch 2 Branch 3

SB4.1 5kA

Control Circuit

10kA

200kA

5kA

200kA

200kA

65kA

100kA

200kA

NA

200kA

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Branch 1 First Pass SCCR =

10kA

200kA

Branch 1

10kA

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Branch 2 First Pass SCCR = 5kA

5kA

200kA

NA

Control Circuit

Branch 2

200kA

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Branch 3 First Pass SCCR = 5kA

Branch 3

SB4.1

5kA

65kA

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Step 3: Assign a SCCR to the Feeder Circuit

Make a pass of the feeder circuit and determine the lowest SCCR value

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Feeder Circuit SCCR = 100kA

Feeder

100kA

200kA

200kA

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Step 4 is pivotal when attempting to increase panel SCCRUL 508A, SB4.3 – Feeder Components that limit the short circuit current available– Power Transformer– Circuit Breaker– Fuse

Compare the Ip of the feeder OCPD to the SCCR of the feeder and each branch circuit– If the Ip of the feeder OCPD is not greater than the SCCR of the

feeder or branch then the feeder or branch will have an SCCR equal to the appropriate Ip column on SB4.2

The panel SCCR would be equal to that of the component with the lowest SCCR; in this case 5kA (Electronic Motor Controller & 12A Motor Starter)

What if we stopped now?

QA

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Step 4: Consider the current-limiting effects of the feeder over-current protection devices

Power Transformer– Use SB4.3.1

Circuit Breaker– The panel builder must select the current limiting breaker based on

the published peak let-thru curve data provided by the breaker manufacturer. Listed breakers rated 15A or 20A, 600V or less, also labeled “current limiting” are able to limit the current to 5kA and 10kA respectively.

Fuse– Refer to UL 508A Table SB4.2 to determine the Ip of the fuse

– Class CC, G, J, L, RK1, RK5 or T

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Feeder OCPD

In this example the feeder overcurrent protection device is an AJT60– Class J

– 60A

– 600V

– Time Delay

Feeder

100kA

200kA

200kA

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Class J 60A

– 50kA peak Ip = 8kA

– 100kA peak Ip = 10kA

– 200kA peak Ip = 16kA

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Ip = 10kA @ 100kA

Branch 1 = 10kA

Branch 2 = 5kA

Branch 3 = 5kA

SB4.1 5kA

Control Circuit

10kA

200kA

5kA

200kA

200kA

65kA

100kA

200kA

NA

200kA

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Branch 1 Second Pass SCCR =

10kA

200kA

Branch 1 = 10kA

Ip = 10kA @ 100kA

Branch 1 = 100kA

100kA

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Branch 1 = 100kA

Branch 2 = 5kA

Branch 3 = 5kA

SB4.1 5kA

Control Circuit

10kA

200kA

5kA

200kA

200kA

65kA

100kA

200kA

NA

200kA

Ip = 10kA @ 100kA

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Panel SCCR is still 5kA

Summary– Feeder SCCR = 100kA

– Branch 1 SCCR = 100kA (was 10kA)

– Branch 2 SCCR = 5kA

– Branch 3 SCCR = 5kA

Branches 2 and 3 are limiting our panel SCCR

Lets examine Branch 2 in more detail…

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Branch 2

In this case the Power Transformer is a component that limits short circuit current available

Using UL508A SB4.3.1 we can determine the current limiting capabilities of the power transformer

Power Transformer Specs– 5KVA– 480V – Pri– 120V – Sec 5kA

200kA

NA

Control Circuit

Branch 2 = 5kA

200kA

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The current limiting effects of a Power Transformer

UL508A SB4.3.1– For branch circuit supplied by a power transformer with an isolated

secondary winding, the short circuit current rating on the line side of the transformer shall be one of the following:

XFMR Rating Secondary Voltage Secondary Devices SCCR on line side of transformerless than or

= 10kVAless than or

= 5kVAgreater than

10kVAUse lowest SCCR of secondary current

components--

not specified greater than or = 5kA Use rating on primary overcurrent device

120V maximum greater than or = 2kA Use rating on primary overcurrent device

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Branch 2 Second Pass SCCR =

According to SB4.3.1 our branch power transformer will limit the let-thru current to 2kA

Branch SCCR is equal to our primary overcurrent device

– ATQR25

200kA

5kA

200kA

NA

Control Circuit

Branch 2 = 5kA

200kA

Branch 2 = 200kA

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Branch 1 = 100kA

Branch 2 = 200kA

Branch 3 = 5kA

SB4.1 5kA

Control Circuit

10kA

200kA

5kA

200kA

200kA

65kA

100kA

200kA

NA

200kA

Ip = 10kA @ 100kAPanel SCCR is 5kA

Limiting Factor is Branch 3

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Panel SCCR is still 5kA

Summary– Feeder SCCR = 100kA

– Branch 1 SCCR = 100kA (was 10kA)

– Branch 2 SCCR = 200kA (was 5kA)

– Branch 3 SCCR = 5kA

Branch 3 is the limiting factor in our panel!

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1. Upgrade to components with higher SCCR

2. Add sub-feeder fuses

3. Upgrade feeder overcurrent protection device with a higher current limiting device

4. Consider type tested components, i.e., components that have been tested by the manufacturer under specific conditions and witnessed by UL

There are four methods that could be utilized

What can I do to increase my panel SCCR?Q

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Option 1: Upgrade to components with higher SCCR

Ip = 10kA @ 100kA

Branch 3 = 5kA

SB4.1

5kA

65kA

10kA

Branch 3 = 65kA

Solution– Replace the un-labeled motor starter

with one that has a SCCR of 10kA

Now we can utilize the current limiting capabilities of the feeder OCPD

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Option 2: Add a sub-feeder fuse

SB4.1

5kA

65kA

Branch 3 = 5kA

By adding a sub-feeder fuse, the standard allows you to use this fuse as your current limiting feeder protection

In this case lets add a Class CC 20A Fuse– ATDR20

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Class CC 20A

– 50kA peak Ip = 3kA

– 100kA peak Ip = 4kA

– 200kA peak Ip = 5kA

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Option 2: Add a sub-feeder fuse

SB4.1

5kA

65kA

Branch 3 = 5kA

Branch 3 = 65kAClass CC, 20A

– ATDR20

– Ip = 4kA @ 100kA Fault

This sub-feeder fuse will allow the panel SCCR to be increased to 65kA because once again the MCCB becomes the limiting device

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Feeder

100kA

200kA

200kA

Option 3: Upgrade feeder OCPD

Not applicable for this example. We are using the most current limiting device

Use option 3 if you are using a circuit breaker as your feeder overcurrent protective device

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Option 4: Consider type tested components

65kA

SB4.1

5kA

Branch 3 = 5kASome manufacturers “type test”

fuses or circuit breakers with their motor starters to achieve a higher SCCR rating when used in combination

Each major controller manufacturer type tests their components on an annual basis. Results are available on UL.com

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Option 4: Consider type tested components

Example:– 12A Motor Starter and 15A MCCB are both Schneider products

(Square D, Telemecanique)

– 12A IEC Contactor• Part # LC1D12

– OVLD Relay, 5.5–8A• Part # LRD12

– 15A Circuit Breaker• Part # GJL36015

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UL provides links to each manufacturers website displaying the SCCR testing results. (www.ul.com/controlequipment/shortcircuit.html)

Component Type

Component Manufacturer Component Catalog Designation

Component kA

Component Amps

Combination kA

Combination Volts

ICB Square D GJL36015M03 - 15 65 480MC Telemecanique LC1D12 5 12OLR Telemecanique LRD10 5 4 - 6OLR Telemecanique LRD12 5 5.5 - 8OLR Telemecanique LRD14 5 7 - 10OLR Telemecanique LRD16 5 9 - 13OLR Telemecanique LRD1510 5 4 - 6OLR Telemecanique LRD1512 5 5.5 - 8OLR Telemecanique LRD1514 5 7 - 10OLR Telemecanique LRD1516 5 9 - 13

F Any Class CC 200 30 100 480MC Telemecanique LC1D12 5 12 1 0 0 4 8 0

OLR Telemecanique LRD01 5 0.10 - 0.16 1 0 0 4 8 0

OLR Telemecanique LRD02 5 0.16 - 0.25 1 0 0 4 8 0

OLR Telemecanique LRD03 5 0.25 - 0.40 1 0 0 4 8 0

OLR Telemecanique LRD04 5 0.40 - 0.63 1 0 0 4 8 0

OLR Telemecanique LRD05 5 0.63 - 1 1 0 0 4 8 0

OLR Telemecanique LRD06 5 1 - 1.6 1 0 0 4 8 0

OLR Telemecanique LRD07 5 1.6 - 2.5 1 0 0 4 8 0

OLR Telemecanique LRD08 5 2.5 - 4 1 0 0 4 8 0

OLR Telemecanique LRD10 5 4 - 6 1 0 0 4 8 0

OLR Telemecanique LRD12 5 5.5 - 8 1 0 0 4 8 0

OLR Telemecanique LRD14 5 7 - 10 1 0 0 4 8 0

OLR Telemecanique LRD16 5 9 - 13 1 0 0 4 8 0

OLR Telemecanique LRD1508 5 2.5 - 4 1 0 0 4 8 0

OLR Telemecanique LRD1510 5 4 - 6 1 0 0 4 8 0

OLR Telemecanique LRD1512 5 5.5 - 8 1 0 0 4 8 0

OLR Telemecanique LRD1514 5 7 - 10 1 0 0 4 8 0

OLR Telemecanique LRD1516 5 9 - 13 1 0 0 4 8 0

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This information is also available by viewing the Mersen USFM vs. Circuit Breaker Comparison Sheet available on our website

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Option 4: Consider using type tested components

Branch 3 = 5kA

200kA

5kA

Branch 3 = 100kAReplace the 15A circuit breaker

with the USFMCCI and the ATDR20

The USFMCCI + ATDR25 combined with the Telemecanique Motor Starter yields a SCCR of 100kA

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Panel SCCR is now 100kA

Summary– Feeder SCCR = 100kA

– Branch 1 SCCR = 100kA (was 10kA)

– Branch 2 SCCR = 200kA (was 5kA)

– Branch 3 SCCR = 100kA (was 5kA)

By using option 4 we were able to remove the circuit breaker in Branch 3 and replace it with fusing in order to increase our panel SCCR

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Questions and Answers