1. Understanding NEC Article 409 & UL508A Short-Circuit Current Ratings
Welcome to Rockwell Automation’s presentation of “Understanding NEC 409 & UL 508A Short Circuit Current Ratings”.
This presentation will cover the new requirements of NEC 409 and UL 508A Supplement SB for panel Short Circuit Current Ratings, SCCR.
A copy of this presentation has been provided with a note section for your convenience.
In addition, if there are any questions or issues that cannot be addressed during this presentation, additional assistance can be attained via RA Technical Services team at address or by calling Rockwell Automation technical support line at , select Allen-Bradley Brand, then select Industrial Control.
Copyright © 2006 Rockwell Automation, Inc. All rights reserved.

2. Understanding NEC Article 409 & UL508A Short-Circuit Current Ratings
DISCLAIMER
This document is intended to provide general technical information on a particular subject or subjects and is not an exhaustive treatment of such subjects. Accordingly, the information in this document is not intended to constitute application, design, software or other professional engineering advice or services. Before making any decision or taking any action, which might affect your equipment, you should consult a qualified professional advisor. ROCKWELL AUTOMATION DOES NOT WARRANT THE COMPLETENESS, TIMELINESS OR ACCURACY OF ANY OF THE DATA CONTAINED IN THIS DOCUMENT AND MAY MAKE CHANGES THERETO AT ANY TIME IN ITS SOLE DISCRETION WITHOUT NOTICE. FURTHER, ALL INFORMATION CONVEYED HEREBY IS PROVIDED TO USERS "AS IS." IN NO EVENT SHALL ROCKWELL BE LIABLE FOR ANY DAMAGES OF ANY KIND INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS PROFIT OR DAMAGE, EVEN IF ROCKWELL AUTOMATION HAVE BEEN ADVISED ON THE POSSIBILITY OF SUCH DAMAGES. ROCKWELL AUTOMATION DISCLAIMS ALL WARRANTIES WHETHER EXPRESSED OR IMPLIED IN RESPECT OF THE INFORMATION (INCLUDING SOFTWARE) PROVIDED HEREBY, INCLUDING THE IMPLIED WARRANTIES OF FITNESS FOR A PARTICULAR PURPOSE, MERCHANTABILITY, AND NON-INFRINGEMENT. Note that certain jurisdictions do not countenance the exclusion of implied warranties; thus, this disclaimer may not apply to you.
Excerpts from “UL Standard for Safety for Industrial Control Panels, UL508A” are copyright Underwriter’s Laboratories Inc.
Excerpts from the “2005 National Electrical Code” are copyright National Fire Protection Association
Hide this overhead during presentations.
If you leave the presentation behind, include it in the hard copy.
Copyright © 2006 Rockwell Automation, Inc. All rights reserved.

3. RA Component Solutions RA Website & Database
UL 508A & NEC 409 Topics
Introduction
Definitions
UL 508A Supplement SB
RA Component Solutions
RA Website & Database
Today’s discussion will cover several key areas that will give you a good understanding of the topic so that you can confidently address the new standard requirements in NEC 409 and UL 508A.
First, we will complete a short Introduction covering the background, relationship and requirements of NEC 409 & UL 508A
Next, we will review some important Definitions to insure that we have a common understanding of the circuit and standards nomenclature.
The majority of our time will be used to review the UL 508A Supplement SB analysis process.
And finally, we will review the Component Solutions and Website/Database the has been developed by Rockwell Automation to assist you in attaining the panel SCCR required in your application.
Copyright © 2006 Rockwell Automation, Inc. All rights reserved.

4. Copyright © 2006 Rockwell Automation, Inc. All rights reserved.
NEC 409 Code Change
2005 Edition of National Electric Code (NEC) contains a new Article 409 covering “Industrial Control Panels”
Why introduce this new article?
Control panels have been evaluated under several different NEC articles
Inconsistencies have led to improper use of control components
New article provides a single set of requirements that can be used by panel builders, installers and inspectors
NEC 409 provides for design and evaluation process to insure safe installation and application of control panels
Panel SCCR must cover the actual available fault level.
The 2005 Edition of the National Electrical Code (NEC) contains a new Article 409 covering “Industrial Control Panels”. As each state adopts the new 2005 Code, the requirements to comply with Article 409 becomes mandatory.
Why develop a new NEC Article 409??
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5. Copyright © 2006 Rockwell Automation, Inc. All rights reserved.
NEC 409 Code Change
Panel SCCR
50kA RMS Symmetrical
480V Maximum
New Article 409 covers “Industrial Control Panels”
“Industrial Control Panel” Definition
Industrial Control Panels, intended for general use, ordinary locations, 600V or less.
Clause (3) “Markings” Industrial Control Panels must be evaluated & marked for their Short-Circuit Current Rating (SCCR)
In the 2005 Edition of the National Electrical Code, a NEW Article 409 has been added that covers the general requirements for “Industrial Control Panels”.
The definition of Industrial Control Panel is very generic, covering general use, ordinary locations, 600V or less. This description covers the vast majority of industrial control panels that RA customers design, build and install on a regular basis.
In Paragraph covering proper “Marking”, Clause (3) states that the “short-circuit current rating (SCCR) of the industrial control panel based on one of the following:”
a) Short-circuit current rating of a listed and labeled assembly, or
b) Short-circuit current rating established utilizing an approved method
FPN: the NEC 409 references that UL 508A-2001, Supplement SB, is an example of an approved method.
This FPN is the direct link between NEC 409 and UL 508A Supplement SB
Additional NEC Articles that also require that SCCR’s be marked on panels include Article “Industrial Machinery” and Article “HVAC Equipment”, (B) Multi-motor and Combination-Load Equipment.
FPN: Fine Print Note
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Definitions
Let’s begin with some important definitions
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Common Terminology
Short-Circuit:
An over-current that exceeds the normal full load current (10x, 100x, 1000x FLC)
Short-Circuit Current Rating – SCCR:
The maximum short-circuit current an electrical component can sustain without the occurrence of excessive damage when protected with an over-current protective device
Interrupting Rating (Breaking Capacity):
The maximum short circuit current at rated voltage that a over-current protective device can safely interrupt and clear.
Current Limiting:
When a fuse or circuit breaker operates in its current limiting range, it will clear a short in less that ½ cycle, typically ¼ cycle
Performance standards such as UL specify let-thru characteristics (Ip & I2t) at rated fault levels and voltages
Power Circuit:
Conductors and components of branch and feeder circuits.
Control Circuit:
A circuit that carries the electric signals directing the performance of a controller, and which does not carry the main power circuit. A control circuit is, in most cases, limited to 15 amperes
It is important that we take some time to review several terms and clearly understand their meaning. This review will insure that we all have a common understanding of terminology.
Short Circuit: Short-circuit technically refers to any over-current that exceeds the normal full load current. In many applications dependent on the type of load on the circuit, normal inrush current can be quite high, but usually lasts for a short amount of time. In most instances, this inrush will never be greater that 20X FLA, typically 6..8 times for starting motors. Short circuits on the other hand are fault level currents, to 10s, 100s and even 1000s of times the normal FLC.
Short Circuit Current Rating: The short circuit current rating of an electrical component has also been referred to in the past as a withstand ratings. We will drop the “withstand” due to the variation of meaning for different types of products. Short Circuit Current Rating (or SCCR) is the maximum short circuit current that a component can sustain without excessive damage, when protected by a specified over current protection device.
An example would be the SCCR of a Size starter when used in combination with a Class J Fuse. The 509 starter has been tested with a Class J fuse and in combination carries a “conditional” SCCR of 100kA, 600V. This rating is conditional since a specific short circuit protection device must be used to achieve this high fault rating.
Interrupting Rating: Interrupting ratings are only applied to protective devices such as fuses and circuit breakers. These are the components that will interrupt the fault and safely open the circuit. In the IEC world, the term breaking capacity is typically used. Fuses and circuit breakers are supplied with maximum interrupting ratings (kA) and maximum voltage (V) ratings.
Current Limiting: Current limiting refers to the ability of a protective device to clear a short circuit fault in less that ½ cycle, and typically within ¼ cycle. Current limiting devices with reduce the levels of Ip (magnetic) and I2t (heat) energies
For many years, fuses were the only protective devices that were considered to provide current limiting performance. Over the last several years, circuit breakers such as the 140M have been developed that can open high current faults in just a few milliseconds, very comparable to fuses.
Power Circuit: The power circuit is considered to be those portion of a panel that contain the feeder circuit and power branch circuits.
Control Circuit: The control circuit is carries the electrical signals the direct the operation and performance of a controller, is not part of the main power circuit and is typically limited to 15A.
Copyright © 2006 Rockwell Automation, Inc. All rights reserved.

8. “Feeder Circuit” Definition
“Feeder Circuit” covers all power conductors and components from incoming feeder disconnect, …. to the line side of the last branch SCPD.
“Feeder Circuit”
“Branch” SCPD
In our example of a typical control panel, the “feeder” circuit will include components and conductors from the incoming power to the panel, including the disconnecting means, down to the line side of the last branch circuit protection device.
In the example, our main fuse switch, power block, conductors, busbar mounting system, down to the line side of the last branch circuit protection device, are all part of the feeder circuit.
Note that the “line side of the branch protection device” for control transformers, power supplies, and PLC’s is limited to the feeder circuit.
Copyright © 2006 Rockwell Automation, Inc. All rights reserved.

9. “Branch Circuit” Definition
“Branch Circuit” covers all power conductors and components from the load-side of the branch circuit, …. to the controller load-side connections.
“Branch Circuit”
“Branch” CPD
Terms used for protection devices:
Over-current Protection Device (OPD)
Branch Circuit Protection Device (BCPD)
Branch Circuit on the other hand includes those components on the load side of the last branch circuit protection device.
Most Rockwell Automation branch circuit solutions will have a conditional short circuit current rating which includes that protective device in combination with the controller.
Example: A Bul. 100 contactor and 193-E overload relay when installed with a recommended short circuit protection device will provide a branch circuit solution having a conditional SCCR
Copyright © 2006 Rockwell Automation, Inc. All rights reserved.

10. UL 508A Supplement SB “How to Comply”
The next part of our discussion will cover UL 508A Supplement SB, which is now used as the standard procedure for determining the SCCR of a complete panel.
Copyright © 2006 Rockwell Automation, Inc. All rights reserved.

11. How to determine SCCR of a panel?
Three potential options:
Option 1: Test each panel design and document in a follow-up procedure
Costly, time consuming, too many variations
Option 2: Purchase previously tested constructions
Practical for repeatable machine/panel design and production
Option 3: Complete analysis according to UL 508A Supplement SB
Recommended procedure for most panels and machines
How can you determine the SCCR of a complete panel??
There are just a few ways that this can be accomplished,
One option would be to build a sample panel, test and document. This is very impractical, expensive and not often used.
Another option is to purchase previously tested constructions. Again, this is not a solution for Rockwell Automation customers
The third option is to utilize the prescribed method as reference in NEC Article 409, UL 508A Supplement SB. For now, this is the only approved method that is available.
Copyright © 2006 Rockwell Automation, Inc. All rights reserved.

12. Determining SCCR’s - 3 Basic Steps
UL 508A Supplement SB
“Short-Circuit Current Ratings for Industrial Control Panels”
Goes into effect in “April 2006”
SB4.1: Short-Circuit Current Rating (SCCR)
Shall be determined by the following evaluation:
First, establish the short circuit current ratings of individual power circuit components as specified in SB4.2
Second, modify (limit) the available short-circuit current within a portion of a circuit in the panel due to the presence of current limiting components as specified in SB4.3
Third, determine the overall panel short-circuit current rating as specified in SB4.4
In UL 508A, a Supplement SB has been added that outlines a procedure to determine the “Short Circuit Current Ratings for Industrial Control Panels”. As we proceed through the process, examples have been included to help clarify the concepts.
SB 4.1 summarizes the three basic steps to completing the panel analysis for determining a panel SCCR
In SB4.2, the first step is to establish the short circuit current ratings of individual power circuit components
In SB4.3, the next step is to modify (or limit) the available short circuit current within a portion of the panel by utilizing current limiting components.
Finally, in SB4.4, the last step is to determine the overall panel short circuit current rating
As you might expect, there is a lot more detail and considerations behind these “three simple steps”
Copyright © 2006 Rockwell Automation, Inc. All rights reserved.

13. SB4.2.1 – Individual Power Circuit Components
SB4.2.1 All power circuit components, 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 voltage.
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.
Exception No. 2: The “S” contactor of a wye-delta motor controller is not required to have a short-circuit current rating.
In SB 4.2, the first step is to determine the SCCR of the individual power circuit components
SB requires that all power circuit components have SCCR’s expressed in amperes, kilo- amperes (kA) and voltage. All power circuit components from RA are provided with SCCR as required by UL, standard fault ratings at a minimum.
Note that there are two exceptions that cover certain types of power circuit components (Ex No. 1) and shorting contactors in wye-delta starters (Ex. No 2)
Copyright © 2006 Rockwell Automation, Inc. All rights reserved.

14. SB4.2.2 – Establish SCCR of Components
SB4.2.2 The short-circuit current rating of a component shall be established by one of the following methods:
a) The short-circuit current rating marked on the component or on instructions provided with the component;
b) The short-circuit current rating determined by the voltage rating of the component and the assumed (assigned) short-circuit current from Table SB4.1; or
c) The short-circuit current rating….
SB4.2.2 states that the SCCR of a component shall be established by one of the following methods:
UL 508A requires that the SCCR information be provided either on the product nameplate or included with the instruction sheets/user manuals that are shipped with the product. All RA products are supplied with SCCR’s, with a trend towards moving to instruction sheets/user manuals as product combinations become more complex. Product nameplates in many situations will reference instruction sheets and user manuals as the source for SCCR information.
Note: Examples of SCCR references on the 100-C contactor nameplate to the instruction sheet data and the PowerFlex 70 Drive SCCR information covered in the user manual
Some power circuit components do not have a SCCR or has not been required to have a SCCR in the past, but is now required. UL has taken the initiative to assume (assign) a SCCR to several types of components as seen in table SB4.1.
(We will return to option C following the next slide)
100-C Contactor Nameplate & Instruction Sheet
PowerFlex 70 Drive User Manual
Copyright © 2006 Rockwell Automation, Inc. All rights reserved.

15. Table SB4.1 - Unmarked Components
508A SB - Table SB4.1
“Assumed” SCCR’s
Line up with UL minimum test SC test requirements to attain listing
These minimum ratings limit “high fault” panel ratings
High Fault SCCR’s on all components & combinations are top priority
Table SB4.1 is the “Assumed maximum short circuit current rating for unmarked products”
The major issue for the these components is that they are very limiting to the panel SCCR that can be achieved. RA has worked extremely hard to insure that our components have been tested for high fault ratings and that we are not limited to the “assumed” ratings.
A good example of this effort is with the 1492-PD power blocks. Extensive testing has been completed to insure that high fault SCCR’s are available and not have to rely on these assigned ratings.
Copyright © 2006 Rockwell Automation, Inc. All rights reserved.

16. SB4.2.2 – Establish SCCR of Components
SB4.2.2 The short-circuit current rating of a component shall be established by one of the following methods:
a) The short-circuit current rating marked on the component or on instructions provided with the component;
b) The short-circuit current rating determined by the voltage rating of the component and the assumed (assigned) short-circuit current from Table SB4.1; or
c) The short-circuit current rating for a load controller, motor overload relay, or combination motor controller that has been investigated (tested) in accordance with the performance requirements, including short-circuit test requirements for standard fault currents or high fault currents specified in the Standard for Industrial Control Equipment, UL 508, and described in the manufacturer’s procedure.
SB4.2.2 states that the SCCR of a component shall be established by one of the following methods:
Another option is use controllers or components that have been investigated (tested) in accordance to their UL 508 Industrial Control Standard for standard or high fault ratings. This is the best solution for our customers, to utilize the high fault ratings that have been tested and covered in the manufacturers procedure.
Copyright © 2006 Rockwell Automation, Inc. All rights reserved.

17. SB4.2.3 – High Fault SCCR of Components
SB4.2.3 A high fault short-circuit current rating for a motor controller, an overload relay, or a combination motor controller, as specified in SB4.2.2 (a) or (c), shall only be used as the short-circuit current rating of the component when the specified branch circuit protective device is provided.
Exception 1:
When the specified BCPD related to the high fault SCCR is a Class CC, G, J, L, RK1, RK5 or T fuse, a fuse of different class is able to be used at the same high fault rating where the Ip let-through current and I2t of the new fuse is not greater than that of the specified fuse. See Table SB4.2 for maximum let-through currents (Ip) and I2t
Exception 2:
The specified BCPD is able to be provided in the field when the panel is marked in accordance to SB5.1.2.
Exception 3:
When the specified BCPD related to the high fault SCCR is a listed circuit breaker marked “current limiting”, a different current-limiting circuit breakers is able to be used at the same high fault rating……
SB specifies that where high fault ratings are utilized for a controller, that the specified branch circuit protective device must be used. In no case can a user indiscriminately exchange fuse types or sizes, while maintaining the high fault rating.
Examples of controllers that have been tested for high fault SCCR’s include NEMA and IEC starters that have conditional SCCR’s based on a specific branch circuit protection device such as fuses and circuit breakers.
SB does however make an allowance for replacing branch circuit protective devices (UL fuses and current limiting circuit breakers) in Exceptions 1 and 3. Here, if the user is able to determine that the Ip and I2t let-through values of the replacement device are at or lower than the original BCPD, a different class fuse or current limiting circuit breaker could be used.
Note: For fuses, table SB4.2 shall be used for the maximum Ip and I2t values, and for circuit breakers, the manufacturers let-through tables will be used.
Exception 2 allows the specified BCPD to be installed in the field when the panel is properly marked.
SCCR: 100kA, 600V with UL Class J or CC Fuses
Copyright © 2006 Rockwell Automation, Inc. All rights reserved.

18. SB4.3 - Feeder Components that Limit SCC
SB4.3 – Feeder components that limit the short-circuit current available
SB4.3.1 – For branch circuits supplied by a power transformer with an isolated secondary winding, the a power transformer with an isolated secondary…
SB4.3.2 – For branch circuits supplied by a Listed circuit breaker marked “current limiting” in the feeder circuit, the short circuit current rating…
SB4.3.3 – For branch circuits supplied by a Class CC, G, J, L, RK1, RK5, or T fuse in the feeder circuit, the short-circuit current rating…
SB 4.3 addresses the current limiting affect of power transformers with an isolated secondary, listed circuit breakers marked “current limiting” and specific UL Class fuses to be used in combination with devices of lower SCCR. We will go review each section thoroughly to see how current limiting can affect your panels SCCR strategies and solutions.
Copyright © 2006 Rockwell Automation, Inc. All rights reserved.

19. SB4.3.1 - Current Limiting w/ Power Transformers
10 kVA Max
SCC Available: < 5kA
OPD SCCR
SB4.3.1 For branch circuits 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:
a) For a power transformer rated not more than 10 kVA, and where the short circuit current rating of all components in the secondary circuit are not less than 5 kA, the short-circuit current rating of the primary over-current protective device (OPD) is able to be assigned to the line side of the power transformer circuit.
b) For a power transformer rated not more than 5 kVA and a 120 V maximum secondary voltage, and where the short circuit current rating of all components in the secondary circuit are not less than 2 kA, the short-circuit current rating of the primary over-current protective device (OPD) is able to be assigned to the line side of the power transformer circuit.
c) For circuits that do not comply with SB4.3.1(a) or SB4.3.1(b), the lowest short-circuit current rating of the components in the secondary circuit is assigned to the line side of the power transformer circuit.
OPD SCCR
SB4.3.1 covers power transformers with an isolated secondary. This section is not specifically talking about control transformers, these are power transformers that typically supply a feeder or power branch circuit(s). Due to the nature of the design of power transformers with an isolated secondary, it is recognized that there is a limited short circuit current available on the load side of the power transformer. This limited energy allows load side components with standard or low fault ratings to be used.
a) Transformers rated 10kVA or less, will be assigned a secondary rating of 5kA. As long as all the components on the load side of the transformer are 5kA rated, the line side over current protective device ratings are assigned to the line side of the power transformer circuit.
And like the 10kVA transformer a similar method is applied to power transformers, 5kVA and less.
b) Transformers rated 5kVA or less and have a 120V secondary, will be assigned a secondary rating of 2kA. As long as all the components on the load side of the transformer are 2kA rated, the line side over current protective device ratings are assigned to the line side of the power transformer circuit.
c) For circuits that do not comply to a or b, then the lowest rated device on the load side of the power transformer will be assigned to the line side of the power transformer circuit.
5 kVA Max, 120V Sec.
SCC Available: < 2kA
All secondary components must have a SCCR > 5kA or > 2kA respectively to maintain SCCR of OPD
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20. Control Transformer Circuits
Control Circuit: “A circuit that carries the electric signals directing the performance of a controller, and which does not carry the main power circuit. A control circuit, in most cases is limited to 15A.”
Control Transformer: “A transformer whose secondary supplies power to control circuit devices only (excluding loads).
Typically limited to 1kVA with a secondary current less than 15A
The OPD provides over-current and short circuit protection for the control transformer branch circuit
Secondary components are not evaluated for SCCR
1 kVA, 15A Max.
SCC Available: Limited
OPD SCCR
The definitions of a control circuit and control transformer are very different that those we just reviewed for Power Transformer. Again, power transformers are typically feeding other “load” circuits, where control circuits are limited to non-loads (control devices) and are typically limited in size to 1kVA and 15A.
The main issue with control transformer circuits is that it is still considered a “branch circuit”, and the line side over-current protection device must have an “interrupting” the is equal or greater than the available short circuit current that is feeding the panel. Additionally, load side devices are not evaluated for SCCR’s.
Secondary components are not required to have a specific SCCR and do not affect the SCCR of OPD.
Copyright © 2006 Rockwell Automation, Inc. All rights reserved.

21. SB3.2 Over-current Protection of Control Circuit
> 65kA
SB3.2.1 For control circuits tapped from the feeder circuit, the over-current protection for the common control or for the primary of a control transformer or power supply shall be provided with branch circuit protective device having a short-circuit current rating not less than the overall panel short-circuit current rating.
Control circuit taps off the feeder circuit must be protected:
Common Control
Control Transformers
Power Supplies
Control Circuit BCPD SCCR must cover the panel SCCR!
SB3.2.1 addresses the requirements of over-current protection devices for control circuit taps, such as common control, control transformers and power supplies. The main requirement is that the BCPD must have a SCCR (interrupting rating) equal to or greater than the panel SCCR.
65kA Panel SCCR
Copyright © 2006 Rockwell Automation, Inc. All rights reserved.

22. SB4.3.2 – Current-Limiting w/ Circuit Breakers
SB4.3.2 For branch circuits supplied by a Listed circuit breaker marked “current-limiting” in the feeder circuit, the short-circuit current rating on the line side of the circuit breaker shall be one of the following:
a) The short-circuit current rating of the feeder circuit breaker when all of the components in the branch circuit have a short-circuit current rating not less than the published peak let-through current of the circuit breaker, see Figure SB4.1, and the short-circuit current rating of all branch circuit protective devices on the load side are not less than the short-circuit current rating of the feeder circuit breaker;
b) The smallest short-circuit current rating of any branch circuit protective device on the load side of the feeder circuit breaker, when the conditions of SB4.3.2(a) exist except the short-circuit current rating of the branch circuit protective devices on the load side are less than the short-circuit current rating of the feeder circuit breaker.
c) The smallest short-circuit current rating of any branch circuit on the load side of the feeder circuit breaker, when the conditions of SB4.3.2(a) or SB4.3.2(b) are not met.
40kA SCC Available
Feeder CB
“Current Limiting”
65kA SCCR
25kA Ip Let-through
Branch Circuit PD
MCCB = 65kA SCCR
SB4.3.2 addresses the use of Listed Circuit Breakers that are marked “current limiting” per UL By using a “current limiting” circuit breaker in the feeder circuit, the known peak let-thru can be utilized to allow lower rated components to be used in the branch circuits, as long as they have a SCCR that is equal to or greater than the feeder Ip let-through value.
Allows the feeder circuit breaker SCCR (interrupting rating) to be claimed for the panel as long as the branch circuit protection device (BCPD) has a SCCR equal to or greater than the feeder breaker
If the BCPD has a lower SCCR than the main feeder and covers the available panel SC level, then the panel SCCR will be the lower branch circuit rating. An example would be if the BCPD had a reduced 50kA SCCR rating, the overall panel rating would be 50kA.
If lower SCCR’s are present in any branch circuit, the panel rating will be reduced accordingly
Branch Controller
30kA SCCR
a) 65kA Panel SCCR
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23. Current Limitation – UL Listed Circuit Breakers
This is an example of a Ip let-through table. This table is used to determine the maximum peak let-through of a circuit breaker applied at three different voltages, 240, 480 & 600V. On the horizontal axis, is listed the available fault level that the circuit breaker will be required to interrupt. Let’s say the breaker will be required to interrupt 42kA at 480V. Find 42,000A on the horizontal axis, move up to the 480V curve, move to the left to the vertical axis and read the peak let-through value, in this case it will be 15kA. This peak value can then be used in SC calculations and determining the capability of downstream devices to “withstand” the peak let-through energy of the molded case circuit breaker.
Per UL 489, circuit breakers that are marked current limiting are tested on a regular basis to insure that the ongoing performance of the device meets or exceeds the let-through characteristics contained in the table.
“Current Limiting Circuit Breakers”
Copyright © 2006 Rockwell Automation, Inc. All rights reserved.

24. SB4.3.3 - Current Limiting w/ Fuses
SB4.3.3 For branch circuits supplied by a Class CC, G, J, L, RK1, RK5, or T fuse in the feeder circuit, the short-circuit current rating on the line side of the fuse shall be one of the following:
a) The short-circuit current rating of the feeder fuse when all of the components in the branch circuit have a short circuit current rating not less than the peak let-through current corresponding to the specific fuse class employed from Table SB4.2 based on the current rating of the fuse and the available short-circuit current on the line side of the fuse, and the short-circuit current rating of all branch circuit protective devices on the load side are not less than the short-circuit current rating of the feeder fuse;
b) The smallest short-circuit current rating of any branch circuit protective device on the load side of the feeder fuse, when the conditions of SB4.3.3(a) exist except the short-circuit current rating of the branch circuit protective devices on the load side are less than the short circuit current rating of the feeder fuse.
c) The smallest short-circuit current rating of any branch circuit on the load side of the feeder fuse, when the conditions of SB4.3.3(a) or SB4.3.3(b) are not met.
40kA SCC Available
Feeder Fuse Switch
200kA SCCR
60A Class J
8kA Ip Let-through
Branch Circuit PD
65kA SCCR
Current Limiting with fuses is very similar to the process that we just reviewed with circuit breakers. With fuses there is typically are lower Ip let-through levels, allowing wider ranges of lower rated components to be used.
SB4.3.3 addresses the use of UL Listed fuses that are typically current limiting in nature. By using a “current limiting” fuses in the feeder circuit, the known peak let-through can be utilized to allow lower rated components to be used in the branch circuits, as long as they have a SCCR that is equal to or greater than the feeder Ip let-through value.
a) Allows the feeder fuse SCCR (interrupting rating) to be claimed for the panel as long as the branch circuit protection device (BCPD) has a SCCR equal to or greater than the feeder breaker
If the BCPD has a lower SCCR than the main feeder and covers the available panel SC level, then the panel SCCR will be the lower branch circuit rating.
If lower SCCR’s are present in any branch circuit, the panel rating will be reduced accordingly.
Branch Controller
14kA SCCR
b) 65kA Panel SCCR
Copyright © 2006 Rockwell Automation, Inc. All rights reserved.

25. Current Limitation – UL Listed Fuses
“Current Limiting Fuses”
Table SB4.2 is the Let-through table for UL listed fuses, by fuse class, cartridge size, voltage rating and fault level. UL class fuses are required to meet these maximum let-through characteristics. Similar to “current limiting” molded case circuit breakers, fuses are test on a regular basis to insure that they meet these performance levels.
Let’s review an example of how the table works. If we would like to know the let-through characteristics of a Class J, 60A fuse, first find the Class J 60A fuse row, then look up the maximum let-through characteristics at a 50, 100 or 200kA faults. For a 50kA fault, this fuse would have a maximum Ip of 8kA and a I2t of 30,000A2s. If the fault level of the application was larger than 50kA and up to 100kA, then the 100kA column would be used. Here the 60A fuse Ip let-through would go up to 10kA and the I2t would remain the same. This information becomes important as we try to utilize the current limiting affect of the fuse to allow down steam components with lower SCCR’s to be used in the circuit.
Copyright © 2006 Rockwell Automation, Inc. All rights reserved.

26. Current Limiting Devices – Fuses or CB’s
25kA Available
SCCR: 50kA
Current Limiting Fuse in a “Feeder” Circuit:
60A Class J Fuse limits Ip to 8kA,
Default SCCR of a power block is 10kA
Feeder “low” let-thru affect on the Branch
Motor Controller SCCR is 10kA
Let’s take a look at an example of how current limiting can effect the components that would be specified in a panel.
In our example, the available fault level is 25kA at 480V. The main feeder is a 60A fused disconnect with 60A Class J fuses. The first part of the circuit that we need to review is the feeder portion that contains the disconnect and power block. In this example, if the power block does not have a SCCR, we learned earlier that the SCCR would be assigned from table SB4.1 where the power block would be assigned a 10kA SC rating. Also, in Table SB4.2, we saw that the Ip of a 60A Class J fuse up to 50kA faults, has a Ip let-through of 8kA. Since the power blocks has a SCCR that exceeds the Ip let-through of the 60 Class J fuse, then this portion of the feeder circuit would have a 50kA rating. In this example, we used the “current limiting affect of the fuse to qualify the 10kA rated power block, and maintaining a 50kA rating on this portion of the circuit.
Let’s take a look at how the current limiting affect of the feeder fuse might affect components in a branch circuit. If we had a soft-starter in a branch circuit that was only rated 10kA, the same consideration can be used, that the feeder fuse limits the let-through to 8kA Ip, thus the soft-starter has a SCCR sufficient to maintain the 50kA rating of this component.
One issue must yet be addressed, all the branch circuit protection devices must have an interrupting rating and voltage rating that covers the available fault level.
The ability to use the current limiting affect of the feeder protection device is ok, but Rockwell Automation has worked to insure that all our branch circuit components and solutions have tested and Listed high fault ratings, then the current limiting affect becomes much less important.
Note: All SCPD’s must cover the available fault level
Copyright © 2006 Rockwell Automation, Inc. All rights reserved.

27. SB4.4 Overall SCCR of the Panel
SB4.4.1 For each branch circuit provided with branch circuit protection within the industrial control panel, the smallest short-circuit current rating of all power circuit components on the load side of a branch circuit protective device and the control circuit over-current protection in SB3.2.1 shall be determined and compared with the short-circuit current rating of the branch circuit protective device. The smaller of the two ratings shall be assigned to the line side of the branch circuit protective device.
In other words, use the lowest rating of a component or branch protective device to determine branch rating
18kA
65kA
70kA
65kA
Finally, once the branch and feeder protection devices and components have been evaluated, the complete panel rating can be determined. SB4.4 provides the guidelines to determine the final panel SCCR.
SB4.4.1 provides the evaluation of all “branch circuits”. Again, the branch component SCCR’s are compared to the BCPD interrupting ratings and the smaller of the two ratings are applied to the line side of the branch circuit protection device. In addition, we need to evaluate the over-current protection device for the control circuit. The smallest rating of all
Copyright © 2006 Rockwell Automation, Inc. All rights reserved.

28. Short Circuit Panel Ratings
SB4.4.4 The overall short circuit current rating of the panel shall be one of the following:
a) For an industrial control panel consisting of a single branch circuit without branch circuit protection….
b) For an industrial control panel consisting of a single branch circuit including branch circuit protection….
c) For an industrial control panel consisting of multiple branch circuits, and feeder components within the panel, such as disconnecting switches, bus bars, terminal blocks, and feeder over current protective devices, the short-circuit current rating shall be the lowest of the following:
1) The lowest short circuit current rating of any branch circuit in accordance with SB4.4.1 that has not been modified by SB4.3.1 – SB4.3.3;
2) The short circuit current rating of any feeder component not covered by SB4.4.4(c)(3) and any control circuit over current protection connected to the feeder as in SB3.2.1; or
3) The modified short-circuit current rating determined from SB4.3.1 – SB4.3.3 for each branch circuit supplied by the associated feeder component.
18kA 2
200kA 2
50kA 3
65kA
70kA
65kA
SB4.4.4 provides the guidelines for determining the overall SCCR of the panel.
Clauses a) and b) are addressed specifically to single circuit cabinets and are not of much importance at this time.
Clause c) considers the requirements of control panels with multiple branch circuits and feeder components.
Basically, to determine the overall panel SCCR, the lowest SCCR of all branch circuits, feeder components and control circuit over current protection connected to the feeder will determine the panel SCCR.
Simple component changes can increase the panel SCCR as needed. 1 1 1
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29. Copyright © 2006 Rockwell Automation, Inc. All rights reserved.
SB5 Markings
SB5.1 General
SB5.1.1 The nameplate rating of an industrial control panel shall include: “Short circuit current: ___kA rms symmetrical, ___V maximum” or the equivalent.
SB5.1.2 An industrial control panel marked with a high fault short circuit current rating and is not provided with the required branch circuit protective device as specified in the Exception to SB4.2.3 shall be marked with the type and size of branch circuit protection required to be installed in the field. This marking shall be included as part of the marking in SB5.1.1.
SB5.2 Cautionary markings
SB5.2.1 An industrial control panel with a short circuit current rating based on the high fault short circuit current ratings of one or more components as specified in SB4.2.3 shall be marked with the word “WARNING” and the following statement: “Risk of Fire or Electric Shock – The opening of the branch-circuit protective device may be an indication that a fault current has been interrupted. All current-carrying parts and other components protected by this device should be examined and replaced if damaged. If burnout of a current element of an overload relay occurs, the complete overload relay must be replaced.”
SB5 defines the “Marking” requirements for the panel.
SB5.1 covers the panel nameplate ratings and requires that the SCCR be expressed in kA rms symmetrical, and that the maximum voltage be stated on the label.
Note that if the branch circuit protection device in not provided, the panel shall be marked with the “type and size” of BCPD to be installed in the field
SB5.2 states that if and industrial control panel has a SCCR based on the high fault SCCR’s of one or more components, then additions markings must be included. The word “Warning” shall be added with the following statement: “Risk of Fie or Electric Shock – The opening ….
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30. Panel SCCR – 4 Checks to Validate the Results
Panel SCCR will be determined by the smallest SCCR of any feeder or branch over-current protective device or component
Branch circuit protection devices must cover panel available fault level
All components and controllers with SCCR’s based on high-fault ratings must be used with the specified branch circuit protective device
And finally, the panel SCCR must cover the available fault current supplying the panel!
Available Fault:
480V
In summary, the four basic checks required to establish a panel SCCR are:
The panel SCCR will be determined by the smallest SCCR of any…..
Brand circuit protection devices must…..
All components and controllers with SCCR’s…..
An finally, the panel SCCR must…..
It will be important that panel builders and OEM’s communicate early with the end user or contractor to insure that their panel design covers the available fault and voltage at the installation site. This will insure a smooth inspection and installation of the panel or machine.
Panel SCCR
65kA rms Symmetrical
480V Maximum
Copyright © 2006 Rockwell Automation, Inc. All rights reserved.

31. Need Additional Assistance with Your Application??
If there are any questions or issues that were not addressed during this presentation, additional assistance can be attained via RA Technical Services Team. They can be contacted by the following means:
Contact by at
Contact by calling Rockwell Automation Technical Support Line at , select Allen-Bradley Brand, then select Industrial Control.
If additional help is required on UL 508A and the panel SCCR procedure, you can contact Rockwell Automation Technical Services with the following information.
Thanks you for your time!!
Copyright © 2006 Rockwell Automation, Inc. All rights reserved.