2007
Issue 1, Volume 1
Frequently Asked Questions Regarding NEC 409 and UL 508A
The National Electric Code requires compliance with NEC article 409 in April 2006. Some states are
currently following this code, and additional states are coming on board every month.
The New Article 409 covers "Industrial Control Panels". Industrial Control Panels are intended for
general use, in ordinary locations, at 600V or less. One critical part of the new article is that Industrial
Control Panels must be evaluated & marked for their Short Circuit Current Rating (SCCR). This is
established by evaluating each feeder and supplemental circuits' SCCR and determining the smallest
kA value. This weakest link will be the panel's kA value. In order to install a panel, the SCCR value
must be greater than that of the incoming source.
How to Comply
The SCCR must be established using an approved method as described by UL 508A, supplement SB.
SCCRs must be evaluated for feeder circuits and branch circuits within the power circuit. There are
three basic steps to this:
1. Establish the short circuit current ratings of individual power circuit components as specified in
SB4.2, including all feeder and branch circuits.
2. 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 Current limiting components
provide an option for obtaining a higher overall panel kA rating with smaller rated devices.
3. Determine the overall panel short circuit current rating as specified in SB4.4
NEC Article 409
Q. What is National Electric Code (NEC) Article 409?
A. Article 409 is a new article in the 2005 edition of the National Electrical Code that sets installation and
construction requirements for Industrial Control Panels 600 volts or less and intended for general use.
Q. Reference 110.21 and 409.110
What are the installation requirements in NEC Article 409 in regard to marking an Industrial Control
Panel?
A. NEC Article 409 is requiring specific marking on industrial control panels including the short circuit current
rating of the panel. The marking is to be clearly visible or easily accessible after installation.
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NEC Article 409 Continued…
Q. How do I know what short circuit current rating to mark my panel?
A. (Reference 409.110)
NEC article 409 states that the short circuit current rating of a panel will be based on either the…
1) Short-circuit current rating of a listed and labeled assembly or…
2) Short-circuit current rating established utilizing an approved method.
Q. What is the relationship between NEC article 409 and UL508A?
A. (Reference 409.1 and 409.110 (3))
NEC article 409 makes a direct link to UL508A as the safety standard for industrial control panels. Article
409 also states the short circuit current rating of an industrial control panel can be based on utilizing an
approved method. In a fine print note article 409 states that UL508A Supplement B is an example of an
approved method.
UL508A Supplement SB
Short Circuit Ratings for Industrial Control Panels
Q. What is UL508A Supplement SB?
A. UL508A Supplement SB sets the requirements for industrial control panels with short circuit markings. For
panel manufacturers who build to UL508A, the safety standard for industrial control panels, these
requirements became effective April 25, 2006.
UL 508A supplement SB is also considered an approved method for evaluating a panel in the 2005 National
Electrical Code. Article 409 of the 2005 National Electrical Code requires that installation of panels be
marked with a short circuit current rating.
Q. How is the overall short circuit rating of the panel determined?
A. (Reference SB4.4)
The overall SCCR is the lower of the following:
1. The lowest short circuit rating of any branch.
2. The short circuit current rating of any power component in the feeder circuit or any control circuit
overcurrent device connected to the feeder.
3. The modified short circuit current rating (SCCR) of a branch circuit when applying current limiting
provisions.
Q. How are short circuit current ratings of power components established?
A. (Reference SB4.2.2)
There are three methods for establishing the short circuit current rating (SSCR) on power components.
1. The short circuit current rating marked on the component or instruction sheet provided with the
component.
2. The assumed short circuit current ratings in Table SB4.1
3. Components such as load controllers, motor overload relays or combination motor controllers that
have been investigated (tested) and are described in the manufacturers procedure file. (High Fault or
Combination Ratings)
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UL508A Supplement SB Continued…
Q. Do all components in the panel require evaluation in determining the marking of the panel?
A. (Reference SB4.2)
Only power circuit components are required to have short circuit current ratings. Exceptions to power circuit
components that are not required to have short circuit current ratings are power transformers, reactors,
current transformers, dry-type capacitors, resistors, varistors and volt meters.
Q. How are control circuits in the panel evaluated?
A. (Reference SB3.2)
Only overcurrent protective devices in the control circuit are considered in the evaluation of the panel. A
short circuit current rating is assigned to overcurrent protective devices in the common control circuit, or on
the primary side of a control transformer or power supply.
Q. How does current limiting work?
A. (Reference SB4.3)
Using power transformers, circuit breakers marked “Current Limiting” and fuses (Class CC, G, J, L, RK1,
RK5, and T) in the feeder circuit can modify the short circuit current rating of branch circuits when applying
current limiting provisions. The current peak let-thru value of these components must be less than or equal
to the short circuit current ratings of all components in the branch.
Q. Can manufacturer’s performance data be used to determine peak let-thru current values?
A. (Reference SB4.3.2 (a) and SB4.3.3 (a))
For fuses, peak let-thru current values must come from Table SB4.2 in supplement SB.
For current limiting circuit breakers peak let-thru current values can be obtained from published
manufacturer data.
Q. Can I apply current limiting provisions to modify the short circuit current ratings of components in
the feeder circuit?
A. (Reference SB4.3)
No, using current limiting components in the feeder circuit only modifies the short circuit current rating of
branch circuits.
Q. Can I apply series ratings to increase the short circuit current rating of my panel?
A. No, the SCCR of an overcurrent protective device cannot be modified by an overcurrent protective device in
the feeder circuit.
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For a FREE copy of Rockwell Automations Short Circuit Current Ratings
Guide contact marketing@frenchgerleman.com. In the subject line reference
Short Circuit Current Ratings Guide – Copy. Or download your FREE copy
from the FG Connections Newsletter.
Article/Information provided by:
Rockwell Automation
Author:
Humberto Vargas
Rockwell Automation
Commercial Engineering
email: Hvargas@ra.rockwell.com
The statements, recommendations, and claims made in this newsletter represent the opinions of the contributing manufacturers and
French Gerleman. They should not be construed as a claim of warranty or performance of equipment. Neither French Gerleman nor
anyone related to or involved with French Gerleman will be held liable for any claims or damages arising out of the publication of this
newsletter, whether made in tort, contract, intellectual property or otherwise.
Reference Section 12, French Gerleman General Terms and Condition of Sale. For a complete copy of the French Gerleman General
Terms and Condition of Sale, visit www.frenchgerleman.com.
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