NEC409/UL 508A Support

Overview of Code Changes in NEC Article 409

The new change in Article 409 covers general use industrial control panels operating less than 600V. Applications other than general use may be recognized and covered by other articles covered within the National Electric Code.

The intent of the article is to ensure safe and reliable products are manufactured by control panel builders with clearly marked short circuit current rating (SCCR) such that the application is within specified SCCR range. This SCCR rating will help reduce fire and safety hazards.

In the past components contained within the control panel may be rated at a lower value than the main overcurrent protective device. If a short circuit was passed through to the lower rated component a fire hazard would exist. This code requirement is intended to reduce these hazards.

What is an industrial control panel and how is it to be marked?

Section 409.2 Defines a Control Panel
An assembly of a systematic and standard arrangement of two or more components such as motor controllers, overload relays, fused disconnect switches, and circuit breakers and related control devices such as pushbutton stations, selector switches, timers, switches, control relays, and the like with associated wiring, terminal blocks, pilot lights, and similar components. The industrial control panel does not include controlled equipment.

Section 409.110 Requires a Control Panel to be Marked
Not only are there high levels of short-circuit current available at the line terminals of many industrial control panels, there is also an interaction of the protective and control components under fault conditions that is assessed as part of the evaluation of control panels by conformity testing organizations. Assembling interrelated and interactive control and protective components in an enclosure presents a dynamic that in many cases can be evaluated for safety only under strict conformity assessment guidelines.

Source: Underwriters Laboratory UL 508A ISBN 0-7626-0398-8

Components NOT requiring an SCCR for use in UL508A industrial control panels

In most cases components located within the control circuit are protected by the fuse and need not be applied to SCCR calculation.
The overall circuit SCCR must still be evaluated.

SB3.2 Overcurrent protection of control circuit

SB3.2.1 For control circuits tapped from the feeder circuit, the overcurrent protection for the common control circuit or for the primary of a control transformer or power supply shall be provided with branch circuit protective devices having a short circuit current rating not less than the overall panel short circuit current rating, see SB4.4.4. For control circuits tapped from the load-side of a motor branch circuit protective device, the overcurrent protection for the common control circuit or for the primary of a control transformer or power supply, the short circuit current rating of the overcurrent protection shall be included in the determination of the branch circuit short circuit current rating in SB4.4.1 and SB4.4.4(a).

Source: Underwriters Laboratory UL 508A ISBN 0-7626-0398-8

Building a Control Panel and need an SCCR?

Option 1: Obtain a SCCR rating for an individually designed assembly

Option 2: Use an approved method (such as UL 508A Supplement SB)

NOTE: Option 1 may be suitable for a manufacturer that produces very few designs but any slight change in the design warrants a new test to be performed to prove SCCR. Option 2 is more suitable for a control panel manufacturer that produces many different designs.)