Ul 508

Short Circuit Current Ratings

TECHNICAL DATA

UNDERSTANDING NEC ARTICLE 409 AND UL 508A, SUPPLEMENT SB SHORT CIRCUIT CURRENT RATINGS

Short Circuit Current Rating Technical Data National Electric Code Changes Overview
National Electrical Code Introduces Article 409 for Industrial Control Panels
In 2005, the National Electrical Code (NEC) introduced Article 409 for Industrial ControlPanels requiring that panel builders and original equipment manufacturers (OEM) analyze and mark their panels with short circuit current ratings (SCCR).

Why Article 409 was developed
Historically, industrial control panels have been evaluated under several different NEC articles, which have led to inconsistencies.Article 409 provides common construction and installation requirements. It alsoestablishes a marking requirement for short circuit current ratings of industrial control panels. The fine print note of NEC Article 409.110 references UL508A, Supplement SB as an approved method to evaluate the short circuit current ratings of their panel. As individual states adopt the 2005 Edition of the NEC, a thorough understanding UL508A, Supplement SB is essential.
FIGURE 1: NEC 2005 Code BookNEC Article 409.110
An industrial control panel shall be marked with the following information that is plainly visible after installation: 1. Manufacturer’s name, trademark, or other descriptive marking by which the organization responsible for the product can be identified. 2. Supply voltage, phase, frequency, and full-load current. 3. Short-circuit current rating of the industrial controlpanel 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 FPN: UL508A-2001, Supplement SB, is an example of an approved method. 4. If the industrial control panel is intended as service equipment, it shall be marked to identify it as being suitable for use as service equipment. 5.Electrical wiring diagram or the number of the index to the electrical drawings showing the electrical wiring diagram. 6. An enclosure type number shall be marked on the industrial control panel enclosure.

TABLE OF CONTENTS
DESCRIPTION PAGE

Determining Your Panel Short Circuit Current Rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3 High Fault ShortCircuit Current Ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 Accessing High Fault Short Circuit Current Ratings Online . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7 Marking Your Panel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8Reference Materials and Contact Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

Short Circuit Current Rating Technical Data Determining Your Panel Short Circuit Current Rating
Per Section 409.110 of the NEC, there are two permitted ways to determine your panel short circuit current rating: purchase a previously listed and labeled assembly orestablish the short circuit current rating of the panel using an approved method. For the latter, the only stated approved method is UL 508A, Supplement SB. According to UL508A, Supplement SB4.1, there are three steps to analyze and mark your panel with a short circuit current rating: STEP 1 Establish the short circuit current ratings of individual power circuit components. STEP 2 Modify theavailable short circuit current within a portion of a circuit in a panel due to the presence of current limiting components. STEP 3 Determine the overall short circuit current rating of the panel.

STEP 1

Establish the short circuit current ratings of individual power circuit components using one of the following methods: a) The short circuit current rating marked on the component or on...