National Electrical Installation Standards

Standards as High as Your Own

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Monday, March 30, 2020

Question:

In reference to Monday's answer (on 2-25-2000) it is important to consider the available fault current of the commercial building when making this judgement call.  Most residential panel main breakers are only rated for 10,000 amps of available fault current.  In many cases, commercial installations can have a fault current that exceeds this rating, which would make the panel unacceptable for the installation. 
 
Jackson Roos
A

Answer:

Hi Jackson, thanks for participating. Good observation. Just as a point of information, circuit breakers are typically not identified for use in only residential applications. The whole idea behind Section 110.24 is to draw closer focus on the requirement that electrical equipment must be used within its short-circuit current ratings. Subdivision (B) emphasizes that any modifications that cause changes to the level of available fault current should trigger an assessment of the system and equipment to ensure that the equipment SCCR are still equal to or greater than the available fault current. There are many older installations in the field with inadequate short circuit current ratings. As a reminder the panel board SCCR is only as high as the lowest SCCR of any installed circuit breaker. Example: A panel board may have an overall SCCR rating of 22,000 amperes, but if the breakers installed are rated 22k, and there is one at 10k, the overall SCCR is reduced to the lower SCCR, which is 10k. 

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