National Electrical Installation Standards

Standards as High as Your Own

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Wednesday, August 21, 2019

Question:

I need to understand how the main breaker (in this case 1200A) can support by code feeders of 400A, 800A, & 1000A. The question my customer posed to me was how does the main of 800A support what appears to be a total of (400 + 800 + 1000A) 2200A. I understand the breakers are sized to protect the conductor size but I find limited information on how the 1200A main can support the 2200A feeder loads.  

Rick Gardner
A

Answer:

Hey Rick thanks for your question. It doesn't but that doesn't mean that the installation is not acceptable. It seems you meant that the second time you mention "main" it should read 1200 instead of 800. Feeder conductors must be sized to carry the load as stated in 215.2(A)(1). Starting at the load end calculating the loads using Article 220 to ensure that the 400, 800, and 1000 ampere feeder conductors are properly sized using 215.2(A)(1). If the total of those loads is 1200 or less then they can be supplied by a 1200 ampere feeder. If it is more then the 1200 ampere size is not sufficient. Consider a typical 100 ampere panelboard in a dwelling unit. If you total all the overcurrent devices it will very likely exceed 100 amperes but they are not all loaded to their full ratings. Overcurrent protection is also necessary as stated in 240.4 but that is separate from the load calculations. If this is not what you are asking please rend in an additional question.

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