National Electrical Installation Standards

Standards as High as Your Own

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Tuesday, May 14, 2019

Question:

Hello, Article 430 gives us the ability to install a 30amp breaker on a 14 gauge wire for example. NM-B is to be figured at 60c for ampacity and only at its higher ampacity of 90c for derating. The way I read the code for the example above you would have to install 10 gauge NM-B if you wanted to put the motor on a 30amp breaker. I can't seem to find an answer to this question anywhere and I believe the latter to be right.

Thanks, Jon
A

Answer:

Hey Jon thanks for your question. An inverse time circuit breaker used for branch circuit short circuit and ground fault protection can be sized at 250% of the motor full load current as stated in Table 430.52. The conductors must be sixed at 125 % of the motor full load current as stated in 430.22 and overload protection must be provided as stated in 430.32. You are correct that NM cable is limited to the ampacities for 60 degree C conductors in Table 310.15(B)(16) as stated in 334.80 which for 14 AWG is 14 amperes. If your motor full load current rating is not more than 12 amperes from Table 430.248 a 30 ampere circuit breaker can be used 12 X 125% = 15 amperes minimum conductor size and 12 X 250% = 30 amperes maximum circuit breaker rating.

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