National Electrical Installation Standards

Standards as High as Your Own

Monday, September 19, 2022


Hello Charlie,

I saw a 240v HVAC unit nameplate with a minimum circuit ampacity of 24.8A and MAX CKT.BKR (HACR) 40A. In the panel, I saw a 12 gauge THHN ran thru conduit and attached to a 40A double pole breaker. Is this acceptable? If so why or why not? THHN is good for 30A at 90º, but can that column be used, or must we use the 75º column? Here is some additional info: 19RLA, 45LRA. What gauge wire should be used with a 40A HACR breaker in this instance?

Thank you so much for your greatness,

James Evans.



James, thanks for your submission, and what a great question. As in all electrical installations, the circuit is only as strong as the weakest point. The temperature rating of all applicable equipment terminals must be confirmed, before the proper temperature column in Table 310.15(B)16 of the 2017 NEC™️ can be applied. The lowest temperature rating of any applicable terminals is the temperature rating we must use for determining conductor ampacity. Without knowing the terminal ratings we will have to assume the worst case which would be 60º C. NEC™️ Section 440.6(A) allows us to size the branch circuit conductors using the nameplate listed current. With a Minimum Circuit Ampacity of 24.8A, the minimum conductor size would be 10 AWG copper. Section 440.22(C) requires use to size the protective device in accordance with the manufacturer's marked value. The nameplate is marked with a maximum circuit breaker of 40A, so a 40A circuit breaker would be permitted.  

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