National Electrical Installation Standards

Standards as High as Your Own

Tuesday, October 31, 2017


Re: CQD answer published Monday, October 23 & 16, 2017 -AC Equipment Branch Circuit

1) 10 awg.... simply put 14 and 12 AWG can never be used on larger than 15 and 20 amp circuits respectively. Except as part of a ul-listed device but as an electrician we should never be installing these wires on anything larger than 15 and 20 amp circuits. The rating of 30 amps for 12 wire is simply for amperage adjustment for multiwire bundling. Oliver Allen

2) Good morning, Maybe it was because it was Friday the 13th or something, but I don't think we have the correct answer to Carl's question yet. If we have terminals rated 75 deg. C, and we use the 25 A. ampacity indicated in the 75 deg. column of Table 310.15(B)(16) per 440.6(A) and 240.4(G), then 12 AWG THHN/THWN is adequate.  440.22(A) would permit up to the 40 A. maximum circuit breaker or fuses specified on the nameplate.  Certainly, the 10 AWG is a better choice but the question was about determining the minimum size required.  Remembering that the NEC is the minimum standard, one can (and maybe sometimes should?) go beyond the minimum required.  Agreed? J Grant Hammett

3) Additionally NEC Article 240.4 Protection of Conductors (D) Small Conductors (5) 12 AWG Copper limits the overcurrent protection to 20 amperes after any correction factors for ambient temperature and number of conductors  have been applied. See 240.4(D)(5). Thanks , Mike Farrell

4) You are aware that all the connections involved are more-than-likely rated 60/75. Thus #10 would be the minimum for NM-B cable. But #12 would be the minimum for MC cable or THHN/THWN conductors. Matthew Hermanson



Hey All Ya All, thanks for your comments.

1) Overcurrent protection can exceed 15 for 14 AWG and 20 for 12 AWG as stated in 240.4(D such as several included in Table 240.4(G).

2) The original question or Keith's detailed explanation didn't mention anything about 75 degree C terminations, although is very possible all the terminations could be 75 degree C. But the question also was based on NM cable and 334.80 limits us to the 60 degree C values.

3) True but as stated in 240.4(D the specific conditions in (E) and (G) allow higher overcurrent protection.

4) Correct if the terminations are rated 75 degrees C as stated in 110.14(C) and Table 240.4(G)

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ABOUT CHARLIE: Charles M. Trout, better known as Charlie, was a nationally known NEC® expert and author. He served on several NEC® technical committees and is past chairman of CMP-12. In 2006 Charlie was awarded the prestigious Coggeshall Award for outstanding contributions to the electrical contracting industry, codes and standards development, and technical training. Charlie was also a member of NECA’s Academy of Electrical Contracting. Charlie’s experienced team of industry experts keep the CQD dialogue and discussions active and informative in the spirit of the man himself, as he wanted.

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