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Thursday, September 14, 2023

Question:

Hello Charlie, I have a question regarding your answer to Tommy for August 29, 2023, CQD on a TESLA wall connector and the required wire and breaker sizes. Wouldn’t Section 240.4(B) permit the use of a 60A OCPD on #6 AWG NM Cable, using the next size up rule? For his question, 240.4(B) can allow for a 60-amp breaker to be used on 55-amp rated #6 NM-B (Romex) cable. Thanks, Rich Cressotti
A

Answer:

Hello Rich, thank you for your comments and feedback regarding 8/29/23 CQD.

Here is some additional information for clarification:

  • Section 210.19(A)(1)(a) requires branch-circuit conductors supplying continuous loads to have a minimum ampacity not less than 125% of the continuous load served.
  • Section 334.80 requires the ampacity of NM Cable to be calculated using the 60º C column of Table 310.16, limiting the ampacity of the #6 AWG NM Cable to 55A.
  • The TESLA Wall Connector has a maximum output is 48A, as a continuous load, the ampacity of this unit calculates to (48 x 1.25) = 60A.
Regardless of what Section 240.4(B) permits for the OCPD, the conductors still need to be sized according to the continuous load served, so 55A rated conductors would not be acceptable for a 60A load, thus #6 AWG NM Cable would not be permitted.

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CHARLIE TROUT: 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. Even though Charlie passed away in October of 2015, his work continues in spirit. NECA continues to maintain this question forum for its many subscribers in memory and recognition of all his significant contributions to making the NEC what it is today.

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