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Thursday, December 17, 2015

Question:

Re: CQD answer published Tuesday, December 8, 2015 -

Charlie,

In most cases the NEC cable ampacites are on the conservative side, but heavily loaded underground duct banks are not one of those cases. For these applications we really should be using the information in Annex B as recommended by the consulting engineer firm. The issue is the excessive heat build-up where there are multiple ducts. The earth acts as insulation, increases the conductor temperature and requires that the conductors operate at an ampacity lower than what is permitted by the tables in Article 310.

Don Ganiere

A

Answer:

Hey Don thanks for your comment. I agree with your concern and mentioned the possibility of mutual heating as a reason they determined a different ampacity in my initial answer. But Annex B is not mandatory. If others are aware of field problems caused by using the Tables for determining conductor ampacities especially for underground parallel conductor installations please share those with us.

ABOUT CQD: The Code Question of the Day (CQD) is NECA and ELECTRICAL CONTRACTOR Magazine’s flagship National Electrical Code (NEC®) public forum for the industry, sponsored by EATON. The daily distribution of Q&A generates a lively dialogue and shares relative Code-based practical responses.

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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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