National Electrical Installation Standards

Standards as High as Your Own

The National Electrical Code is the bedrock of the electrical construction business.

Do you know all the ins and outs of the Code? NECA and Electrical Contractor magazine are pleased to present their daily online feature, “Code Question of the Day,” Sponsored by Eaton.


  • ?
    Friday, July 19, 2019

    Question:

    Hello all, I have a new storage building at a local high school being fed from an existing panel in the school. The engineer had me pull a grounding conductor in with the feeders, but my inspector is saying I need to bond my neutral at the disconnect for the new building.... my take is that the neutral is already bonded at the main switchgear for the school and bonding the neutral again would cause the grounding conductor to be a parallel run with the neutral back to the panel the building is fed from (not the switchgear). For this on whether or not the neutral should be bonded at the new building and if so, should I eliminate the ground I pulled in with the feeders...

    Fyi the service is  225A 480/277V 3 phase 4 wire, not that that should change anything.

    Thank you,

    Jeremy Buxman
  • ?
    Thursday, July 18, 2019

    Question:

    Re: CQD answer published Wednesday, July 10, 2019 -NM Cable Adjustment Factor

    Hi Charlie 2.0, Regarding the answer provided to the question for Tuesday 7/9/2019, just wanted to point out the 80% adjustment factor mentioned in the answer is applied to the ampacity allowed for the maximum conductor temperature. For NM-B Cable, the conductor insulation is rated 90C therefore the ampacity adjustment is applied to the 90C ampacity. In this situation (12/4 or 12/2/2 NM-B), that would give an allowable ampacity of 0.8 x 30 Amps = 24 Amps. Since this calculated result is higher than the 60C ampacity, we must use the lowest of the two values which is the 60C ampacity.  The net result is that a 4 conductor 12 AWG NM-B Cable (whether it is one circuit or two circuit) has an ampacity of 20 Amps – just like the 12/2 and 12/3 NM-B Cables. If multiple cables are bundled together, further ampacity adjustment may be required.

    Dave Watson, Southwire
    A

    Answer:

    Hey Dave thanks for your comment, it is a point we didn't cover well enough. The adjustment factor for more than 3 current carrying conductors applies based on 310.15(B)(3)(a) but the value for the 90-degree insulation in Table 310.15(B)(16) can be used for applying that factor as stated in 334.80.

ABOUT CQD: NECA’s Code Question of the Day (CQD) is a leading National Electrical Code® forum for NECA and the industry. The CQD generates a lively dialogue and relative practical and Code-based responses to an ever-increasing and interactive audience.

IMPORTANT NOTICE: All answers are based on the latest edition of the National Electrical Code®, unless the question requests a response based on a specific edition. This correspondence is not a formal interpretation of the NEC®. Any responses expressed to the questions are opinions and do not necessarily represent the official position of NECA, NFPA, or any technical committee. In addition, this correspondence is neither intended, nor should it be relied upon, to provide professional consultation or services.

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.

NECA STANDARDS: NECA publishes the National Electrical Installation Standards™ (NEIS™), a series of ANSI-approved performance and quality standards for electrical construction. Visit NECA-NEIS.org for more information. NEISÔ can be purchased in three formats: as paper books, on CD, or as electronic downloads.

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