National Electrical Installation Standards

Standards as High as Your Own

Thursday, August 23, 2018


Re: CQD answer published Tuesday, August 14, 2018 - Feeder and Service Neutral Size

Charlie, I very much enjoy the CQD and start each of my days with reading the questions and answers.  This is a very valuable forum for the industry.

Regarding the answer provided for the sizing of the neutral for dwelling services, the 2017 NEC does not allow the reduced neutral to be used for single phase 208/120 volt services or feeders from a 208/120 three-phase system. Small nuance in the text that has to be read carefully. 310.15(B)(7)(1) to (4) applies for 240/120 volt services, but only 310.15(B)(7)(1) to (3) apply to 208/120 volt services. The list item 4 is where the neutral is allowed to be reduced.

Keep up the good work, Chuck Mello



Hey Chuck it's good to hear from you. Thanks for your comment, the kind words, and for letting me off gently. You are correct. Only the neutrals of single phase services and feeders supplied from 120/240-volt systems are allowed to be sized using 310.15(B)(7)(4). The reference in the second paragraph was changed after the First Draft Report by Second Revision 1505 which included the following statement:

"The 208Y/120-volt application is limited to (1) through (3) due to the fact that the neutral of that system is a current-carrying conductor and must always be full sized in accordance with 220.61, and therefore the reduced neutral sizing allowed by (4) can never be applied."

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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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