National Electrical Installation Standards

Standards as High as Your Own

Wednesday, March 7, 2018


Hello Charlie, the building I work in is old and most of the wiring that has not been updated by myself is quite old. The question I have has to do with shared neutrals. Many of the existing circuits are sharing neutrals with several ungrounded conductors. When I have worked with these circuits I have re-pulled wire to have one neutral with one ungrounded conductor. Which is how I understand it in the code book. Exceptions I think being breaker ties or being able to shut all circuits off sharing that neutral. I was told some wiring can be grandfathered in and if this is true where does this apply? Can I tie into existing circuits in a junction box sharing a neutral between circuits if I know the load will not surpass my overload protection or overload my neutral? Can this be done without breaker ties? Last if I am disturbing the existing wiring where do I draw the line as needing to update this wiring to current code? Thank You for your time, Brad Wray



Hey Brad thanks for your question. The use of shared neutrals for branch circuits are allowed and that is a "Branch Circuit, Multiwire" as it is defined in Article 100. Single pole circuit breakers can be used if a means that will simultaneously disconnect all ungrounded conductors of the circuit as stated in 210.4(B). Identified handle ties are permitted for that purpose as stated in 240.15(B)(1). Prior to the 2005 NEC the simultaneous disconnect wording in 210.4(B) was limited to multiple devices or equipment on the same yoke - and only for dwelling units.

The NEC does not specifically address modifications of existing multiwire branch circuits regarding having a simultaneous disconnecting means. Although Annex H does not apply unless specifically adopted it gives us some guidance in 80.9 for existing installations (sometimes called grandfathering) and the determination to be made by an AHJ.

It seems the judgement of "if" and "what" needs to be done, for other than a new installation, is left to someone. Consider the safety concerns, the risk and questions that could be asked if there is an incident involving an injury or worse and there was an opportunity to make the installation compliant during the modification. Even for existing circuits that are not modified, creating work practices and procedures that help ensure incidents are prevented will help reduce the risk. If identified circuit breaker handle ties are available for the brand and type of circuit breakers you have it seems to be a low cost solution to reduce risk even if not specifically required.

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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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