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.”


  • ?
    Tuesday, April 24, 2018

    Question:

    For a system can be supplied either strictly from a utility source or strictly from a generator, the 2014 NEC code specifically addresses where to ground the generator neutral based on using either a 3 pole transfer switch or a 4 pole transfer switch. However, each of these scenarios appear to assume that there are neutral loads and therefore, a neutral will be ran from both the utility service entrance equipment to the transfer switch and from the generator to the transfer switch. If a 3 pole transfer switch is used, the generator neutral is not grounded at the generator but is ran to the transfer switch where it is solidly connected to the neutral from the utility service entrance equipment (i.e a non-separately derived system). If a 4 pole transfer switch is used, the generator neutral is grounded at the generator and both the generator neutral and the utility service neutral are connected to the 4th pole of the transfer switch where they are switched (i.e. a separately derived system). However, where in the code does it address what to do when there are no neutral loads and therefore, there is no need for the utility neutral to continue beyond the service entrance equipment or for the generator neutral to extend beyond the generator? The code seems to imply that there must be a neutral wire between the generator and the utility service entrance equipment, but I can't find where it specifically states this. If you have a 4 pole transfer switch and you ground the generator neutral at the generator, this appears to be the definition of a separately derived system. However, in this situation, if you do not run a neutral from the utility service to the 4 pole transfer switch or from the generator to the 4 pole transfer switch, is this a code violation? If this is not a code violation, would it become a code violation if you replaced the 4 pole transfer switch with a 3 pole transfer switch under this situation (i.e. no neutral loads - therefore, no neutral from the generator or the utility service entrance equipment)? In each of these situations, please assume ground fault protection/indication has been addressed when operating solely off the utility and solely off generator.

    Thanks, Chris D.

  • ?
    Monday, April 23, 2018

    Question:

    An existing installation has (3) separate oil-filled single phase transformers (pad-mounted) wired together as delta-wye in an overhead bus structure. All (3) transformers are protected with (1) overcurrent relay. My question is: does each transformer require it's own independent overcurrent protection? How does NEC-450.7 relate to this?

    Barry Calloway

    A

    Answer:

    Hey Barry thanks for your question. What you are describing is not operating transformers in parallel as covered by 450.7. If you connected the three separate single-phase transformers in parallel, to have multiple sources or for additional capacity, then they would need to be switched as a unit as stated in 450.7. If they were not then deenergizing only one transformer would still leave the others supplying power.

    For overcurrent protection multiple single-phase transformers operating as a unit is considered "a transformer" for overcurrent protection as stated in 450.3.

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.

NECA SAFETY PRODUCTS: NECA publishes valuable electrical safety books and CDs for the industry. Visit necanet.org/store to view or purchase NECA safety products.

CONTACT US: To submit a Code question, subscribe, or unsubscribe from this list, please e-mail: codequestion@necanet.org.