National Electrical Installation Standards

Standards as High as Your Own

Tuesday, December 3, 2019


A client owns a 12.6 KV X 480Y277 transformer. This transformer primary is supplied by the utility and is the sole power source for the building. The project electrical engineer shows bonding and grounding of  X 0 at the transformer, then just 3 phase conductors to the first disconnecting means. We objected. He replied the the grounded conductor will not serve any neutral load loads, and seems to be treating this like a separately derived system instead of what we think it is, a service entrance. We cited the code requiring that the grounded conductor was required to be brought to the first disconnecting means regardless, and grounded and bonded there in addition to the bonding at the transformer. He is adamant, and we are not willing to knuckel under. The argument seems to center on the private ownership of the transformer. In our opinion, who owns the transformer doesn't matter. Your thoughts might help us bring some sanity.

Ron Robertson




Hey Ron thanks for your question. Actually, ownership makes a considerable difference. If the customer owns the transformer then the secondary is not a Service as that term is defined in Article 100. The service ends at the point where the primary conductors terminate, which should be the Service Equipment (disconnecting means) as that term is defined in Article 100. See also the definition of Service Point in Article 100.

The transformer in your example is a Separately Derived System as that term is defined in Article 100. The word "if" in 250.30(A)(3) allows a grounded conductor but does not require one for a separately derived system. But in your case a supply side bonding jumper must be installed between the transformer and disconnecting means it is supplying as stated in 250.30(A)(2), which can be a metal nonflexible raceway.


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.

SUBMIT YOUR CODE QUESTION: Click here to submit a question to for inclusion in an upcoming edition of the Code Question of the Day, or email

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.

NECA STANDARDS: NECA publishes the National Electrical Installation Standards™ (NEIS™), a series of ANSI-approved performance and quality standards for electrical construction. NEIS can be purchased in the NECA Store in three formats: a printed or  PDF download of a standard or, as an  annual subscription service.

NECA SAFETY PRODUCTS & PUBLICATIONS: NECA produces electrical safety publications and products for the industry including jobsite safety guides, handbooks and resource kits. View a full listing of available resources and products »

IMPORTANT NOTICE: Unless the question requests a response based on a specific edition, all answers are based on the latest edition of NFPA 70® National Electrical Code®.

This correspondence is not a formal interpretation of the NEC® and any responses expressed to the questions are opinions and do not necessarily represent the official position of NECA, NFPA, the NEC Correlating Committee any Code-making panel or other electrical technical committee. In addition, this correspondence is neither intended, nor should it be relied upon, to provide professional consultation or services. 

UPDATE YOUR SUBSCRIPTION PREFERENCES: Subscribe or Unsubscribe from this list.