National Electrical Installation Standards

Standards as High as Your Own

Monday, April 23, 2018


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



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 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 to view or purchase NECA safety products.

 CONTACT US: To submit a Code question, subscribe, or unsubscribe from this list, please e-mail: