National Electrical Installation Standards

Standards as High as Your Own

?
Tuesday, January 14, 2020

Question:

What is the minimum size branch circuit conductor for a 50 HP, 3-phase, 460-volt motor that runs continuous and is installed in a steady ambient temperature of 85-90 degrees F? Our office is having a debate about this and we were hoping your forum could provide another view/opinion.

Thomas Clark

A

Answer:

Thanks for the question Thomas.

The short answer is 4 AWG THWN copper. This one involves a few rules in the NEC. First let me advise that Example 8 in Annex D can prove to be very useful when dealing with Code issues/questions related to electric motors. Here is a brief outline of the rules that apply. Section 430.6(A)(1) says to use the table values to determine conductor ampacities (generally). Table 430.250 indicates a value of 65 amperes for the motor in the question. Section 430.22(A) indicates that a 125% factor has to be included with the ampacity values derived from the 430ntable values. 65 amperes times 125% = 81.25 amperes. Referring to Table 310.16 in the copper THWN column one can see that 6 AWG is too small (at 65 amperes) but 4 AWG is adequate as it will carry 85 amperes. The question did clarify there was no ambient temperature correction needed and the question was silent on the number of current-carrying conductors in the wiring method supplying the motor. The assumption was that there were only three current-carrying conductor and an equipment grounding conductor supplying the motor.

Hope this information is useful. Again, Example 8, Annex D is very helpful is solving motor questions. 

 

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.