National Electrical Installation Standards

Standards as High as Your Own

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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. 

 

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