- Thread starter shaun
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Shawn

First of all the method you used was incorrect.

You get the motor amperage form table 430.150

which shows a 15 hp with 46.2 amps.

If the method you used had been correct you said

"OK 15hp=11185.5w, so in turn 11185.5w/208v=I, which is 53.8amps. You forgot to divide by the

sq. root of 3. 11185w/{208x1.73) = 31 amps.

Conductor size requires 46.2 x 1.25 = 58 amps or

# 4 cu thhn or equal. Breaker size is from table

430.52. NEC allows up to 250% of the FLA for breaker size. 2.50 x 46.2 = 115 amps max.

I have no idea what he means by usine two sets of

conductors. You need to quiz him for more info.

I assume this will be running for more than three hours at a time, continuous load derating?

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Ohm's Law is very useful in theory class, direct current systems, and AC systems that are purely resistive. Once you start try to apply it to typical circuits and installations, it doesn't quite work.

Anytime you see motor, or A/C unit, welder, etc.. you need to remember Article 240.4(G). Unlike genral purpose circuits and the like, specific loads have specific rules. In many of the cases, conductor sizing, overload sizing, and ground-fault/short-circuit protection are figured independently.

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