Commercial mains wiring: 12ga wire for 30 amp load?

Since he went with an autotransformer, he should derate the wiring for NEC "general use" and that puts 12ga THHN at 20A. He needs to reinstall at 10ga- maybe- depending on circuit run length. 10ga would be good for up to 100', 8ga would be recommended for over 150', 6ga for up to 300' etc at 2.5% voltage drop..This may explain why he went with an autotransformer- burned the old motor out with overload at low voltage and upped the breaker from 20A motor time delay to 30A to eliminate LRA trips. He should therefore reinstall the 20A breaker with the 10ga or larger and get rid of the autotransformer unless the motor specifically calls out 230V which is rare these days.

That is what the NEC says to use for the circuit to the motor. They probably said 16 ".5" so you wouldn't use 14 ga wire and a 40a brealer

That is usually a question on the inspector test. FLA=16 with internal O/L protection, what is the min wire size & max breaker. Answer #14cu, 40a. Now days breakers are usually HACR so you don't need one that big to handle the locked rotor on start up but the code hasn't changed.

The 14ga = 15a, 12ga = 20a 10ga = 30a we all know is really 240.4(D) and is aimed at the circuits that are likely to have receptacles, where the installer has no control over what gets plugged in so the

True, but why would anyone undersize the wire? If it's a 30A breaker - you best be using at least 10AWG wire! Even larger if youre 50' or more from the source. Why would you Jeopardize burning out the motor from a large voltage drop in the wire? Stall current (starting current to break inertia) is typically 20* running I...

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What does Homer Simpson use to bake bread?

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Doh!

;-) Rich

Marge.

frequency? what mechanical load is it driving?

yeah 10% more voltage out than in means 10% more current in than out (+ losses).

Thus spake Jasen Betts:

FBt

A measurement between the load and transformer should tell you where the extra current is going.

Try a smaller pulley on the motor (or larger on the compressor)

That'll increase the duty cycle but reduce the load on the motor (and thefore the current it draws)