NEC reference says that 12ga THHN wire can carry 30A max current. Motor load is currently drawing constant 20A (230v 1ph circuit). Breaker is 30A. (A bit of history: Previous motor (running from 208v) was drawing less (15A?) current until machine broke. New machine's motor (running from 230v via boost autotransformer) is drawing 20A (measured at breaker). Motor spec plate says 16A.)Is 12ga wire sufficient, or should I bump it up to 10ga? Would the autotransformer account for the increased current draw?FBt

You have 2 issues. Foiust, is the motor supposed to be running on 230or 208? That couild be the extra current.If the motor is really exceeding the nameplate amps I wouldinvestigate futher. The second question is ampacity of the conductor 12ga THHN is onlygood for 30a if you have all 90c terminations and that is just notgoing to true so you have to use 25a. That is still 125% of the 20ayou measured so that is OK.For you "20a voters", article 430 allows you to use the 310.16ampacity on a dedicated motor circuit, not 240.4(D) which says 20.It also allows a breaker sized to 175% of FLA so the 30a breaker isOK.

...What does Homer Simpson use to bake bread?vvvvvvvvDoh!;-)Rich

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

Fred Bloggs 20 years ago

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.

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scada 20 years ago

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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Rich, Under the Affluence 20 years ago

...

What does Homer Simpson use to bake bread?

Doh!

;-) Rich

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keith 20 years ago

Marge.

Keith > > v > > > > v > > > > v > > > > v > > > > v > > > > > v > > > > > v > > > > v > > > > Doh! > > ;-) > Rich

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Jasen Betts 20 years ago

frequency? what mechanical load is it driving?

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

Bye. Jasen

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Esther & Fester Bestertester 20 years ago

Thus spake Jasen Betts:

60hz. 4hp cap. start motor driving 2-stage compressor, light duty cycle (20-25%).

FBt

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Long Ranger 20 years ago

Strangely the clock tower does not seem to show any artifacts, the hands of the clock seem fairly focused for the maximum zoom level.

Must be out of order ? can anyone confirm that ? :)

QuackQuack

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Jasen Betts 20 years ago

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)

Bye. Jasen

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