What size breaker and wire for a 220v 18 amp motor

[JAMES SAKLAS]

Thanks to all you that responded.

[Jim Neeley]

James,

At 240V, your motor is almost certainly an induction motor. My statements below will also assume you don't have a variable speed drive or other advanced "very soft start" circuitry attached.

Motor loads are very different from lighting or electric heating loads in that there is a surge of electricity that flows for a very short period of time (a tiny fraction of a second). This surge is typically on the order of 6x the nameplate rating. The good thing is our circuit breakers follow an "inverse-time curve" for these "small" [much less than short circuit] loads.

Nevertheless, this is the reason why the NEC treats motors in a special way, typically permitting larger circuit breakers on motors than you would use for other large loads.

Will it run on 18A? Perhaps, if it doesn't trip on startup but good engineering practice tells us to go with a 30A breaker and #10 wire to be pretty certain it will plug and play.

Because some motor/loads are so hard to start, the NEC permits an 18A motor to size the breaker UP TO: (full load = 18A * 225%) and then round up to the next size, which would be 50A (and require large wire).

Don't do that though, as the go on to say you should use nothing larger than "necessary" and require the motor to have its own thermal overload protection. Additionally, this would require using a giant 50A plug on the tool, likely replace the wire from the motor to the plug, and perhaps more.

FWIW, my Unisaw draws 20A and Delta recommends a 40A breaker.

Read more here: http://en.wikipedia.org/wiki/Circuit_breaker

Just my $0.02.. YMMV..

Jim in Alaska
(Also EE)

[Wade Lippman]

FWIW, my Unisaw draws 20A and Delta recommends a 40A breaker.

My cyclone draws 22a (it actually does, I thought they were exaggerating, but I measured it). Grizzly recommends a 40a breaker.
I used #10 and a 30a breaker after finding out that everyone else does that and verifying the startup current is well within the trip curve of the breaker.
Beats me why they say 40a; maybe to cover defective breakers?