Wiring

#12 wire would be fine as long you are not more than 60 feet from the breaker.

-- Gary, East TX -- The longest journey begins with a single step.

In most areas I’d say Use 10-2

14 → 15A
12 → 20A
10 → 30A

-- Tom, Central New York

Not to mention “code”, if it applies.

-- Tim -- http://tmuli.com

Ordinarily to comply with the National Electrical Code (NEC), you cannot have a 30A breaker protecting #12 wire, but motors have different requirements where protection can be higher to prevent breaker tripping during motor start-up. Motors draw approximatel 6-8 times full load current for up to 15 cycles (15/60 = 1/4 second). In order to size wire correctly for a single motor, you must multiply the motor namplate full load amps (FLA) by 125%. You can use the NEC tables if the nameplate does not list FLA. This will give you the minimum circuit amps (MCA). For example; If your MCA was 24A then you would need at least #10 wire. Then you select the next closets wire size that matches (but not smaller). Then you size the motor feeder protection (circuit breaker or fuse). Molded case circuit breakers can be sized up to 250% of the motor FLA. I usually go as high as 175% depending on the motor and its use, since some motors draw more on start-up dpending on their design and use.

For a 3HP 240V single phase moter the NEC lists 8.3A, 1.25×8.3=10.4A, so #12 wire should be fine. 8.3×2.5=20.8A, the code allows the next higher size breaker to be used as long as it does not exceed 400% of the motor FLA (for motors under 100A), since 30A is less than the 400% you are okay with a 30A breaker.