motor supply conductors

Not enough information for a good analysis.

Find a copy of the NEC at your library and read the parts about circuit conductors

You gain nothing by using #12 and violating the code in the process. Use #10 with a 30 amp breaker. The few bucks extra for the #10 is well worth it. Ed

Is 30A (or 25A) a large enough breaker to start the motors? You should use info from the manufacturer where available, sometimes they recommend a breaker size or give a MOCP (max over current protection) value. I think the general rule is to have a breaker that's up to 250% of the FLA, e.g. 40A in your case. Admittedly that's in Canada. Probably you can use wiring smaller than the breaker for motor loads in general, but I think 17FLA is too much for #12. 80% of #12's 20A rating would limit you to 16A. I'm not sure if you can use a receptacle rated lower than the breaker size. If that's not allowed, you may have to use wiring and receptacle rated equivalent to the breaker.

The ampacity of 12 AWG copper is at least 25 amps. The rule requiring number 12 AWG to have over current protection not greater than 20 amperes does not apply to motor branch circuits. The size of the Over Current Protective Device (OCPD) should be 1.25 times the FLA for that horsepower if that will allow the motor to start. In this case that would be 25 amperes. On a dedicated motor branch circuit the receptacle can be sized to the next standard size above the FLA current of the motor.

You'd think the Ampacity would be higher here, where it's so very much colder.