What size motor can be used on a 20 amp breaker at 460 volt?

Answer for USA, Canada and countries running a 60 Hz supply service.

Motors are a complicated part of the NEC. I will give you general rules but what type of motor you have, what it's "duty rating" is, how it is going to be used, etc. etc., can make huge differences in the answer to your question.

Assuming your 20 amp breaker is either 2 poles or 3 poles, the code allows a motor up to 10hp to operate on that breaker.

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As always, if you are in doubt about what to do, the best advice anyone should give you is to call a licensed electrician to advise what work is needed.

Before you do any work yourself,

on electrical circuits, equipment or appliances,

always use a test meter to ensure the circuit is, in fact, de-energized.

IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOB

SAFELY AND COMPETENTLY

REFER THIS WORK TO QUALIFIED PROFESSIONALS.