What size breaker is needed for feeding 10 kva transformer at 460 v single phase?

One horsepower (electric motor) is 746 watts, so ten horsepower is 7460 watts. Divide that by a power factor (assumed) of 0.8 and you get 9325 volt-amps. Divide that by a voltage (again, assumed) of 240 and you get 39 amperes.

For calculation purposes the electrical code book states that a 10 horsepower motor at 240 volts will draw 50 amps. The motor's nameplate amperage that is driving the rotary phase converter should all ways be used first.

The electrical code requires a breaker for a motor to be 250% of the motor's operating full load current. 250% of 39 amperes is 97 amperes. I would use a 100 ampere breaker for a 10 horsepower motor.

If the motor rating is used from the code book values, then 50 x 250% would equal 125 amps and that is the size of the breaker that must be used. This is the reason that the motor's exact name plate amperage rating should be used. In the long run feeder wire costs could make a saving in the overall projected costs.