I've always thought P = U * I (units watt, volt and ampere, respectively), but I ran across this discussion on a forum:
If you look in your panel there are 2 “hot wires coming in from the
pole”. 1 is the 120v A phase and 1 is the 120v B phase. you need a hot
leg from each one to make 240V. if you have a machine drawing 20 amps
on 120V it is drawing it from 1 phase. If you have the same machine
drawing 10 amps on 240V it is drawing 10 from the A phase and 10 from
the B phase.The easiest motor for me to read the name plate on is a 1.5 HP 3450
rpm Baldor motor. It was on the shelf. It states at 115V it draws
13.2 amps. next it states 230V it draws 6.6 amps.“amps x volts = watts”
13.2×115 = 1518
6.6 x 230 = 1518
no matter what you do it will draw the same amperage total.
To me this explanation contradicts itself since it says that the motor draws both the same current and the same power. Is the motor actually drawing 6.6 A from each hot wire (for a total of 13.2 A), or is it drawing 3.3 A from each hot wire? If it's 6.6 A from each hot wire, it seems the classic "current x voltage = power" equation is misleading for 240 V "single-phase" circuits.
Update: to clarify, this question applies to 120 V and 240 V current in the US.
Best Answer
A 240 volt motor will only be connected to the two live wires, not to neutral, so your example draws 6.6 amps at 240 volts. If it is connected to operate at 120 volts, it will be connected between one live wire and neutral, and will draw 13.2 amps at 120 volts.
I think it is misleading to say that, in the 240 volt case, the motor draws 6.6 amps from each live wire. A more correct description would be that it draws 6.6 amps from one live wire, and returns that 6.6 amps through the other live wire.
Using the same terminology, in the 120 volt case, the motor will draw 13.2 amps from the live wire, and return that 13.2 amps through the neutral wire.