There are two common types of 240volt devices in the United States: Those that use singlephase circuits and those that use threephase circuits. The method of deriving amperage differs among the types of circuits being used, by way of an additional voltage multiplier for the threephase circuits. Except for this multiplier, each calculation follows Ohm's Law, which states watts will equal volts times amps. By reconfiguring this equation, you can calculate the amperage of a 240volt system, provided you know its wattage.

Look on the motor's identification plate, information booklet or specifications. You will need to know the wattage of the device and whether it uses a single or threephase circuit. If you cannot find the phase circuit, look at the power connection. Four wires is a dead giveaway that it is a threephase circuit. If there are only three wires, then it is probably a single phase.

Divide the number of watts by the voltage to calculate the amperage for a singlephase circuit. Since this is a 240volt device, you would divide the watts by 240 volts. For example, a 1,800watt device would use 7.5 amps.

Divide this figure by 1.732 to calculate the amps for a threephase circuit. In the example, 7.5 divided by 1.732 would give you 4.33 amps.
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