How to Calculate the Amperage of 240 Volts

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There are two common types of 240-volt devices in the United States: Those that use single-phase circuits and those that use three-phase circuits. The method of deriving amperage differs among the types of circuits being used, by way of an additional voltage multiplier for the three-phase 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 240-volt 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 three-phase circuit. If you cannot find the phase circuit, look at the power connection. Four wires is a dead giveaway that it is a three-phase 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 single-phase circuit. Since this is a 240-volt device, you would divide the watts by 240 volts. For example, a 1,800-watt device would use 7.5 amps.

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

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  • Photo Credit Watt image by Ewe Degiampietro from Fotolia.com
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