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How to calculate power amp current draw from an output wattage

Updated February 21, 2017

Every time you use an electrical appliance, it consumes power. The power consumption of appliances is usually printed on them, expressed as a value in watts. U.S. homes are supplied with a constant 120-volt electricity supply, although some appliances like dishwashers may use 240 volts. Calculating the current drawn in amps necessitates knowing both the appliance wattage and the voltage. These values are substituted into a simple mathematical formula to calculate the current drawn.

Ascertain the power rating of the appliance, or appliances, for which you want to find the current. Read the label or plate attached to the appliance(s). If more than one appliance is involved, use the combined value in watts.

Determine whether the appliance is on a 120-volt or a 240-volt circuit. The formula to calculate the current drawn in amps is: amps = watts/volts.

Using this equation, for a 1200-watt steam iron running on 120 volts: 1200/120 = 10, so the current drawn is 10 amps.

If the voltage is 240 volts, and a dishwasher uses 1200 w, 1200/240 = 5, so the current drawn is 5 amps.

Check for errors. Multiply your answer in amps by the voltage. The answer should be the wattage of the appliance. Using the dishwasher example, 5 * 240 = 1200. If the answer is not the value with which you began, there was an error in your calculations.

Tip

Appliances often show voltage as well as wattage on the information label. The symbols for amps, watts and volts are A, w and v. Appliances using a motor, like fridges and freezers, may use three times as much power when switching on than when running. Use a sustained wattage value rather than a peak wattage.

Warning

Electricity is dangerous. It can cause fires and can kill. If in doubt, consult a professional electrician.

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About the Author

David Robinson has written professionally since 2000. He is a Fellow of the Royal Geographical Society and the Royal Meteorological Society. He has written for the "Telegraph" and "Guardian" newspapers in the U.K., government publications, websites, magazines and school textbooks. He holds an honors Bachelor of Arts in geography and education and a teaching certificate from Durham University, England.