How to calculate power cable size

Updated February 21, 2017

Every electrical appliance must be safely connected to a power supply. The cable must be capable of carrying all the power needed by the appliance and of coping with the heat generated as electricity flows through the wires. Calculating the size of your power cable necessitates knowing the voltage of the power supply and the power rating of your appliances. Most households in the United States have a 120-volt supply. A straight forward mathematical formula tells you the power rating, or size, of the cable.

Determine the wattage of each appliance to be attached to the power cable. Each appliance has a label showing its power consumption in amps, watts or kilowatts.

Calculate the power rating of the cable, in amps, by using this formula: amps = watts/volts. For example, a 120-volt supply and a total of 2 kilowatts of appliances attached to the cable indicates that the minimum cable size will be (2,000/120) amps, or 16.67 amps.

Check your calculations for errors. Divide the amp rating by the voltage, and the answer should be the wattage of the appliances you expect to run from the cable. If it isn't, there was an error in your calculations.


Current can be calculated using an online electricity calculator. The thickness of the wires within a cable determine its current-carrying capacity. You can find tables showing wire thickness and current capacity on the Internet. Commercially available power cables are rated with a maximum current. Buy one rated higher than your calculated needs. The power rating is printed on the insulating cover of most cables.


Electricity passing through a cable generates heat. If a cable has a low current rating and you pass a high current through it, it may catch on fire. Electricity can kill; if in doubt consult a qualified 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.