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How to calculate an electrical load for wire size

Updated February 21, 2017

The relationship between the size of a wire and the electrical load it can safely carry is an important one. As electricity flows through a wire it encounters resistance; resistance generates heat. When too large a load is passed through a wire it heats up excessively and may break down the insulating cover or catch fire. Calculating the safe electrical load for a wire requires basic mathematical skills and the use of a simple formula.

Enter, into your calculator, the current rating of the wire. The rating will be printed on the insulating covering, and is expressed in amps. For example, if your wire is rated at 20 amps, enter 20 into the calculator.

Multiply the current rating by your home voltage. Most North American homes have a 120-volt supply. Using our example, multiply the current rating of 20 by the voltage of 120. The answer, 2400, is the maximum safe electrical load in watts. The wire can be used to supply electricity to an appliance using no more than 2400 watts.

Check for errors. The formula for load is voltage multiplied by current (in amps) = load (in watts). Divide your calculated load by the voltage. The answer should be the current rating of the wire. If your answer is different there was an error in your calculations.

Tip

Use an online electrical calculator to quickly calculate load and current in a wire. Wire size is also measured by its cross-sectional area, commonly known as American Wire Gauge. Online tables relate AWG to current carrying capacity.

Warning

Electricity is dangerous. It can cause fires, electrocute, or kill you. If in doubt always consult an electrician. The safe load for a cable bundled with other cables is lower than for a cable used on its own in an open space. The cable is said to be derated. Never use a wire rated lower than the anticipated load. It will overheat and may catch fire.

Things You'll Need

  • Calculator
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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.