How to calculate an electrical load for wire size

Written by david robinson
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How to calculate an electrical load for wire size
Matching wire size to load is important for electrical safety. (plug in the socket #1 image by Alexander Zub from

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.

Skill level:

Things you need

  • Calculator

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  1. 1

    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.

  2. 2

    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.

  3. 3

    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.

Tips and warnings

  • 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.
  • 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.

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