The United States uses a system called the American Wire Gauge (AWG) which makes calculating the correct wire size to connect between your electrical appliance and the power source easier and safer. Two general principles apply when working out the correct wire size: amperes, which is the electrical current flowing through the wire, and distance. The higher the amperes your appliance uses to operate, the thicker the wire needs to be. The same applies to distance: the longer the distance the thicker the wire.
Things You'll Need
 Tape measure
 Pen and paper
 Calculator
 Internetconnected computer

Measure the distance between the appliance and the power source using a tape measure. Multiply the distance by two. For example, if the figure you've measured is 40 foot, then multiply 40 by 2 to get 80 foot. Use a pen and paper and write down the figure.

Check the label on the electrical appliance to determine the wattage. The wattage is the total electrical requirement the appliance needs to operate. The label may give a number followed by "watts", or it may have a number followed by "kilowatts." Write down the number, if it says watts. If it says kilowatts, then multiply the number by 1,000 to get the watts. For example, if the label says 2 kilowatts then multiply 2 by 1,000 to get 2,000 watts and write down the number.

Look on the label to find the voltage the appliance uses. It will say between 110 and 120 volts, which is the standard U.S. voltage supplied to your home.

Divide the watts you wrote down by the voltage on the label to ascertain the amperes. For example, if the watts are 2,000 and the voltage is 120, then divide 2,000 by 120 to get 16.66 amperes. Round up the figure to the nearest whole number and write it on the sheet of paper.

Use an internetconnected computer. Click the "Engineering Toolbox: Amps and Wire Gauge" link in the Resources section below.

Look at the table displayed on your computer screen. The yellow line across the top of the table is the amperes and the green column to the left of the table is the distance. The central white area of the table refers to the AWG size.

Refer to your sheet of paper. Find the distance you wrote down and then look at the green column to find the line that corresponds to the distance. If the number you wrote down doesn't directly correspond, then use the next highest number in the column.

Find the amperes you wrote down and find the corresponding number from the yellow column. Use the next highest figure in the line, if there's not an exact match.

Follow the line across from the number in the green column and follow the column down from the number in the yellow line until the two meet. The figure in the box is the AWG wire size you need to safely connect your appliance to the power source.
References
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