What Size Extension Cord Do I Need?

Ever use a power tool pulled into an extension cord on a project and it not work right. To test it, you plug the power tool directly into the wall outlet and it works fine. What’s this? Another bad extension cord? Maybe, maybe not! You could just be using the wrong extension cord.

Extension cords are measured by their length in feet and the size of wire used in their construction measured in units referred to as the wire’s gauge (the larger the number the smaller the wire). Both of these numbers work together to determine how much power (measured in amperes, amps or amperage and most commonly represented by the letter “A”) can be safely provided to the equipment pulled into the extension cord. The longer the extension cord and/or the smaller the wire, the less power can flow through it. If you use an extension cord that is not rated to provide the amperage needed to properly run the equipment attached to it, the extension cord can cause equipment on the extension cord to not function properly, cause damage to the equipment, cause breakers to trip and cause a fire hazard.

Below is a table to help you determine which extension cord you need for the job. First determine how long of an extension cord you need. If you are working 20 feet from an outlet, use the shortest extension you can based on the table below. Longer extension cords just waste electricity. Next, determine how much amperage the piece of equipment you are using needs to function properly. This is usually found on a sticker or metal plate on the piece of equipment itself (if not, look in your owner’s manual). Using these two numbers (distance and amperage), you can use the table below to determine the size of the wire needed inside the extension cord to have your equipment function properly.

Extension cord length Amperage Required
0-2 amps 2-5 amps 5-7 amps 7-10 amps 10-12 amps 12-15 amps
25 ft. 16 ga. 16 ga. 16 ga. 16 ga. 14 ga. 14 ga.
50 ft. 16 ga. 16 ga. 16 ga. 14 ga. 14 ga. 12 ga.
100 ft. 16 ga. 16 ga. 14 ga. 12 ga. 12 ga. 10 ga.
150 ft. 16 ga. 14 ga. 12 ga. 12 ga. 10 ga. -
200 ft. 14 ga. 14 ga. 12 ga. 10 ga. - -
Table courtesy of Planet Christmas

Think of it as a hose. The bigger the hose, the more water will flow. If you need a 50ft extension cord to provide a total of 6.5amps, you need to use a 16gauge or larger extension cord (a larger gauge means a smaller number, in this case, 14, 12 or 10 gauge). Extension cords are expensive, matching the right extension cord for your needs could save you quite a bit of money!

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