Extension cord

Ever use a power tool plugged into an extension cord and the power tool did 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 the wire used in their construction measured in units referred to as the wire’s gauge (the larger the gauge number the smaller the wire). Both of these numbers work together to determine how much amperage (also called amperes or amps and most commonly represented by the letter “A”) can be safely provided to the equipment plugged into the extension cord. The longer the extension cord and/or the smaller the wire, the less amperage 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 the equipment to not function properly, cause damage to the equipment, cause breakers to trip and/or cause a fire hazard from overheating.

How to size an extension cord

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 cord you can based on the table below. 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).

Power Tool Label

Helpful Hint: In the picture to the right, you see part of a typical power tool label. It shows the model number, serial number, voltage and amperage needed to operate the power tool properly (circled in blue).

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/Size Chart

Amperage Required
25 feet 16 gauge 16 gauge 16 gauge 16 gauge 14 gauge 14 gauge 12 gauge
50 feet 16 gauge 16 gauge 16 gauge 14 gauge 14 gauge 12 gauge 12 gauge
100 feet 16 gauge 16 gauge 14 gauge 12 gauge 12 gauge 10 gauge 8 gauge
150 feet 16 gauge 14 gauge 12 gauge 12 gauge 10 gauge 8 gauge
200 feet 14 gauge 14 gauge 12 gauge 10 gauge 8 gauge
300 feet 14 gauge 10 gauge 8 gauge
400 feet 12 gauge 8 gauge
500 feet 12 gauge 8 gauge

This table inspired by Planet Christmas

Think of the wire inside the extension cord as a hose. The bigger the hose, the more water will flow. If you need a 50ft extension cord to provide a total of 6.0 amps, you need to use a 16 gauge or larger extension cord (a larger gauge means a smaller number, in this case, 14, 12, 10 or 8 gauge). See our article entitled “What Gauge Is My Extension Cord?” if you need help determining what gauge wire is used in your extension cord.

A few words of caution

Here are some things to look out for when purchasing an extension cord:

  • Don’t go smaller than 16 gauge – Long extension cords smaller than 16 gauge with a load on them can heat up quickly (and possibly catch fire)
  • Beware the “big cord” – Just because an extension cord is bigger in diameter than another doesn’t mean that it can handle more amperage. Some extension cords are made for heavy abuse and have more plastic on them to protect the wires inside. Always read the label before buying
  • Buy quality – Some inexpensive, Chinese extension cords are made with cheap components and are fire hazards. Buy from a store that specializes in hardware or electrical components. Sometimes that deal at the “dollar” store isn’t such a good deal


Extension cords are expensive, matching the right extension cord for your needs could save you quite a bit of money!