How to Choose the Correct Extension Cord Sizes

Extension cords offer a means of extending a home's or workshop's electrical circuits when a lamp, appliance, or tool has an attached cord that is too short to reach the needed location. Most people have several extension cords they keep on hand for this purpose. What is often not considered, though, is that these extension cords need to be of an appropriate size for the power load being placed on them.

Extension Cord Ratings

Extension cords vary in rating, based upon the amperage they can safely carry and the wattage they can handle—both of which are determined by the gauge of the wire. The amperage rating for each extension cord should be labeled on a tag attached to each cord, but this label may not always be present or visible. 

What’s important when choosing an extension cord is to determine the amount of the wattage and amperage that will be drawn by the tool or appliance connected to the extension cord. With simple devices, such as lamps, radios, televisions, and other electronics, the familiar basic household extension cord is perfectly acceptable in most cases. Usually, these basic extension cords use 16-gauge wire.

Things get more complicated when you are using an extension cord for devices with motors, or which produce heat of any kind. The electrical load of these items is substantially higher, which means light-weight extension cords should not be used with them. Further complicating the issue is the length of the extension cord. In longer extension cords, the resistance of electrical current is higher, and the potential for overheating is greater. A 5-foot-long heavy-duty extension cord powering a table saw rarely overheats, but a 50-foot-long extension cord of the same wire gauge may well be undersized for the job. 

Matching Extension Cord to Load

Extension cords should be rated for loads that are at least equal to, and preferably greater than, the requirements needed to run a tool effectively. Extension cord lengths of 50 feet or less can be used based upon the chart shown below, but runs over 50 feet should implement the next heavier-sized cord to accommodate voltage drop due to the natural resistance of the wire.

Safety Reminder

Special caution should be taken with electric space heaters. You should never use an extension cord to plug one of these heaters into a power source. Numerous house fires have occurred when heaters are plugged into extension cords, due to their high wattage and amperage demands. Instead, space heaters should be plugged directly into electrical outlets, but even there you must be careful not to exceed the amperage rating of that circuit. If you find that a circuit breaker trips or a fuse blows when you plug in a space heater, you should try another circuit with heavier wire and a larger circuit breaker amp rating. 

Discard or repair extension cords that are missing ground connections, show signs of extreme wear, or have cracks or cuts, or if you can see bare wire exposed through the outer jacket of the cord. Cords without ground connections can be dangerous to the user. Nicks and bare exposed wires can cause electrical shock.

Also, make sure to examine the plug ends on your extension cords, looking for damage. Your local hardware store has replacement plugs that can be used to extend an extension cord's life, provided the cord itself is in good condition.