How to Size Your Extension Cords to Lengths of the Run

Two people trying to plug two extension cords together

PM Images/Getty Images

Electrical extension cords are a great way to extend circuits to areas that you are working, away from available outlets. Sometimes we only need a short extension cord to give us the ability to plug our tools in and complete the project. Then again, we've all had many times where we've had to tie many extension cords together to reach the area that we needed to reach. But how many extension cords can one tie together and still have the amount of power to power up those tools? Is it actually safe to keep adding extension cords to an existing circuit?

Well, there are some other factors to consider as well. How many feet from the electrical circuit breaker panel is the outlet that you are connecting to? It could be 50 feet away on the other side of the house. What is the size circuit breaker the circuit is connected to? What size is wiring actually feeding the outlet that you are about to plug into? As you can see, there are many factors to consider beyond the reality of the extension cord.

Voltage Drop

Many times, the user doesn't heed the extension cord size compared to the length of the run. They just grab any and all sizes of extension cords and put them together to get to the work at hand. Long runs of wire encounter a variable that you may not have considered, resistance. Even though the copper wire is a very good conductor, it does have some resistance that causes heat. Heat does damage, not only to the extension cord itself, but also to the power tools connected to them. The voltage drop that occurs can heat up the motors of the tools that are attached to them.

If you've ever burnt up a drill while using it in the series of examples we have provided, you now know what not to do. When you power up a tool like a drill and it runs slower than normal, it should now give you a warning sign of voltage drop and cause you to stop before ruining the tool. Most times, having a good, heavy extension cord or two to get to the area you need power can solve that problem. Never run long runs of undersized extension cords and power heavy-load equipment like sump pumps, compressors, etc.

Why It Matters

The longer the run of wire, the more resistance, and thus, more heat. So how does this affect you? Well, power tools draw a certain amount of amperage to run both correctly and efficiently. This electrical load may be too great for the size of the extension cord it is attached to. Because of wire size, the resistance of the wire, and the voltage drop due to the distance, these variables can damage both extension cords and the power tools that are connected to them. Be sure to choose extension cords wisely by choosing a heavy gauge wire by following the chart that we have provided. You can plainly see the maximum amperage and wire gauge that can supply each over a number of feet of cord. As you add additional feet of cord, the amperage availability gets less.

Voltage drop in the cord often doesn't allow the power tools to run at full speed. This causes them to heat up and often inflicts damage on them. Wiring inside the tools can melt as well as inflicting damage to the contacts. As the power tools heat up, so does the extension cords that they are connected to. That, in turn, leads to circuit breakers tripping, often due to a heating up of the breaker and/or overloading of the circuit breaker.

To be safe, try not to exceed the extension cord chart below. Be safe, not sorry.

Extension Cord Usage Chart

Extension Cord Length (Feet) Maximum Amperage Wire Gauge
25 10 18
25 13 16
25 15 14
50 5 18
50 10 16
50 15 14
75 5 18
75 10 16
75 15 14
100 5 16
100 15 12