After searching the web recently, we realized that people need a definitive extension cord size chart. In some cases, understanding wire gauge can be difficult. This chart details how the gauge and length of an extension cord affects its ability to deliver power to a corded tool. Running a 15 amp tool? How about a full 20 amp tool? We can help you understand what length and gauge of extension cord will meet your needs and keep you up and running.
That's right, we gave you the diagram directly without making you read through the text paragraph. If you want to know more, like what is AWG or American Wire Gauge, see below. However, if you just want to know what gauge extension cord you need to support a certain amount of amps, or how long an extension cord can run without power, look here. Having a solid grasp of wire gauges and amps and how they relate to each other can protect your tools and keep you safe.
We've broken down the chart by extension cord length, since that's where most people start when looking for the right gauge cord:
100ft extension cord
If you want to use a 100 foot extension cord, you have to take into account that you are actually making a giant resistor. With that in mind, you typically won't see 100 feet of wire with more than 15 amp (15A) capacity.
| current consumption | Minimum Wire Gauge | voltage drop |
|---|---|---|
| 20A | No. 10 | 4.00% |
| 15A | 12th | 5.00%* |
| 12A | 12th | 4.00% |
| 10A | 12th | 3.33% |
*Gives a 5% pressure drop – the maximum allowed in our calculations. Consider using a size up for anything at or near 5%, or – even better – go for shorter wire if possible.
50ft extension cord
| current consumption | Minimum Wire Gauge | voltage drop |
|---|---|---|
| 20A | 12th | 3.33% |
| 15A | No. 14 | 3.88% |
| 12A | No. 14 | 3.10% |
| 10A | No. 14 | 2.58% |
25ft extension cord
| current consumption | Minimum Wire Gauge | voltage drop |
|---|---|---|
| 20A | No. 14 | 2.58% |
| 15A | No. 14 | 1.94% |
| 12A | No. 14 | 1.55% |
| 10A | No. 14 | 1.29% |
See below for some suggestions for extension cords that should meet the above requirements.
Assumptions for Determining Extension Cord Length
For our extension cord sizing chart calculations we assume 120V single phase with a power factor of 1. We also used the numbers in Table 9 of Chapter 9 of the 2014 NEC for impedance and voltage drop calculations.
We also allow up to 5% voltage drop – some may think this is too high. In this case, only one of our suggestions made it to that level, a 100-foot 12-gauge extension cord with a full 15A. This might be an unusual application for some, but we think it represents a good scenario. It can help you see what happens when you use a 100 foot extension cord on a high current draw tool.
Why Knowing Wire Gauge and Current Is Important
Everyone on the job site or remodeling site has experience with extension cords. For the larger tools that need them, it's not enough to plug into a temporary power pole or a nearby outlet. You have to make sure that if your tool needs 15 amps, it gets 15 amps. Using an extension cord that doesn't carry the correct thickness (gauge) of wire, you're "starving" your tools.
But, you've also done worse things. First, you can tax the tool motor – make it work harder to get the energy it needs to run. Think of this as trying to breathe through a straw. If the straw isn't big enough to deliver air, you'll have trouble breathing. That's the tool you use on an improperly sized extension cord.
Second, you can create dangerous situations. An extension cord that is undersized can get hot over time. If used in this state for an extended period of time, the wire insulation may melt. This is especially true if you put the wire in a coil, which creates a resistive and heated magnetic field.
some useful notes
Do:
- Find out how much current your tool or equipment draws
- Properly adjust the wire gauge for the desired length and current draw
- Keep cables as short as possible for higher current tools
- Fully unroll the extension cord to avoid curling during use
No:
- Use a cable that is not suitable for your tool and cord length
- Keep extension cord coiled during use
- Using a power cord without a grounded plug (lost or damaged)
- Run multiple high current tools with a single wire or circuit
- Use a long extension cord if you have a short one on hand
We hope you found our extension cord size chart helpful and on point. Knowing wire gauge and amperage and how to properly size wire for the tool and distance can keep your tools running longer and more optimally. You may also want to check out our article on which extension cord do I need for more information.