Extension Cord Gauge Calculator: Amps, Length and Voltage Drop
A thin extension cord starves your tool of voltage. The motor runs hot, loses power and wears out early. This calculator tells you the cord gauge you need for your tool and your run.
Extension Cord Gauge Calculator
Enter your tool and the cord length. You get the gauge that keeps your tool at full power.
| Gauge | Voltage drop | Verdict |
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Voltage drop uses copper wire resistance at 20°C for the full out and back run, with 3 percent as the target for motors. Current limits are typical ratings for flexible cord. Always check the amp rating printed on the cord jacket, and never use a cord rated below your tool.
How to Read Wire Gauge
Cord gauge runs backwards. The lower the number, the thicker the wire, so a 12 AWG cord carries more current than a 16 AWG cord.
You will see it printed on the jacket as something like 12/3. The first number is the gauge and the second is the number of wires inside.
Why Length Matters
Every foot of wire has resistance. Double the length and you double the voltage you lose on the way. A cord that is fine at 25 feet can be too thin at 100.
Aim for 3 percent drop or less for motors. Lights and chargers can live with more.
Quick Rules You Can Remember
- 16 AWG: light tools and chargers on short runs.
- 14 AWG: most handheld power tools up to 50 feet.
- 12 AWG: saws, compressors and anything over 50 feet.
- 10 AWG: big loads on long runs.
Powering your tools from a generator? Size it first with the generator wattage calculator.
Frequently Asked Questions
What gauge extension cord do I need for a 15 amp tool?
At 25 feet, 14 AWG works. At 50 feet you want 12 AWG, and at 100 feet you want 10 AWG to keep voltage drop near 3 percent.
Can I join two extension cords together?
You can, but the voltage drop adds up across the full length and every plug joint adds resistance. One cord of the right length and gauge is better.
Why does my extension cord get warm?
A warm cord is carrying close to its limit or is too thin for the load. Unplug it, uncoil it fully and switch to a thicker gauge.