Right gaugeGenerator extension cord
Outdoor-rated cord with the gauge, length and plugs your load needs.
Enter the load and length to see the gauge you need.
AmazonCheck price
Generator extension cord size calculator
What gauge cord do you need from the generator to your appliances? Enter the load and the length. Results update as you type.
- Shows the voltage drop
- Free, no sign-up
- Runs in your browser
Generator extension cord size calculator
What the cord carries
How long is the run?
The cord you need
The minimum wire gauge, the voltage drop for each gauge, and the outlet and plug the load needs.
How it works
Current runs from the generator to the appliance and back, and the copper in the cord resists it. The longer and thinner the cord, the more voltage is lost on the way. Too much loss makes motors run hot and appliances misbehave.
- 1Currentamps = watts ÷ volts. For motors, the amps on the nameplate are more accurate.
- 2Voltage drop2 × length × wire resistance × amps, using the resistance of warm copper so the figure isn't flattered.
- 3GaugeThe thinnest wire that keeps the drop within 5% (or 3%) and the load within 80% of the cord's amp rating.
The cord's marked amp rating and both plugs must cover the load as well. Lower gauge numbers are thicker wire. Feeding home circuits needs a transfer switch or interlock fitted by a licensed electrician, never a cord into a wall outlet.
Worked example: 1,500 W at 120 V over 100 ft
A 1,500 W space heater on a 100 ft cord, allowing 5% voltage drop:
Current1,500 W ÷ 120 V = 12.5 A
14 AWG2 × 100 ft × 3.14 Ω/1,000 ft × 12.5 A = 7.85 V = 6.5%, too much
12 AWG2 × 100 ft × 1.98 Ω/1,000 ft × 12.5 A = 4.95 V = 4.1%, fits
Cord to buy12 AWG, 100 ft, rated 20 A, on the generator's 20 A outlet
12.5 A is above the 12 A continuous limit of a 15 A plug, so the cord and outlet need to be 20 A rated. At the stricter 3% limit, the answer is 10 AWG.
Frequently asked questions
What gauge extension cord do I need for a generator?
It depends on the current and the length. For a 1,500 W load at 120 V (12.5 A) over 100 ft, 12 AWG keeps the voltage drop near 4%. Lower gauge numbers are thicker wire. Longer runs and heavier loads need thicker cord.
Why does cord length matter?
Current flows out and back along the cord, and the copper resists it, so the voltage at the far end drops. Too much drop makes motors run hot and can stop appliances working. A thicker wire has less resistance.
Can I use a regular household extension cord with a generator?
For small loads, yes, if it is rated for outdoor use and its amp rating covers the load. Most household cords have 15 A plugs and thin 16 AWG wire, which is fine for a fridge but not for a space heater 100 ft away.
Can I plug the generator into a wall outlet to power the house?
No. Backfeeding through an outlet can electrocute utility workers and neighbors, and can start a fire. To power home circuits, have a licensed electrician fit a transfer switch or an interlock and an inlet box.
More backup power tools
Cords and connections that fit
What to look for. Sizes appear once the calculator has a result.
We may earn a commission from links on this page. How this works
For home circuitsPower inlet box
The safe way to feed home circuits, with a transfer switch or interlock. Fitted by a licensed electrician.
For feeding home circuits through a transfer switch, fitted by an electrician.
AmazonCheck price