Your device and cord
Your result
| Device load | |
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| Cord length | |
| Minimum gauge | |
| Max safe load at this gauge & length | |
| Use |
Safety checklist
Why cord gauge matters
Thin wire + heavy load = heat
Every wire resists electric current a little, and that resistance turns electricity into heat. A thin (high-AWG-number) cord carrying a heavy load gets hot — hot enough to soften insulation and start a fire. The same thin cord also causes voltage drop: less voltage reaches your device, so motors strain, run hot, and can burn out. Matching the gauge to the load and the length keeps both the cord and the device safe.
Longer runs need thicker wire
Resistance grows with length. Double the cord length and you roughly double the voltage drop — so a gauge that is fine at 25 feet can be dangerous at 100 feet for the same device. That is why the chart below gets thicker (lower AWG number) as you move right, and why the shortest cord that reaches is always the safest choice.
How to find your device's amps
Check the nameplate, sticker, or manual for "A" or "Amps" — that number already accounts for how the device really behaves. If you only have watts, divide by 120 (standard US voltage): a 1,500 W heater draws about 12.5 A. One caution: for motors and compressors, watts ÷ volts can understate the true current, so prefer the nameplate amps when it is printed.
The full reference chart
Minimum recommended gauge by device amps and cord length. "—" means no extension cord is recommended for that combination. When your values fall between cells, this tool rounds up to the thicker gauge.
| Device amps | 25 ft | 50 ft | 75 ft | 100 ft | 150 ft | 200 ft |
|---|
Merged from three published charts (Vantecable, AEANET, and TradeSafe citing ESFI). Where the sources disagreed, the thicker gauge won — so this chart is never thinner than any of them. The 75 ft column is interpolated from the 50 and 100 ft columns: three of its cells (marked †) were thickened one step to preserve monotonicity, so a longer cord never recommends a thinner gauge than a shorter one at the same load.
Frequently asked questions
What does AWG mean, and why is a lower number thicker?
AWG stands for American Wire Gauge, the US system for measuring wire thickness. The scale runs backwards: a lower AWG number means a thicker wire. A 10 AWG cord has much thicker copper conductors than a 16 AWG cord, so it carries more current with less resistance and less heat. When buying an extension cord, a lower number is the heavier-duty choice.
I only know my device's watts. How do I find amps?
Divide watts by 120 (standard US voltage): amps = watts / 120. A 1,500-watt space heater draws about 12.5 amps. This calculator does the conversion for you if you enter watts. One caution: for motors and compressors, watts / volts can understate the real current draw, so use the amperage printed on the device's nameplate whenever it is available.
What gauge extension cord do I need for a space heater?
A typical 1,500-watt space heater draws 12.5 amps, which lands in the 12 to under 15 amp band. At 25 feet that means a minimum of 12 AWG, and at 50 feet or more you need 10 AWG or a shorter run. Space heaters are one of the highest-draw household devices, so never run one on a thin 16 AWG cord.
Can I plug two extension cords together to reach farther?
No. Daisy-chaining extension cords is unsafe: every plug-to-socket connection adds resistance and a potential heat point, and the combined run usually exceeds the safe length for the gauge. Buy a single cord of the correct gauge that reaches the full distance in one run.
Can I use an indoor extension cord outside?
No. Outdoor use requires a cord rated for it — look for a W in the jacket code (such as SJTW) and a UL listing for outdoor use. Outdoor-rated jackets resist moisture, sunlight, and temperature extremes that break down indoor cord insulation. For outdoor or damp locations, also plug into a GFCI-protected outlet.
Why should I uncoil the cord fully before using it?
A coiled cord traps its own heat. The warmth from current flowing through the wire has nowhere to escape when the cord is wound tight, so under heavy load the coil can overheat and melt insulation. Always uncoil the cord completely before powering a high-draw device.
What should I do if the calculator says "not recommended"?
It means no standard extension cord is a safe choice for that combination of amps and distance — the voltage drop and heat risk are too high. Do not solve it with a thinner cord or by chaining cords together. Call a licensed electrician about permanent wiring or a closer outlet instead.