What Gauge Extension Cord for a Space Heater?
A typical 1500W/120V space heater draws about 12.5A, and per our Appliance Amp Draw → Gauge table needs a minimum of 16 AWG at 25ft, 14 AWG at 50ft, 10 AWG at 100ft, and 8 AWG at 150ft — and because a heater is a continuous, unattended heat load, we recommend sizing up one gauge from that floor.
Why a heater is a worst-case load for a thin cord
A 1500W heater pulls its ~12.5A continuously — not in bursts like a saw. Continuous draw is what heats a cord, and the voltage drop that builds with length compounds it: the same heater that is fine on a 16 AWG cord at 25ft needs a far heavier 8 AWG cord at 150ft, per the derived figures in our Cord Gauge Ampacity Index. The heater's draw never changes — the cord's resistance over the distance does.
The floor, by run length (1500W / 120V / 12.5A)
| Run length | Minimum gauge | Our recommendation |
|---|---|---|
| 25 ft | 16 AWG | 14 AWG (size up — continuous heat load) |
| 50 ft | 14 AWG | 12 AWG |
| 100 ft | 10 AWG | 10 AWG or heavier |
| 150 ft | 8 AWG | 8 AWG |
Minimums from our Appliance Amp Draw → Gauge dataset (v1.0); treat them as a floor, not a target.
If you need a cord for this job, a 12 AWG SJTW heavy-duty extension cord covers the common 25–50ft heater runs with a gauge of margin in hand.
Reels and jackets: the two other checks
Run a heater from a cord reel only with the reel fully unwound — coiled wraps trap heat far worse than a straight run. And remember the jacket rating is a separate spec from gauge: SJTW/SJOOW cords are outdoor/wet-rated, SJT or SPT without the W are indoor-only, regardless of AWG — see indoor vs outdoor cord ratings.
This page is general reference, not an electrical code determination. Local code governs.
Frequently Asked Questions
A typical 1500W/120V space heater draws about 12.5A. Our Appliance Amp Draw to Gauge table calls for a minimum of 16 AWG at 25ft, 14 AWG at 50ft, 10 AWG at 100ft, and 8 AWG at 150ft. Check your heater's own nameplate wattage and amps first — these figures assume the common 1500W rating.
Size up. A space heater is a continuous, high-heat load that often runs unattended, and our sizing tables are a floor, not a target — we deliberately recommend stepping up one gauge for continuous, safety-relevant, or high-heat loads like heaters.
Only with the reel fully unwound. A coiled reel traps heat far worse than a straight cord, and a heater's continuous 12.5A draw is exactly the kind of load that overheats a still-wound reel even when the gauge is otherwise correct.
Yes, and it is a separate spec from gauge. Jacket rating (SJTW/SJOOW = outdoor/wet-rated; SJT or SPT without a W = indoor-only) is independent of AWG. For a heater used indoors an indoor-rated cord is fine, but a cord used in a damp garage or outbuilding needs the W (weather) rating regardless of how thick it is.