Don't plug a space heater into an extension cord.

Just don’t. Voltage drop is the least of your worries.

A 1500W space heater pulls about 12.5 amps. Even a heavy-duty 12-gauge extension cord is a fire risk if the cord gets coiled, pinched, or runs under a rug. The voltage drop at 50–100 feet makes the heater work harder, but the real danger is the cord overheating because a space heater is a continuous, high-draw load. Most extension cords aren’t rated for that.

If you absolutely must use one—for a few minutes, not hours—use the shortest, thickest cord you can find: 10 or 12 gauge, no longer than 25 feet. And inspect it every time. But honestly, move the heater closer to the wall outlet instead.

A space heater belongs in the wall. Not on a cord.