Use 12-gauge for a 1500W space heater.

No, 14-gauge isn’t sufficient for a continuous load like a space heater. At 25 feet, voltage drop might be okay (about 2.5%), but the real issue is ampacity derating. A 1500W heater draws 12.5 amps. A 14-gauge cord is rated for 15 amps, but for continuous loads (running more than three hours), codes derate to 80% — that’s only 12 amps. So you’re already over.

A 12-gauge cord handles 20 amps (16 amps derated), so you’ve got plenty of headroom. Yes, the 14-gauge cord might not melt immediately, but it’s a fire risk you don’t need. Space heaters already pull max current; don’t cheap out on the cord.

This is not where you save ten bucks.