15 amps is the safe limit for a 100-foot 12-gauge extension cord.

15 amps, but voltage drop will be noticeable.

A 12-gauge copper wire can technically handle 20 amps under ideal conditions (short run, proper insulation). But at 100 feet, you get about 3% voltage drop at 15 amps and nearly 5% at 20 amps. For power tools or anything with a motor, that drop can cause overheating or poor performance. Extension cords also have less forgiving insulation than permanent wiring, so you want a margin.

Stick to 15 amps or less for a 100-foot 12-gauge cord. If you need 20 amps, go with 10-gauge at that length.

Don’t push it. The cord might not melt, but your tools will thank you.