What is the safe amp limit for a 100-foot 12-gauge extension cord?

15 amps is the safe limit. While 12-gauge copper wire can technically handle 20 amps under ideal conditions, at 100 feet you get noticeable voltage drop—about 3% at 15 amps and nearly 5% at 20 amps. For power tools or motors, that drop can cause overheating or poor performance. Stick to 15 amps or less; if you need 20 amps, use a 10-gauge cord at that length.