Go with 12-gauge for a 15-amp saw at 100 feet.

12-gauge. A 14-gauge cord at 100 feet will drop enough voltage to make that saw run hot and slow, which cooks the motor over time. A 10-gauge is overkill unless you’re running a tablesaw or something heavier.

The math: 15 amps × 100 feet needs at least 12 AWG to keep voltage drop under 3% (the usual recommendation for power tools). 14-gauge at that distance drops about 5%, which the motor can’t compensate for. It’ll still spin, but not well. The extra resistance turns into heat inside the motor windings. That’s how you burn up a circular saw that’s supposed to last years.

A 50-foot 12-gauge cord is fine too. But once you hit 100 feet, don’t cheat with 14-gauge just because it’s cheaper. The saw will thank you by not dying mid-cut.