Gauge matters more than length for ampacity.

Short version: 14-gauge is fine for household stuff under 15 amps, 12-gauge for 20 amps, and 10-gauge for 30 amps. Length matters, but only because voltage drop becomes an issue on long runs.

Ampacity is primarily determined by the wire gauge (thickness). Thicker wire = more current without overheating. The NEC tables tell you the safe current limit for each gauge, but for extension cords, the rule of thumb is:

• 16 gauge – light duty (lamps, phone chargers), max 10 amps, keep under 50 ft.
• 14 gauge – standard household tools (circular saw, miter saw, shop vac), max 15 amps, fine up to 100 ft.
• 12 gauge – heavy tools (table saw, air compressor), max 20 amps, good up to 100 ft.
• 10 gauge – big draw stuff (welder, RV, heater), max 30 amps, can go 150 ft+.

Length does lower the ampacity indirectly because voltage drop increases. If you need to run a 15-amp saw 150 feet away on a 14-gauge cord, the voltage drop will make the motor run hot and slow. That’s why for long runs you might need to step up a gauge to keep voltage loss under 3%. But for typical garage/house use under 50 ft, gauge is the only number you really need to check.

Don’t guess—look at the cord’s tag or the stamped info on the plug. It should list the gauge and amp rating. If it doesn’t, throw it away and buy one that does.

You can get away with a lot, but picking the right gauge keeps things safe and your tools happy.