Extension Cord Size Calculator
Find the correct extension cord gauge (AWG) from amperage draw and cord length.
Prevents overheating and voltage drop for any device.
Recommended Cord
Extension cord gauge selection is a safety-critical decision. An undersized cord creates resistance that generates heat — a leading cause of residential electrical fires. The two key factors are the current draw of the connected device and the length of the cord run.
The wattage-to-amperage formula:
Current (A) = Wattage (W) ÷ Voltage (V)
For US standard 120V outlets:
Amperage = Watts ÷ 120
Maximum safe wattage by cord gauge and length:
| AWG | 25 ft | 50 ft | 100 ft | 150 ft |
|---|---|---|---|---|
| 16 | 1,625 W | 1,250 W | 625 W | 500 W |
| 14 | 1,875 W | 1,625 W | 1,250 W | 1,000 W |
| 12 | 2,500 W | 2,500 W | 1,875 W | 1,250 W |
| 10 | 3,750 W | 3,750 W | 2,500 W | 1,875 W |
Worked example: A contractor uses a circular saw (1,800 W) on a 75-foot extension cord.
- Current needed: 1,800 ÷ 120 = 15 A
- At 75 ft, AWG 16 is rated for about 800 W — dangerously undersized
- AWG 14 at 75 ft supports about 1,400 W — still too small
- AWG 12 at 75 ft supports 2,500 W — correct choice
Safety rules:
- Never use a lower AWG number as a substitute for a higher one — AWG 16 is thinner than AWG 12
- Never daisy-chain extension cords (one plugged into another)
- Fully unroll coiled extension cords before use — a coiled cord acts as a heat trap
- Never run cords under rugs or carpets
- Replace any cord with cracked insulation, exposed wire, or a warm plug immediately
Cord ratings to look for:
- SJ / SJT — light-duty indoor cords
- SO / SOW — heavy-duty outdoor cords
- UL Listed — verified safety standard
- SJTW — all-weather rated