Extension Cord Gauge Calculator
Find the wire gauge a generator cord run actually needs, based on load, distance and acceptable voltage drop.
A 10 AWG cord 50 feet long carries 25 amps within a 3 percent drop. Anything thinner will run hot or starve the load.
The run
Every gauge at this load and length
| AWG | Cord ampacity | Drop | Drop % | Verdict |
|---|---|---|---|---|
| 16 | 13 A | 10 V | 8.4% | Overloaded |
| 14 | 18 A | 6.3 V | 5.3% | Overloaded |
| 12 | 25 A | 4 V | 3.3% | Too much drop |
| 10 | 30 A | 2.5 V | 2.1% | OK |
| 8 | 40 A | 1.6 V | 1.3% | OK |
| 6 | 55 A | 1 V | 0.8% | OK |
| 4 | 70 A | 0.6 V | 0.5% | OK |
| 2 | 95 A | 0.4 V | 0.3% | OK |
How this is calculated
Voltage drop is calculated as V = 2 × length × amps × ohms per 1000 ft ÷ 1000, doubled because current travels out and back. Resistance figures are for copper conductors at 75 degrees Celsius. A cord passes only if it clears both the drop limit and the ampacity ordinarily assigned to flexible cord of that size.
Three percent is the usual target for a branch circuit. Motors are the reason it matters: sustained low voltage makes them draw more current, which makes them run hot, which is how a cheap cord ends up destroying an expensive pump.