A 50 ft cord for a 13 A tool needs at least 14 AWG to stay within a 3% voltage drop; a 100 ft cord at 15 A needs 10 AWG, or 12 AWG if you accept 5%.
| Cord length (ft) | 5 A | 7 A | 10 A | 13 A | 15 A | 20 A |
|---|---|---|---|---|---|---|
| 10 | 18 | 18 | 18 | 16 | 14 | 12 |
| 15 | 18 | 18 | 18 | 16 | 14 | 12 |
| 25 | 18 | 18 | 18 | 16 | 14 | 12 |
| 50 | 18 | 16 | 14 | 14 | 12 | 12 |
| 75 | 16 | 14 | 12 | 12 | 12 | 10 |
| 100 | 14 | 14 | 12 | 10 | 10 | — |
| 125 | 14 | 12 | 10 | 10 | — | — |
| 150 | 12 | 12 | 10 | — | — | — |
How it is computed
For each length and current, the table picks the thinnest copper gauge whose cord ampacity covers the load and whose round-trip resistance keeps the drop under the limit at 120 V: drop = amps × resistance of 2 × length ÷ 120. A lower AWG number is a thicker wire. A dash means no common cord gauge meets the limit.
FAQ
Is a thicker cord ever a problem?+
No. A lower AWG number than the table shows is always safe; it only costs more and weighs more.
Can I plug two cords together?+
Avoid it. Each connection adds resistance and heat; use one cord long enough, of the gauge for the total length.