Extension cord gauge chart
Find the current the tool draws on its nameplate, then read across to the cord length. If the tool sits between two rows, use the higher current.
| Tool amps | 25 ft | 50 ft | 100 ft | 150 ft |
|---|---|---|---|---|
| 0 to 5 | 16 | 16 | 16 | 14 |
| 5 to 10 | 16 | 16 | 14 | 12 |
| 10 to 13 | 16 | 14 | 12 | 10 |
| 13 to 15 | 14 | 12 | 12 | 10 |
| 15 to 20 | 12 | 12 | 10 | 10 |
| 20 to 30 | 10 | 10 | 8 | 8 |
Why a thin cord damages tools
A cord that is too thin for the run drops voltage along its length. A universal motor responds to low voltage by drawing more current to make the same power, and that extra current is what overheats the windings. The tool feels weak, runs hot, and its life is shortened.
The cord itself gets warm as well. A 16 gauge cord feeding a 15 amp saw at 100 feet is dropping around 8 percent, and that heat is going into the jacket. If a cord is warm to the touch it is undersized.
Reading the jacket markings
The jacket carries a code such as SJTW 12/3. The number before the slash is the gauge and the number after is the conductor count, so 12/3 is 12 gauge with three conductors including ground.
The letters describe the jacket. S is a flexible service cord; J means junior, rated to 300 volts, which covers all household and workshop use; T is thermoplastic; W is weather and water resistant for outdoor use. SJTW is the common outdoor workshop cord. A cord without W is indoor only.
| Code | Meaning | Use |
|---|---|---|
| SJT | Junior service, thermoplastic | Indoor |
| SJTW | Junior service, thermoplastic, weather resistant | Indoor and outdoor, the usual choice |
| SJEOW | Junior, elastomer, oil resistant, weather resistant | Cold weather, stays flexible |
| SOOW | Heavy service, oil resistant jacket and insulation | Industrial, 600 V |
