Use the extension cord table in your tool’s manual. Find the amps on the tool’s nameplate, then read the gauge for the length of cord you need. For a 15 amp tool such as DeWalt’s DWS713 miter saw, its manual asks for 14 gauge on a 25 foot cord and 12 gauge on a 50 foot cord.
Read your tool’s cord table
The manuals for DeWalt’s DWS713 miter saw and DWE7491RS table saw, Makita’s 5007F circular saw, Bosch’s 11253VSR rotary hammer and Milwaukee’s 5268-21 rotary hammer each have a table of the minimum gauge by the tool’s nameplate amps and the cord’s length. The lower the gauge number, the heavier the cord. If in doubt, use the next heavier gauge.
The amps are printed on the tool’s nameplate, the label with its ratings. If it lists only watts, the watts to amps calculator gives the amps. A cord that’s too light drops the voltage, so the tool loses power, can overheat and can be damaged.
Without the manual, use the extension cord gauge finder. It reads DeWalt’s and Milwaukee’s tables next to two cord makers’ charts.
Where the tool makers’ tables differ
These DeWalt, Makita and Bosch manuals print the same gauges for 120 volt tools. Milwaukee’s manual works its table out a different way, by limiting the voltage drop to five volts at one and a half times the tool’s rated amps. For an 8 amp tool, such as Milwaukee’s 5268-21 rotary hammer, its table asks for a heavier cord at 25, 50 and 100 feet, and gives none at 150 feet. For a 15 amp tool all four give the same gauges.
| Tool’s amps | 25 ft | 50 ft | 100 ft | 150 ft |
|---|---|---|---|---|
| 8 A: DeWalt, Makita, Bosch | 18 | 16 | 14 | 12 |
| 8 A: Milwaukee | 16 | 14 | 10 | None |
| 15 A: all four | 14 | 12 | None | None |
Wire gauge (AWG) for 120 volt tools. None: DeWalt and Makita mark these lengths “Not Recommended”, and Bosch and Milwaukee print a dash. Milwaukee’s table also has a 75 ft column, with 12 gauge at 8 A and 10 gauge at 15 A.
Air compressors
DeWalt’s manual for its DXCMLA1983012 30 gallon compressor says to add more air hose instead of an extension cord, using hose at least 3/8 inch across and rated for 300 psi. If a cord has to be used, that manual calls for one no longer than 50 feet and 12 gauge or heavier, never 14 or 16 gauge. Check your own compressor’s manual for its rule.
Plugs and grounding
- A tool with a three-prong plug needs a three-wire extension cord with three-prong ends. Milwaukee’s manual says double insulated tools, which have two-prong plugs, can use a two- or three-wire cord.
- Never remove the grounding prong or alter the plug, and don’t use an adapter plug on a three-prong tool.
- If the plug doesn’t fit the outlet, have a qualified electrician install the right outlet.
- Unplug by pulling the plug, not the cord.
Outdoors and damp places
Outside, use an extension cord made for outdoor use. A W in the letters on the jacket is the outdoor (weather) rating. Check the cord or its tag for the outdoor marking too. See what the letters on an extension cord mean.
If you can’t avoid working in a damp place, plug into a supply protected by a GFCI, such as a portable GFCI. Keep cords away from sharp edges, heat and wet areas.
Replace a damaged extension cord instead of taping it up. Use one cord long enough for the job instead of joining two.