Extension Cord Gauge Table
| Gauge (rating) | 25 ft | 50 ft | 100 ft |
|---|---|---|---|
| 16 AWG (13A max) | 2.2% | 4.4% | 8.7% |
| 14 AWG (15A max) | 1.6% | 3.2% | 6.3% |
| 12 AWG (20A max) | 1.3% | 2.6% | 5.3% |
| 10 AWG (30A max) | 1.2% | 2.5% | 5.0% |
| Gauge | Rating | Max length for 15A under 3% drop |
|---|---|---|
| 16 AWG | 13A | 30 ft |
| 14 AWG | 15A | 45 ft |
| 12 AWG | 20A | 55 ft |
| 10 AWG | 30A | 60 ft |
The extension cord aisle sells you length and hides the number that matters: gauge. A 100-foot 16-gauge cord and a 100-foot 12-gauge cord plug into the same outlet, but the 16-gauge delivers 10% less voltage at the far end - and that missing 12 volts comes back as heat in the cord, a sluggish motor, and electronics that reboot when the compressor kicks in.
Bottom line: for anything over 10 amps or about 50 feet, 12 AWG is the default answer; a generator owner buys one 10 AWG cord instead of a longer cord twice. Under 3% drop (3.6 volts on 120V) is healthy; over 5% you are heating the jacket and starving the motor.
The honest part: the drop numbers here are computed from standard copper resistance (VD% = 2 x L x I x R / V), not marketing copy, and they assume one tool per cord. Daisy-chaining two cords doubles the length and the drop - the 'I already have a cord' instinct is how 16-gauge 100-foot cords end up on table saws.
How to use
- Find your tool's amps (watts / 120) and your real cord length - the distance to the far outlet, not the cord's advertised stretch.
- Read your gauge row at that length: green is fine, over 3% is a warning for sustained loads, over 5% means size up or shorten.
- Check the rating column too: a 16-gauge cord is a 13-amp cord on paper - running a 15-amp heater through it fails twice, on heat and on code.
Frequently asked questions
Can I use a 16-gauge cord for a 1500-watt space heater?
No. 1500 watts is 12.5 amps; a 16 AWG cord is rated 13 amps and drops over 5% at just 50 feet (6 volts lost as heat in the cord). For a sustained 12.5-amp load the safe chart pick is 12 AWG up to 75 feet - the heater's own manual says 'plug directly into a wall outlet' for exactly this reason.
Does extension cord length really affect power tools?
Yes, measurably: on a 100-foot 16-gauge cord a 13-amp saw loses about 10% of its voltage (12 volts), which shows up as bogging under load, slower recovery, and hotter windings. The same saw on 12 AWG at 100 feet loses only 4% - the tool feels like itself again.
What gauge extension cord do I need for a generator?
One 10 AWG cord handles 30 amps (3,600 watts) at 100 feet with only 5% drop - it is the single-cord answer for running a fridge, a heater and lights from an outdoor generator. Size the unit with the generator size calculator, then buy the cord for the amps, not the distance you wish you had.
Why do cords get hot at the connections?
Heat follows resistance: a loose or corroded plug blade, a worn receptacle, or an underrated gauge all concentrate it. Warm jacket along the length means the cord is too small for the load - unplug it, let it cool, and read the table again with your real amps. Warm plugs with an in-range cord mean the connection, not the wire, needs replacing.