Extension Cord Size Calculator
Minimum AWG cord gauge from tool amps and cord length.
What it does
Extension cords starve motors when undersized: thin wire over long runs drops voltage, heats up, and stalls saws. Gauge must rise with amps and with feet together, which is why a 25 ft lamp cord fails a 100 ft saw.
The tool above takes tool amps with cord length in feet or meters. It returns minimum AWG with duty class and voltage drop beside it. The default 15 A over 100 ft needs 10 AWG extra heavy duty at about 3.73 V (3.1%) drop.
Tables match university safety charts drawn from NEC lineage: 16 gauge to 13 A, 14 to 15 A, 12 to 20 A at short lengths, stepping up per length band. Voltage drop math through copper resistivity confirms the edges, with 5% as the design budget and 3% preferred for motors.
Temporary use rules close the loop. One portable appliance plugs straight into the wall, never daisy chained, on UL listed cord rated for indoor or outdoor duty, and permanent loads get wired circuits instead.
How to use
- Read tool amps from its plate.
- Pick the full cord run length.
- Read minimum AWG first, never thinner.
- Check drop stays inside 5%.
- Use outdoor W rated cord outside.
- Retire nicked, hot, or stiff cords.
Formula
- AWG
- Wire gauge; lower number is thicker.
- 12.9
- Copper K factor in ohm circular mil per foot.
- 5%
- Round trip drop budget, 6 V at 120 V.
Meters convert first (divide by 0.3048) into the foot band table. Above 20 A the answer is a permanent circuit, not a cord.
Worked example
Example: 15 A, 100 ft
Imperial defaults: 15 A, 100 ft.
- Band: 100 ft row, 12 to 15 A column.
- Gauge: 10 AWG minimum.
- Drop: 2 x 12.9 x 15 x 100 / 10,380 = 3.73 V.
- Percent: 3.73 / 120 = 3.1%, inside budget.
- 14 AWG would drop 9.42 V (7.8%), rejected.
- Tool shows 10 AWG, extra heavy duty, 3.73 V (3.1%).
Reference table
| Length | 7 to 10 A | 10 to 12 A | 12 to 15 A |
|---|---|---|---|
| 25 ft | 16 ga | 14 ga | 14 ga |
| 50 ft | 14 ga | 14 ga | 12 ga |
| 100 ft | 12 ga | 12 ga | 10 ga |
| 150 ft, 12 to 15 A | 12 ga at 10 A | 10 ga at 12 A | 8 ga |
Limits
- 120 V portable tools; 240 V differs.
- Copper only; aluminum needs other tables.
- Coiled cords trap heat; uncoil fully.
- Table bands, not continuous interpolation.
Common mistakes
- Reading higher AWG as thicker.
- Running 100 ft on a 25 ft rated cord.
- Daisy chaining two cords.
- Permanent shop loads on cords.
- Indoor cord in rain.
Try it yourself
Quick checks. Answers sit tucked under each question.
10 A, 50 ft?
14 AWG minimum, medium duty.
7 A, 25 ft?
16 AWG minimum, light duty.
25 A tools?
Rejected: above 20 A, wire a permanent circuit.
Questions
Default cord?
10 AWG minimum for 15 A at 100 ft.
Bigger number thicker?
No: lower AWG is thicker wire.
Acceptable drop?
Inside 5% at 120 V; motors prefer 3%.
Two cords joined?
Never daisy chain; buy one full length cord.
Outdoor cord?
W rated, UL listed for outdoor duty.
Metric match?
Yes: 30.48 m converts to the same 100 ft band.
Sources
- University of Kansas: Extension Cords and Power Strip Requirements
- UC San Diego: Electrical Safety Guidelines