Master chart
Extension cord gauge by amps and length
Use the nameplate amps and the actual run. Then pick AWG so voltage drop stays in a range a motor tool can live with. The printed “heavy duty” is not a spec.
Smaller AWG numbers are thicker copper. A 12 AWG cord has more metal than a 14 AWG cord, so it drops less voltage over the same feet. That is the whole comparison. Jacket type (SJTW, SJT, SOOW) is a weather and oil story — see outdoor / SJTW — not a substitute for gauge.
What these numbers are
Drop figures use copper resistance from NEC Chapter 9, Table 8 (uncoated, 75 °C) and the two-way path on 120 V: 2 × length × amps × Ω/kft ÷ 1,000. They are planning estimates. We did not bench-test a named SKU. Temperature, connector quality, and how much cord is still coiled all move the real number.
Gauge × amps × length
Shop pick is the thinnest AWG in this set that keeps a 120 V motor-tool load near 3% at that length — then rounded thicker when the math is close. Confirm the cord’s marked ampacity and plug rating before you plug in.
| Load | Run | Shop pick | 16 AWG | 14 AWG | 12 AWG | 10 AWG |
|---|---|---|---|---|---|---|
| 10 A | 25 ft | 16 AWG | 2.4 V (2.0%) | 1.5 V (1.3%) | 1.0 V (0.8%) | 0.6 V (0.5%) |
| 10 A | 50 ft | 14 AWG | 4.9 V (4.1%) | 3.1 V (2.6%) | 1.9 V (1.6%) | 1.2 V (1.0%) |
| 10 A | 100 ft | 12 AWG | 9.8 V (8.1%) | 6.1 V (5.1%) | 3.9 V (3.2%) | 2.4 V (2.0%) |
| 13 A | 25 ft | 14 AWG | 3.2 V (2.6%) | 2.0 V (1.7%) | 1.3 V (1.0%) | 0.8 V (0.7%) |
| 13 A | 50 ft | 12 AWG | 6.4 V (5.3%) | 4.0 V (3.3%) | 2.5 V (2.1%) | 1.6 V (1.3%) |
| 13 A | 100 ft | 10 AWG | 12.7 V (10.6%) | 8.0 V (6.7%) | 5.0 V (4.2%) | 3.1 V (2.6%) |
| 15 A | 25 ft | 12 AWG | 3.7 V (3.1%) | 2.3 V (1.9%) | 1.4 V (1.2%) | 0.9 V (0.8%) |
| 15 A | 50 ft | 12 AWG | 7.3 V (6.1%) | 4.6 V (3.8%) | 2.9 V (2.4%) | 1.8 V (1.5%) |
| 15 A | 100 ft | 10 AWG | 14.7 V (12.2%) | 9.2 V (7.7%) | 5.8 V (4.8%) | 3.6 V (3.0%) |
What the listing usually allows
Flexible-cord ampacity is a listing and NEC Table 400.5 problem, not a wire-gauge daydream. Typical big-box marks:
- 16 AWG. Commonly marked 10–13 A. Lights, chargers, and small tools — not a 15 A saw.
- 14 AWG. Commonly marked 13–15 A. Fine on a short 15 A run; long runs need thicker copper.
- 12 AWG. Often 15 A or 20 A depending on the plug. The default shop cord for circular saws and long 15 A runs.
- 10 AWG. Often 20–30 A depending on the ends. The 100-foot answer for 15 A motors.
A 12 AWG cord with a 15 A plug is a 15 A cord. Do not put a 20 A load on it because the copper could theoretically take more.
How to use the chart on a real job
- Read the tool nameplate. A 15 A circular saw is the saw guide, not a 10 A row.
- Measure outlet to tool. For 100 feet, start at the long-run guide.
- If you are stuck choosing 12 or 14 on a 15 A circuit, read 12 vs 14.
- Do not daisy-chain thin cords to “make” length. Buy one cord in the gauge you need.
What we will not invent
No SKU watt-loss lab scores. No “best cord of 2026” ranking. No claim that a yellow jacket is thicker than an orange one. If two cords share AWG, length, and listing, the useful differences are jacket type, flexibility in the cold, and whether the ends are 15 A or 20 A — not a star rating.
50 ft 12 AWG SJTW is the usual shop buy for a 15 A tool at 50 feet. At 100 feet the chart pick is 10 AWG.
Shop outdoor cords
Named SJTW cords checked against the Amazon listing for gauge, length, and a live buy box. Links go to Amazon (Associates tag laqaer-20). We may earn a commission. No scores and no prices.
- View on Amazon
Southwire 12/3 SJTW 50 ft outdoor
12 AWG · 50 ft · SJTW outdoor, lighted end, marked 15 A
- View on Amazon
Southwire 12/3 SJTW 100 ft outdoor
12 AWG · 100 ft · SJTW outdoor, lighted end, marked 15 A
This is a 12 AWG cord. At 100 ft and 15 A the copper estimate is about 5.8 V (4.8%). The chart pick for that run is 10 AWG, which we are not linking until a checked in-stock listing exists.