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CordGaugeGuide

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.

LoadRunShop pick16 AWG14 AWG12 AWG10 AWG
10 A25 ft16 AWG2.4 V (2.0%)1.5 V (1.3%)1.0 V (0.8%)0.6 V (0.5%)
10 A50 ft14 AWG4.9 V (4.1%)3.1 V (2.6%)1.9 V (1.6%)1.2 V (1.0%)
10 A100 ft12 AWG9.8 V (8.1%)6.1 V (5.1%)3.9 V (3.2%)2.4 V (2.0%)
13 A25 ft14 AWG3.2 V (2.6%)2.0 V (1.7%)1.3 V (1.0%)0.8 V (0.7%)
13 A50 ft12 AWG6.4 V (5.3%)4.0 V (3.3%)2.5 V (2.1%)1.6 V (1.3%)
13 A100 ft10 AWG12.7 V (10.6%)8.0 V (6.7%)5.0 V (4.2%)3.1 V (2.6%)
15 A25 ft12 AWG3.7 V (3.1%)2.3 V (1.9%)1.4 V (1.2%)0.9 V (0.8%)
15 A50 ft12 AWG7.3 V (6.1%)4.6 V (3.8%)2.9 V (2.4%)1.8 V (1.5%)
15 A100 ft10 AWG14.7 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

  1. Read the tool nameplate. A 15 A circular saw is the saw guide, not a 10 A row.
  2. Measure outlet to tool. For 100 feet, start at the long-run guide.
  3. If you are stuck choosing 12 or 14 on a 15 A circuit, read 12 vs 14.
  4. 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.

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