Calculator
Extension cord gauge from amps and length
Enter the nameplate, not the box. The result is a copper planning pick for a 120 V cord, plus a product link only when we have checked that the listing is that gauge and length and can be bought.
Worked examples, without the form
Voltage drop here is the two-way path: 2 × length (ft) × amps × resistance (Ω per 1,000 ft) ÷ 1,000. Resistance is uncoated copper from NEC Chapter 9, Table 8, at 75 °C. These are planning estimates, not a lab test of a named cord.
- 15 A at 50 ft → 12 AWG, 2.9 V (2.4%) on 12 AWG and 4.6 V (3.8%) on 14 AWG. 50 ft 12 AWG SJTW is the usual shop buy.
- 15 A at 100 ft → 10 AWG, 3.6 V (3.0%) on 10 AWG and 5.8 V (4.8%) on 12 AWG. We do not currently link a 10 AWG cord. The Southwire 100 ft cord is 12 AWG, so it is not that pick. View on Amazon
- 10 A at 25 ft → 16 AWG. A heavier cord will also carry it. We do not link a 16 AWG cord, because that is the light-duty aisle these guides tell you to leave for a saw.
The full grid is the gauge chart. A 15 A circular saw has its own notes on the saw page.
What this calculator will not do
- It does not replace the tool nameplate or the amp rating molded into the cord.
- It does not cover 240 V tools, 20 A plugs, or a new branch circuit. Hire a licensed electrician for new wiring.
- It does not treat jacket color, “heavy duty,” or SJTW as a gauge.
- Two cords in series are a longer run plus another plug. See the daisy-chain page before you add a second cord.
Questions
What gauge extension cord do I need?
Read the tool nameplate amps and measure the run from the outlet to the tool. For a 15 A motor, CordGauge’s planning pick is 12 AWG up to 50 feet and 10 AWG at 100 feet. A lighter cord can be enough for a 10 A load on a short run. The amp rating printed on the cord still wins if it is lower than the gauge chart.
Is 12 gauge or 14 gauge better for a circular saw?
For a 15 A circular saw, 12 AWG is the shop pick at 25 and 50 feet. At 50 feet and 15 A, 14 AWG drops about 4.6 V (3.8%) and 12 AWG drops about 2.9 V (2.4%). At 100 feet, 14 AWG is the wrong aisle and 10 AWG is the chart pick.
Does an SJTW jacket mean a thicker cord?
No. SJTW is an outdoor jacket rating. A 16 AWG SJTW cord is still 16 AWG. Match amps and length first, then confirm the jacket is rated for where the cord will lie.
Questions about a specific cord you already own can go to hello@cordgaugeguide.com. We can explain the chart. We cannot inspect your circuit from an email.
Checked 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.