Extension Cord Size Calculator

Find the minimum extension-cord gauge from the amperage and cord length, checked against cord ampacity and a 5% voltage-drop target — a planning estimate to confirm with a licensed electrician.

Your cord run
A
ft
Results update as you type — no submit
ResultsLIVE
Minimum cord gauge
12 AWG
20 A cord rating · 4% drop over 100 ft (30.5 m)
Cord ampacity20 A
Voltage drop at 120 V4%
5% targetWithin 5% ✓
Formula
gauge = thinnest cord with ampacity ≥ 12 A and ≤5% drop
result = 12 AWG (4% at 120 V)
How it works

A long extension cord is a small wire run, and the same two limits apply: the cord must carry the current safely, and it must not lose too much voltage along the way. This calculator checks both and returns the thinnest cord gauge that satisfies them. It is a planning aid for choosing a cord — for permanent wiring, and for any doubt about a tool or appliance, consult a licensed electrician and the equipment's own ratings.

The first limit is ampacity. The calculator uses NEC Table 400.5(A)(1), Column A, the ampacity of flexible cords with three current-carrying conductors: `18 AWG` carries `7 A`, `16 AWG` `10 A`, `14 AWG` `15 A`, `12 AWG` `20 A`, and `10 AWG` `25 A`. A cord may never run above its rating, so a `15 A` tool rules out `16` and `18` gauge immediately.

The second limit is voltage drop, which grows with length. Using the circular-mils method at `120 V`, the calculator keeps the round-trip drop at or below `5%`, the total-drop figure in the Informational Note to NEC 210.19(A). This is why the same tool needs a heavier cord on a `100 ft` run than on a `25 ft` one. If even a `10 AWG` cord cannot stay under `5%`, the tool tells you to shorten the run or move the source closer rather than recommending an unsafe cord. For motors and heat guns, size up one gauge for margin.

Worked example
Load `12 A` · cord length `100 ft`
`14 AWG`: carries `15 A`, but drop = `2 × 12.9 × 12 × 100 ÷ 4110` = `7.5 V` = `6.3%` (over `5%`)
`12 AWG`: carries `20 A`, drop = `2 × 12.9 × 12 × 100 ÷ 6530` = `4.7 V` = `3.95%` (within target)
result = `12 AWG` cord
Questions about extension cords
It depends on both the amperage and the length. For a 12 A tool at 100 ft, use 12 AWG; at 25 ft, 14 AWG is fine. The calculator picks the thinnest gauge that stays within cord ampacity and a 5% voltage-drop target. Heavier is always safer, so when in doubt, size up.
SOURCES — Cord ampacity NEC Table 400.5(A)(1) Col A · voltage drop K/CM method at 120 V, ≤5% per NEC 210.19(A) Informational Note
NEC-referenced estimate for planning only — electrical work must meet local code and be done or inspected by a licensed electrician; local code governs.
Related tools
Wire size / AWG helperELECTRICALVoltage dropELECTRICALAppliance kWh costELECTRICALUnit price comparisonQUICK CONVERTERSFeet-inches fraction calcCONVERTERS