Extension Cord Gauge Calculator — AWG & Percent Drop
Given a tool's amp draw and cord length, see which gauges clear ampacity and a 5% voltage-drop target — a licensed electrician should confirm your pick.
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.
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