What gauge extension cord do I need for my amps and length?
Find the amps on the tool's nameplate. If it only lists watts, divide watts by 120.
How this is calculated
Formula: Voltage drop = 2 × length × amps × ohms per 1,000 ft ÷ 1,000. Copper resistance: 16 AWG 4.016, 14 AWG 2.525, 12 AWG 1.588, 10 AWG 0.999 Ω/1,000 ft.
Assumptions: 120 V, copper conductors, cord ampacity limits of about 13 A (16 AWG), 15 A (14 AWG), 20 A (12 AWG), 25 A (10 AWG). Always follow the cord's printed rating, which wins over this estimate.
Frequently asked questions
Lower gauge number means thicker?
Yes. 12 AWG is thicker and carries more current than 16 AWG.
What gauge for a 15 amp tool at 100 ft?
12 AWG keeps the drop under 5%; 10 AWG keeps it near 3%.
Why does voltage drop matter?
Low voltage makes motors run hot, lose power and wear out sooner. Undersized cords can overheat.
Can I plug two cords together?
Add the lengths and use the thinnest cord's gauge. A single cord of the right length is safer.