Extension Cord Amp Rating Calculator
Find the safe amp rating of an extension cord from its gauge and length, limited by conductor ampacity and a 5% voltage drop, and check it against your load.
How it works
max amps (drop) = 5% × volts × circular mils ÷ (2 × 12.9 × length) · safe amps = min(conductor ampacity, that)
A cord’s safe current is the lesser of two limits: the conductor’s own ampacity, and the current at which voltage drop over the length becomes excessive (about 5% is the practical ceiling for portable cords). Long cords fail the voltage-drop test long before the ampacity test, which is why a 100 ft 16-gauge cord starves a saw even though it "handles" a few amps. Heavier gauge (lower number) means more copper, a higher ampacity and less drop.
Worked example
A 14 AWG cord (4,110 cmil), 50 ft, 120 V: allowed drop = 6 V, so drop-limited current = 6 × 4,110 ÷ (2 × 12.9 × 50) = 24,660 ÷ 1,290 ≈ 19.1 A. The conductor ampacity of 15 A is lower, so the cord is safe to 15 A. A 10 A load drops only about 3.1 V (2.6%) — comfortably fine.
Frequently asked questions
Why does a longer cord carry less?
Voltage drop rises with length, so at the same current a long cord loses more voltage — tools run hot and weak, and the cord itself warms up. Past a certain length you must step up to a heavier gauge to stay under the drop limit.
Can I daisy-chain two cords?
It is discouraged: the connection adds resistance and the combined length worsens voltage drop and heating. Use a single cord of the right gauge and length, and never coil a loaded cord tightly — trapped heat can melt the insulation.
What gauge for a long run to power tools?
For 100 ft feeding a 10–15 A tool, use 12 AWG; for lighter loads 14 AWG is fine at shorter lengths. When in doubt, go one gauge heavier — the cost difference is small and the tool runs better.
Related calculators
Educational estimates only — electrical work must follow the National Electrical Code (NEC) and local amendments, and be done or inspected by a licensed electrician.