Extension cord voltage drop calculator
How many volts a cord loses at a given length and current, and the thinnest gauge that keeps the drop under 3%.
How it works
Voltage drop = 2 × length × current × resistance per foot. The factor 2 covers the out-and-back conductors. The resistance figures are for uncoated copper from NEC Chapter 9, Table 8: 16 AWG 4.016 Ω, 14 AWG 2.525 Ω, 12 AWG 1.588 Ω and 10 AWG 0.9989 Ω per 1,000 ft. A large drop makes motors run hot and slow, and an overloaded cord can overheat.
Quick reference
| 16 AWG | 13 A | 25–50 ft light loads |
| 14 AWG | 15 A | Up to 50 ft for most tools |
| 12 AWG | 20 A | 50–100 ft, saws and compressors |
| 10 AWG | 30 A | 100 ft+ and heavy motors |
Ampacity is a guide only. Always use the cord rating printed on the jacket and never exceed it.
Questions
Where do I find the amps?
On the tool’s nameplate. If it shows watts, divide by the voltage.
Is 5% acceptable?
Many electricians aim for 3% on a branch and 5% overall. Motors start more easily with less drop.
Sources
Injury data for these tools
Other power saws (miter, tile, unspecified)Laceration33,665
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Circular sawsLaceration10,961
Extension cordsFracture5,056
Other power toolsLaceration3,409ER visits in 2025
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