Voltage Drop Calculator
Find the voltage drop, drop percentage and voltage at the load for any wire size, current and distance — copper or aluminum, AC or DC.
Calculations are built on the 2023 National Electrical Code (NEC) and cross-checked against the published code tables. How we verify. Last reviewed June 2026.
How voltage drop works
Every conductor has resistance, so some voltage is lost over the run. The longer the wire and the higher the current, the bigger the loss. The formula is Vdrop = 2 × I × R × L ÷ 1000 (use 1.732 instead of 2 for three-phase), where R is the conductor’s resistance per 1,000 ft. The factor of 2 covers the out-and-back length of the circuit.
NEC informational notes recommend keeping drop at or below 3% on a branch circuit and 5% for feeder plus branch combined. Low-voltage DC circuits (12–48V) are far more sensitive — a small loss is a large percentage — so they often need much larger wire than ampacity alone requires.