Voltage Drop Calculator
Calculate voltage drop and percentage for copper or aluminum conductors — DC, single-phase and three-phase — using NEC conductor resistance values.
Voltage drop is the voltage lost as current travels along a conductor. For a single-phase or DC circuit it equals 2 × L × I × R ÷ 1000, where L is the one-way length in feet, I is the current in amps and R is the conductor resistance in ohms per 1,000 ft. The NEC recommends keeping voltage drop at or below 3% on a branch circuit (and 5% total for feeder plus branch).
Inputs
AWG · kcmilResult
2·L·I·R ÷ 1000Resistance values from NEC Chapter 9, Table 8 (DC resistance, stranded, 75 °C). Results are estimates for planning — verify against the applicable code before installation.
How to use this calculator
Pick your current type, conductor material and wire size, then enter the load current, the one-way run length and the system voltage. The result updates as you type. The drop percentage is compared against the NEC 3% branch-circuit recommendation, and the gauge turns red when the circuit exceeds it.
The voltage drop formula
Voltage drop depends on how much current flows, how far it travels and how much the conductor resists that flow. The standard resistance method is:
L is the one-way length in feet, I is the current in amps, and R is the conductor resistance in ohms per 1,000 ft. The factor of 2 accounts for current travelling out and back on a single-phase or DC circuit; three-phase uses √3 because of the phase relationship between conductors.
Worked example
A 20 A load on a 120 V single-phase circuit, run 120 ft one-way in 12 AWG copper (R = 1.98 Ω/1000 ft):
At 7.9% this run is well over the 3% target, so you would move to 10 AWG or larger, or shorten the run. Try it in the calculator above by switching the wire size to 10 AWG.
Common questions
What is the maximum voltage drop allowed by the NEC?
The NEC recommends a maximum of 3% on a branch circuit and 5% total for feeder and branch combined.
These figures are recommendations in NEC informational notes (210.19 and 215.2), not hard requirements, but most designers treat the 3% branch target as a working limit.
How do you calculate voltage drop by hand?
Multiply 2 (or √3 for three-phase) by the one-way length, the current and the conductor resistance per 1,000 ft, then divide by 1,000.
Conductor resistance comes from NEC Chapter 9, Table 8. Divide the result by the system voltage to get the percentage.
Does aluminum wire have more voltage drop than copper?
Yes. Aluminum has roughly 60% higher resistance than copper of the same size, so it drops more voltage over the same run.
That is why aluminum feeders are usually sized one or two gauges larger than the copper equivalent for the same drop.
Why multiply by 2 for single-phase circuits?
Current flows out on one conductor and back on the other, so it travels twice the one-way length.
Assumptions and limitations
This tool uses DC resistance values from NEC Chapter 9, Table 8 at 75 °C. For most branch circuits that is accurate enough. On long AC runs with large conductors, inductive reactance also matters — use the NEC Table 9 effective-impedance method for those. Always confirm against the code that applies to your installation.