Voltage Drop Calculator
Estimate the voltage lost in a run of wire for DC, single-phase, or three-phase circuits. Enter current, length, AWG size, and material to see the drop and the voltage at the load.
Circuit details
How to use this calculator
1. Choose the system type
Pick DC, AC single-phase, or AC three-phase. This sets the multiplier in the formula — 2 for DC and single-phase, √3 for three-phase.
2. Enter voltage and current
Source voltage is the nominal supply (e.g. 12 V, 120 V, 240 V). Current is the load the circuit will carry in amps.
3. Set length, material, and AWG
Use the one-way distance from source to load, switch feet or metres as needed, then choose copper or aluminum and the wire gauge.
4. Check the percentage
If the drop exceeds 3% the result flags it. To reduce drop, use a larger conductor, shorten the run, or raise the system voltage. See also Ohm's Law.
About voltage drop
The formula
Voltage drop = K × I × ρ × L ÷ A. Here ρ is resistivity (copper 1.68×10⁻⁸ Ω·m, aluminum 2.82×10⁻⁸ Ω·m), L is the one-way length in metres, A is the conductor area in m², and K accounts for the return path and phase.
Why it matters
Long runs and small wires lose voltage as heat. Too much drop dims lights, slows motors, and wastes energy. Keeping it low protects equipment and efficiency.
Rule of thumb
The NEC recommends 3% maximum on a branch circuit and 5% total including the feeder. This tool flags branch-circuit drops above the 3% guideline. Always follow local code and a qualified electrician for real installations.