About this tool
Voltage Drop Calculator is a free, in-browser tool that estimates how much voltage a wire run loses under load. It runs entirely in your browser and uploads nothing.
The conductor cross-section comes from the AWG number using A = π/4·(0.127 mm × 92^((36 − n)/39))². The drop is Vdrop = factor · L · I · ρ / (A·10⁻⁶), where L is the one-way length in metres, I the current, ρ the resistivity (copper 1.724×10⁻⁸, aluminum 2.82×10⁻⁸ Ω·m), and the factor accounts for the circuit: 2 for DC or single-phase (there and back) and √3 ≈ 1.732 for three-phase. The percentage drop is Vdrop / Vsource × 100 and the voltage at the load is Vsource − Vdrop. A flag warns when the drop exceeds 3%, the level NEC recommends staying under for branch circuits.
Use it to check whether a cable run is thick enough: if the percentage is over 3%, step up to a larger wire (a lower AWG number), shorten the run, or raise the supply voltage.
Frequently asked questions
What is the voltage drop formula?
Vdrop = factor · L · I · ρ / A, where L is the one-way length in metres, I the current, ρ the conductor resistivity and A the cross-section in m². The factor is 2 for DC and single-phase circuits and √3 for three-phase.
Why is the factor 2 for DC and single-phase?
Current flows out along one conductor and back along another, so the resistive path is twice the one-way length — hence a factor of 2. Three-phase uses √3 because of the phase relationship between conductors, not a simple round trip.
What does the 3% flag mean?
The US National Electrical Code recommends keeping branch-circuit voltage drop under 3% (and about 5% total including the feeder). The tool flags any result above 3% so you know to use a heavier conductor or a shorter run.
Is my data uploaded?
No. The current, length, gauge and voltage are processed entirely in your browser; nothing is sent to a server and the calculator works offline.
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