Calculate the voltage drop along a wire from the current, run length, conductor cross-section and material. Supports single- and three-phase circuits and shows the percentage drop against your supply voltage.
Current flowing through a conductor loses voltage to the wire's resistance. The drop is V = k·ρ·L·I / A, where ρ is the material resistivity, L the one-way length, I the current and A the cross-sectional area. The factor k accounts for the return path: 2 for single-phase, √3 for three-phase.
Copper (ρ ≈ 1.724×10⁻⁸ Ω·m) has lower resistance than aluminium (2.65×10⁻⁸), so it drops less voltage for the same size. Electrical codes typically recommend keeping the drop under about 3% of the supply voltage; if yours is higher, use a larger conductor or shorter run.
If you already know the wire resistance, use the Ohm's law calculator to calculate the voltage drop from current and resistance.
Voltage drop = k·ρ·L·I/A — the factor (2 for single-phase, √3 for three-phase) times resistivity times one-way length times current, divided by the conductor's cross-sectional area.
A common guideline is to keep the drop under 3% for branch circuits (and under 5% total including feeders). Enter your supply voltage above to see the percentage.
Copper. Its resistivity (1.724×10⁻⁸ Ω·m) is lower than aluminium's (2.65×10⁻⁸), so a copper conductor of the same size drops roughly 35% less voltage.
Use a larger conductor (bigger cross-section), shorten the run length, reduce the current, or switch aluminium to copper. Voltage drop is inversely proportional to conductor area.