Voltage Drop Calculator
Enter the load, distance, and wire size. Get the drop in volts and percent — and the smallest wire that keeps you inside the NEC's recommended 3%.
Uses VD = 2 × K × I × D ÷ CM with DC resistance values — accurate for typical branch-circuit sizes. For large conductors, use NEC Chapter 9 Table 9 impedance. Results are estimates; verify against the current NEC and local amendments.
How it works
The calculator uses the standard K-factor formula: VD = 2 × K × I × D ÷ CM for single-phase (1.732 instead of 2 for three-phase), with K ≈ 12.9 for copper and 21.2 for aluminum, and conductor areas from NEC Chapter 9, Table 8. Distance is one-way — the formula already accounts for the return path.
Want the full explanation, worked examples, and field rules of thumb? Read the companion guide: Voltage Drop: Why It Matters and How to Calculate It.