Residential AC Voltage Drop Calculator

Why voltage drop matters at home

Long or undersized copper runs add resistance. Loads see less than nominal voltage—motors overheat, chargers throttle, and lights dim. Most guides target ≤3% drop on branch circuits and ≤5% total feeder plus branch.

AC drop formula (copper)

Drop (V) = current (A) × resistance × 2 (out-and-back) × one-way length (m). Drop % = 100 × drop ÷ supply voltage. Uses typical copper Ω/m at 20 °C for AWG or mm².

AWG vs. mm²

Pick the conductor you are installing. Metric installs use mm²; North American branch wiring often lists AWG. When in doubt, size up one step for continuous loads (EVSE, HVAC, heat pump).

Frequently asked questions

Q: 120 V or 240 V? A: Use the voltage the load actually sees (usually 120 V line-to-neutral in US branch circuits). Q: Power factor? A: This is a resistive planning estimate—inductive motor starts may need larger wire. Q: Aluminum wire? A: Copper only here; AL needs larger area.