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.