Voltage Drop Calculator
Estimate the voltage lost along a run of wire so the load still sees an acceptable voltage. Enter the conductor material, size, one-way length, load current, and system voltage: a copper conductor's resistance is R = ρ·L/A (resistivity × length ÷ cross-section), temperature-corrected by ρ(T) = ρ₂₀·(1 + α·(T − 20)). For a single-phase circuit the go-and-return path drops Vd = 2·I·R; for a balanced three-phase circuit Vd = √3·I·R. The remaining voltage at the load is V − Vd and the drop as a percentage of supply is 100·Vd/V — rules of thumb call for under ~3% for branch circuits and ~5% for feeders. You can enter the cross-section in mm² or pick an AWG size (the AWG area is derived from the gauge formula). Everything runs locally in your browser.
Conductor
Circuit & load
Result
The math. A wire is not a perfect conductor: it has a small resistance proportional to its length and inversely proportional to its cross-sectional area, R = ρ·L/A, where ρ is the material's resistivity (copper ≈ 1.68×10⁻⁸ Ω·m at 20 °C, aluminum ≈ 2.82×10⁻⁸ Ω·m). Resistance rises with temperature as ρ(T) = ρ₂₀·(1 + α·(T − 20)) with α ≈ 0.00393/°C for copper (0.00403 for aluminum), so a hot conductor drops more voltage. In a single-phase circuit current travels out and back, so the total loop resistance is 2R and the drop is Vd = 2·I·R; in a balanced three-phase circuit the three line conductors share the load and the geometry gives Vd = √3·I·R. The voltage that actually reaches the load is V − Vd, and engineers usually want the percentage drop 100·Vd/V kept below ~3% for branch circuits and ~5% total (feeder + branch). AWG sizes follow a geometric series: a conductor of gauge n has diameter d = 0.005·92^((36−n)/39) inches, so its area in mm² is (1000·d)²·5.067×10⁻⁴ — each 3-gauge step roughly halves the area. This tool uses the idealized DC resistance; real AC circuits add a small skin-effect and reactance component, and installed ampacity also depends on insulation rating and bundling. Pairs with the AWG Wire Gauge, Ohm's Law, Reactance & Impedance, and Voltage Divider tools. Everything runs locally — nothing leaves your browser.