Wire Resistance Calculator
Resistance is what turns a wire into a small heater and steals voltage along the run. It depends on just three things: the material (copper or aluminium), the length, and the cross-section area. This calculator gives the resistance of a single conductor and of the go-and-return loop, plus the voltage drop at your current.
Formulas: R = ρ × L ÷ A (ρ at 20 °C) · Loop resistance = 2 × R · Voltage drop = I × loop resistance
Worked example: 50 m of 4 mm² copper: R = 0.0172 × 50 ÷ 4 = 0.215 Ω one-way, 0.43 Ω for the loop, and at 20 A the drop is 20 × 0.43 = 8.6 V. On a 230 V circuit that is 3.7% — inside the usual 5% limit, but already more than half of it, before any other losses. Aluminium at the same size would give 0.3525 Ω one-way — about 63% more resistance, which is why aluminium runs need a larger cross-section for the same job.
For the full sizing decision (ampacity plus drop limit), use the wire size calculator; for drop as a percentage of system voltage, use the voltage drop calculator.
Frequently asked questions
Why 0.0172 for copper?
That is copper's resistivity at 20 °C in Ω·mm²/m. At operating temperature (70–90 °C) resistance rises about 20–30%, so treat 20 °C figures as best-case and keep margin.
Do I use one-way or loop resistance for voltage drop?
Loop — current travels out and back, so both conductors drop voltage. Single-phase and DC circuits always use the loop figure; three-phase uses a different (√3) form, covered in the voltage drop calculator.
Why is aluminium worse?
Aluminium's resistivity (0.0282) is about 64% higher than copper's, so the same cross-section carries less current and drops more voltage. It wins on weight and price, which is why overhead lines use it.