The formula
R = ρ × L ÷ A
ρ(T) = ρ₂₀ × (1 + α × (T − 20))
| Material | ρ at 20 °C (Ω·mm²/m) | α (per °C) |
|---|---|---|
| Copper | 0.01724 | 0.00393 |
| Aluminum | 0.0282 | 0.00403 |
Worked example
10 m of 2.5 mm² copper at 20 °C: 0.01724 × 10 ÷ 2.5 = 0.069 Ω (69 mΩ). At 60 °C, resistance rises about 16% to 80 mΩ.
Aluminum has about 64% more resistance than copper for the same size, which is why aluminum cable must be larger. It's rarely used in vehicle DC systems because it's harder to terminate reliably.
Frequently asked questions
What is the resistance of 6mm² copper cable?
About 2.87 mΩ per metre (2.87 Ω per km) at 20 °C.
Does wire resistance change with temperature?
Yes. Copper's resistance rises about 0.39% for every °C, so a cable at 70 °C has about 20% more resistance than at 20 °C.