Cable Resistance Calculator
Calculate the DC resistance of a copper or aluminium conductor at any temperature, and the voltage drop and heat loss at a given current.
- ρ = 0.017241 × (1 + 0.00393 × (20 − 20)) = 0.017241 Ω·mm²/m
- R = ρ × L ÷ A = 0.017241 × 50 ÷ 2.5 = 0.3448 Ω
Conductor Resistance Formula
ρ20 = 0.017241 Ω·mm²/m for annealed copper and 0.028264 for aluminium; α = 0.00393 and 0.00403 per °C.
Worked example
50 m of 2.5 mm² copper at 20 °C: R = 0.017241 × 50 ÷ 2.5 = 0.345 Ω per conductor, 0.69 Ω for the loop. At 16 A the loop drops 11 V and dissipates 177 W.
Frequently Asked Questions
What is the resistance of 1 mm² copper wire?
About 0.0172 Ω per metre (17.2 Ω/km) at 20 °C.
How much does temperature change cable resistance?
Copper resistance rises by about 0.39 % per °C, so at 70 °C it is about 20 % higher than at 20 °C.
Why is aluminium cable larger for the same current?
Aluminium has about 1.64 times the resistivity of copper, so it needs a larger cross section for the same resistance.
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