Conductor resistance — temperature correction

Resistance is corrected from its 20 °C reference with R(θ) = R20 × (1 + α × (θ − 20)). Copper rises about 0.4% per °C (α = 0.00393 /°C, IEC 60028), so a 2.5 mm² copper run that measures about 0.69 Ω per 100 m at 20 °C climbs to about 0.84 Ω at 75 °C — a 22% increase.

Aluminium has a higher temperature coefficient (0.00403 /°C, IEC 60228) and higher resistivity (ρ = 2.82 × 10⁻⁸ Ω·m vs copper 1.724 × 10⁻⁸ Ω·m), so a conductor of the same cross-section has about 64% more resistance. That is why aluminium feeders are oversized in area compared with copper for the same current.

When no 20 °C value is entered, the calculator derives it from geometry with R = ρ × L / A — using L in metres and A in mm². The model is DC and ignores skin effect, which becomes significant at higher frequencies and in large conductors.