Conductor Resistance

Resistance increases with temperature. When calculating for heavily loaded circuits, you must account for the conductor operating at 75°C or 90°C, not standard room temperature.

Base Resistivity (ρ) at 20°C

• Copper (Annealed): 1.724 × 10⁻⁸ Ω·m
• Aluminum: 2.82 × 10⁻⁸ Ω·m

Temperature Coefficient (α)

Use the coefficient to adjust base resistance to operating temperature.

R_new = R_ref × [1 + α(T_new - T_ref)]

• Copper: α = 0.00393 per °C
• Aluminum: α = 0.00403 per °C

If calculating voltage drop for an industrial motor running near full load capacity, use the 75°C resistance values. Doing otherwise will understate the drop.