Cable resistance drives voltage drop and I²R loss—both rise with temperature. This guide walks through the conductor resistance calculator: material, mm² size, one-way length in meters, and field operating temperature.
Benefits
- R₂₀ = ρL/A with copper ρ ≈ 0.0175 Ω·mm²/m, aluminum ρ ≈ 0.0282 Ω·mm²/m.
- Temperature correction: ~0.4%/°C above 20°C reference.
- Outputs Ω at temperature plus R at 20°C and Ω/m for BOM checks.
How it works
- Select copper or aluminum and cross-section in mm² (AWG equivalents listed).
- Enter one-way conductor length in meters and expected operating °C.
- Read resistance in ohms—use with voltage-drop and loss calculators.
FAQ
How do I calculate conductor resistance?
At 20°C: R = ρ × L ÷ A (Ω). Example: 6 mm² copper, 15 m one-way → R₂₀ ≈ 0.0175 × 15 ÷ 6 ≈ 0.0438 Ω. At 40°C multiply by [1 + 0.00393 × (40 − 20)] ≈ 1.079 → ~0.0472 Ω. Round-trip DC drop uses 2× length.
Why does temperature change wire resistance?
Metal resistivity rises with temperature—copper α ≈ 0.00393/°C. A cable in a hot attic or engine bay can be 10–20% higher resistance than a 20°C datasheet value. Size and loss calculations should use field temperature, not lab conditions.
Copper vs. aluminum for the same mm²?
Aluminum ρ is higher—about 61% more resistance than copper at the same cross-section. NEC often upsizes aluminum one breaker step. Enter the actual material in the calculator; do not assume copper resistivity on AL cable.
Technical specifications
- R₂₀ (Ω) = ρ × L(m) ÷ A(mm²).
- R(T) = R₂₀ × [1 + α(T − 20°C)].
- Cu ρ ≈ 0.0175 Ω·mm²/m; Al ρ ≈ 0.0282 Ω·mm²/m at 20°C.
- Related: dc-cable-voltage-drop, dc-cable-size, ohms-law.