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.
ρ, length, and area set baseline R
Every conductor is a resistor. Cross-section in mm² sets the area; one-way length is how far current travels before the return path in DC systems doubles it for drop math. Copper and aluminum use different resistivity constants—pick the material that matches the installed cable, not the cheapest quote on the spreadsheet.
Hot runs are not 20°C datasheet runs
Solar DC homeruns in conduit, battery interconnects near inverters, and long RV runs through uninsulated bays routinely exceed 25°C ambient at the conductor. The temperature factor in this calculator adjusts R₂₀ to operating °C so I²R loss and voltage-drop estimates are not optimistic. A few ohms of underestimate on a high-current string becomes meaningful watts and sag.
Resistance feeds the rest of the wire stack
Once R is known, multiply by current for drop (V = I × R) or by I² for loss. Pair results with DC Cable Voltage Drop or Cable Size tools when you are iterating gauge versus length. Resistance per meter from the tool snapshot helps compare two routing options without re-entering full length.