Resistance is not fixed—it climbs with conductor temperature. This guide shows how to calculate conductor resistance vs temperature from 20°C baseline R, material temperature coefficient α, and field operating °C.
Benefits
- Temperature factor: R(T) ÷ R₂₀ = 1 + α(T − 20°C).
- Copper α ≈ 0.00393/°C; aluminum α ≈ 0.00403/°C.
- Side-by-side R at 20°C and R at operating T in the tool output.
How it works
- Compute or enter R₂₀ from ρ, length, and cross-section at 20°C.
- Apply operating temperature in °C—ambient plus heating under load if known.
- Read R(T) and temperature factor; use hot R for conservative drop math.
FAQ
How do I calculate resistance at temperature?
R(T) = R₂₀ × [1 + α(T − 20)]. Example: R₂₀ = 0.050 Ω copper at 60°C → factor = 1 + 0.00393 × 40 ≈ 1.157 → R(60) ≈ 0.0579 Ω (~16% higher than 20°C).
What temperature should I use—ambient or conductor?
Use conductor temperature under load when you have it (IR scan, ampacity tables). Otherwise ambient + 20–30°C margin for sun-loaded conduit is a common conservative estimate. Underestimating T underestimates R and voltage drop.
Does AC vs DC change the temperature formula?
The R(T) relationship is the same—resistivity vs temperature is a material property. AC adds skin effect at high frequency; this calculator uses DC resistance geometry (ρL/A) with temperature correction, typical for PV DC, battery, and low-frequency sizing.
Technical specifications
- R(T) = R₂₀ × [1 + α(T − 20°C)].
- Approx. +0.39%/°C (Cu) and +0.40%/°C (Al) above 20°C.
- ΔR = R₂₀ × α × (T − 20).
- Related: conductor-resistance-calculator, dc-cable-voltage-drop.
20°C is a reference, not your roof in July
Datasheets and NEC tables often anchor resistivity at 20°C. Installed conductors in conduit on a sunny wall may sit at 50–70°C under load. The vs-temperature calculation scales baseline R by a linear factor in (T − 20). Ten degrees hotter is not negligible—it is roughly 4% more resistance for copper, compounding on long homeruns.
Plot two points: cold start and hot steady state
Calculate R at 25°C for morning MPPT current and again at 60°C for afternoon peak. Voltage drop and loss spread between those points bracket real performance. Solar designers who only use 20°C R understate afternoon sag; battery installers who ignore inverter bay heat do the same on interconnects.
Temperature factor exports to other tools
The calculator reports R at temperature, R at 20°C, and the dimensionless temperature factor. Multiply any known R₂₀ by the same factor when you have resistance from a different length but the same cable type and T. Then feed hot R into Ohm's law or DC voltage-drop workflows for field-realistic numbers.