Cable Resistance Calculator
Estimate DC resistance, voltage drop, and heat loss for common conductor materials.
Why cable resistance matters
Every electrical conductor has resistance. As current flows through a cable, some electrical energy is converted into heat. This affects performance, efficiency, and safety. If resistance is too high for your circuit, you can get excessive voltage drop, poor equipment behavior, and unnecessary power loss.
An electrical cable resistance calculator helps you estimate these effects early in the design process. Whether you are working on low-voltage systems, battery wiring, control panels, or longer feeder runs, a quick resistance estimate saves time and helps size conductors more confidently.
Formula used in this calculator
Base resistance at 20°C
R = ρ × L / A
- R = resistance (ohms, Ω)
- ρ = resistivity of the material (Ω·mm²/m)
- L = conductor length (m)
- A = cross-sectional area (mm²)
Temperature correction
Resistance changes with temperature. This calculator adjusts resistance from 20°C using a linear approximation:
RT = R20 × (1 + α × (T - 20))
Where α is the temperature coefficient of resistance and T is conductor temperature in °C.
Material constants used
| Material | Resistivity at 20°C (Ω·mm²/m) | Temperature Coefficient α (/°C) |
|---|---|---|
| Copper | 0.017241 | 0.00393 |
| Aluminum | 0.028264 | 0.00403 |
| Silver | 0.01587 | 0.00380 |
| Gold | 0.02214 | 0.00340 |
How to use this calculator
Step-by-step
- Select the conductor material.
- Enter cable length and choose the unit.
- Enter cross-sectional area and unit.
- Set expected conductor temperature.
- Enable round-trip if you want total path resistance for a complete circuit loop.
- Optionally add current and supply voltage to see voltage drop and loss.
For most two-wire DC circuits (or single-phase line-and-return calculations), enabling round-trip gives a more practical total resistance.
Practical example
Example: 25 m copper run, 2.5 mm², 20 A load
If the run is 25 m one-way and you need loop resistance (out-and-back), use 50 m effective length. The tool calculates resistance, then voltage drop as V = I × R, and heat loss as P = I² × R. This quickly tells you if your chosen wire size is acceptable.
How to reduce cable resistance and voltage drop
- Use a larger conductor cross-sectional area.
- Shorten cable length where possible.
- Choose copper instead of aluminum when practical.
- Lower operating temperature through better routing and ventilation.
- Run at higher voltage (when system design allows) to reduce current for the same power.
Common mistakes to avoid
- Using one-way length when circuit loop resistance is needed.
- Mixing units (for example entering cm² while assuming mm²).
- Ignoring temperature effects in high-current applications.
- Forgetting termination and connector resistance in very low-resistance systems.
FAQ
Is this calculator for AC or DC?
It is primarily a DC resistance estimator and also useful for low-frequency AC approximation. At higher AC frequencies, skin effect and reactance can become significant and require additional analysis.
Does stranded vs solid wire change resistance?
For the same material and effective copper/aluminum cross-sectional area, DC resistance is very similar. Manufacturing details can produce small differences.
Is this a replacement for electrical code requirements?
No. Use this as an engineering estimation tool. Always verify with local electrical code, ampacity tables, insulation temperature ratings, installation method, and safety factors.
Final note
Accurate cable resistance estimation is one of the fastest ways to improve electrical system reliability. Use this calculator early, compare options, and validate final designs against standards and field constraints.