If you need to estimate wire resistance for electrical design, voltage drop checks, battery wiring, solar installations, or low-voltage systems, this copper cable resistance calculator gives you a fast and accurate result. Enter cable length, conductor size, temperature, and current to compute resistance and optional voltage drop/power loss.
Copper Cable Resistance Calculator
How the calculator works
The calculation is based on the standard conductor resistance equation:
R = ρ × L / A
Where R is resistance in ohms, ρ is resistivity in Ω·m, L is total conductor length in meters, and A is cross-sectional area in square meters. Because most cable sizes are specified in mm², this tool converts mm² to m² automatically.
Temperature correction for copper
Copper resistance rises with temperature. The calculator adjusts resistivity using:
ρ(T) = ρ20 × [1 + α × (T - 20)]
- ρ20 = copper resistivity at 20°C (default 1.724×10-8 Ω·m)
- α = temperature coefficient (default 0.00393 /°C)
- T = conductor temperature in °C
Input guide
1) Cable size
You can enter area directly in mm² or choose AWG from a dropdown list. Both modes lead to the same formula; AWG values are just pre-converted areas.
2) Length and path type
If your load is connected by two conductors (positive and return, or line and neutral), use round-trip so the effective length is doubled. This is essential for realistic voltage drop results.
3) Current (optional but useful)
With current, the calculator also returns:
- Voltage drop:
V = I × R - Power loss in cable:
P = I² × R
Worked example
Suppose you have a 25 m one-way run of 2.5 mm² copper cable carrying 10 A at 40°C. Using round-trip length, total current path becomes 50 m.
- Area = 2.5 mm² = 2.5×10-6 m²
- ρ20 = 1.724×10-8 Ω·m
- α = 0.00393 /°C
- T = 40°C
First correct resistivity for temperature, then compute resistance. Finally apply Ohm’s law for voltage drop. This is exactly what the calculator automates.
Why this matters in real systems
Cable resistance affects efficiency, heat generation, and delivered voltage. In low-voltage DC systems (12V/24V/48V), even small resistance can cause meaningful voltage drop. In higher-power AC systems, resistance still impacts I²R losses and conductor heating.
- Long cable runs increase resistance linearly.
- Smaller cable cross-section increases resistance sharply.
- Higher conductor temperature increases resistance.
- Higher current increases voltage drop and heat loss.
Quick practical tips
- Use round-trip length for most two-wire circuits.
- Upsize cable when runs are long or current is high.
- Check resistance at realistic operating temperature, not only room temperature.
- For critical loads, keep voltage drop within your design target (often 2% to 5% depending on application).
Frequently asked questions
Is this for copper only?
Yes, defaults are for copper. You can still model other conductors by replacing resistivity and temperature coefficient values.
Do I enter physical cable length or circuit length?
Enter one-way physical length, then choose one-way or round-trip path. Most power circuits should use round-trip.
Does stranded vs solid copper change resistance?
For most power-frequency and DC calculations, resistance is mainly based on total copper area and temperature. Construction style has smaller secondary effects.
Can I use this for voltage drop design?
Yes. Enter current and use the voltage-drop result as an early-stage sizing check before final code compliance and thermal derating review.