Copper Wire Resistance Calculator
Estimate wire resistance at 20°C and at your operating temperature using length and conductor size.
What this copper resistance calculator does
This calculator estimates the electrical resistance of a copper conductor based on wire length, cross-sectional area (or AWG), and temperature. It is useful for low-voltage wiring, battery cables, electronics projects, solar installations, and general electrical design where voltage drop and heat matter.
In practical design, resistance directly affects how much voltage is lost in a wire and how much power is wasted as heat. Even a small change in wire size can significantly reduce losses in longer runs.
Copper resistance formula
Base equation at 20°C
- R = resistance in ohms (Ω)
- ρ = copper resistivity at 20°C (1.724 × 10-8 Ω·m)
- L = conductor length in meters
- A = conductor area in square meters
Temperature correction
Where α (temperature coefficient for copper) is approximately 0.00393 /°C. As copper gets hotter, resistance increases. That means higher voltage drop and more power loss for the same current.
How to use the calculator
- Enter wire length and choose meters or feet.
- Select conductor size using either mm² or AWG.
- Choose one-way or round-trip path (round-trip doubles effective length).
- Set operating temperature.
- Optionally enter current to estimate voltage drop and power loss.
If you are sizing a DC cable between a source and load, round-trip mode is usually the right choice because current travels out and returns.
Worked example
Suppose you have a 20 meter round-trip run using 2.5 mm² copper wire at 40°C and carrying 10 A. First, resistance at 20°C is calculated from geometry and resistivity. Then temperature correction is applied. Finally:
- Voltage drop = Current × Resistance
- Power loss = Current² × Resistance
This quickly tells you whether the chosen wire size is acceptable or if you should move to a larger conductor.
Quick reference: copper resistance at 20°C (one-way)
| Area (mm²) | Approx. Ω per km | Typical use |
|---|---|---|
| 1.5 | 11.49 Ω/km | Lighting, control wiring |
| 2.5 | 6.90 Ω/km | Sockets, moderate loads |
| 4 | 4.31 Ω/km | Heavier branch circuits |
| 6 | 2.87 Ω/km | Sub-feeds, medium DC runs |
| 10 | 1.72 Ω/km | High-current low-voltage wiring |
| 16 | 1.08 Ω/km | Battery and inverter cabling |
Practical design tips
- For long runs, prioritize larger conductor area to reduce voltage drop.
- Use realistic operating temperature, not only ambient air temperature.
- Account for both outgoing and return path in DC circuits.
- Remember that connectors and terminals add extra resistance not included in this model.
- For critical systems, confirm with local code limits and manufacturer data.
Frequently asked questions
Does stranded vs solid copper change resistance?
For the same effective copper cross-sectional area and temperature, resistance is very similar. Small differences can appear due to manufacturing tolerances and compaction.
Why is measured resistance sometimes higher than calculated?
Real systems include terminal resistance, splice losses, oxidation, contact pressure issues, and temperature rise under load. The calculator models the conductor itself, not all connection losses.
Can I use this for aluminum wire?
Not directly. Aluminum has higher resistivity than copper and a different temperature coefficient. You would need aluminum-specific material constants.
Final note
A copper resistance calculator is a simple tool, but it can prevent costly design mistakes. By estimating resistance early, you can choose better wire sizes, control heating, and keep voltage within safe and efficient limits.