Wire & voltage
Wire Resistance Calculator
Wire resistance equals material resistivity multiplied by conductor length and divided by cross-sectional area. The temperature adjustment changes resistivity from its 20°C reference value.
Estimate conductor resistance, resistance per length, voltage loss, and heat loss from material, size, length, and temperature.
Calculated result
Calculated conductor resistance
Calculating…
Calculated locally in your browser
Planning math only. Verify equipment specifications and installation requirements separately.
What this calculator returns
Real cable construction, terminations, strand geometry, and AC effects can add resistance. Use the custom project specification when it is available.
Formula and variables
The calculation runs entirely in your browser. Static formulas, definitions, examples, and tables remain readable without JavaScript.
R = ρ20 × [1 + α(T - 20°C)] × L / A; Vloss = I × R; Ploss = I² × R.
- Vdrop
- Voltage lost across the conductor path, in volts.
- I
- Current through the conductor, in amperes.
- R
- Electrical resistance of the modeled conductor path, in ohms.
- ρ
- Material resistivity at the modeled temperature, in ohm-metres.
- L
- Conductor length used by the selected path model.
- A
- Metal cross-sectional area of the conductor.
ρ20 is resistivity at 20°C, α is the linear temperature coefficient, L is total conductor length, and A is area.
Worked example
| Input | Value |
|---|---|
| Conductor material | Copper |
| Conductor size | 12 AWG |
| Conductor length | 100 ft |
| Length describes | Total conductor path |
A 100 ft total path of 12 AWG copper at 20°C is about 0.16 Ω using the stated resistivity model.
Reference table
| AWG | Cu 20°C Ω/1000 ft | Cu 75°C Ω/1000 ft | Al 20°C Ω/1000 ft | Al 75°C Ω/1000 ft | Cu 20°C Ω/km | Al 20°C Ω/km |
|---|---|---|---|---|---|---|
| 14 | 2.5254 | 3.0712 | 4.1400 | 5.0576 | 8.2853 | 13.5825 |
| 12 | 1.5882 | 1.9315 | 2.6036 | 3.1807 | 5.2107 | 8.5421 |
| 10 | 0.9988 | 1.2147 | 1.6374 | 2.0004 | 3.2770 | 5.3722 |
| 8 | 0.6282 | 0.7640 | 1.0298 | 1.2581 | 2.0609 | 3.3786 |
| 6 | 0.3951 | 0.4805 | 0.6476 | 0.7912 | 1.2961 | 2.1248 |
| 4 | 0.2485 | 0.3022 | 0.4073 | 0.4976 | 0.8152 | 1.3363 |
Frequently asked questions
Should I enter one-way or total conductor length?
Choose the matching length mode. Total path uses the entered length directly, while one-way mode includes the return conductor.
Why is AC resistance not included?
The calculator is a DC resistance model and does not add frequency-dependent skin, proximity, or reactance effects.
Why does temperature matter?
Metal resistance changes with temperature; the calculator applies a visible linear correction around 20°C.
Assumptions and limitations
- Representative material constants and their technical sources are listed on the Sources page.
- This is a DC resistance model.
- Connection resistance is excluded.
Method and sources
Review the AWG reference, calculation methodology, and standards status for the references most relevant to this calculation. The broader technical sources index records source scope and verification. Last reviewed .