Wire Resistance Calculator
Find the electrical resistance of any copper or aluminum conductor from the AWG table — round trip and per-unit-length values for voltage drop math.
Last updated: 2026-09-15
How the calculation works
- The tool reads the DC resistance at 75 °C from the NEC Chapter 9 Table 8 copper series.
- Aluminum applies a 1.64 multiplier — aluminum's resistivity is about 64% higher than copper's at the same geometry.
- The round-trip figure doubles the one-way length since both conductors carry current.
Formula
R = ρ × L / A tabulated: Ω per 1000 ft at 75 °C (NEC Ch. 9 Table 8)
| Symbol | Meaning | Unit |
|---|---|---|
R | Resistance | Ω |
ρ | Resistivity (material constant) | Ω·cmil/ft |
L | Conductor length | ft |
A | Cross-section | circular mils |
Worked example
100 ft of 12 AWG copper, out and back: 1.98 Ω/kft × 0.2 kft × 2 = 0.396 Ω. The same run in aluminum would be 0.649 Ω — 64% more, which is why aluminum circuits need larger conductors for the same drop.
Interpreting the result
Resistance is the raw material of every voltage-drop and power-loss calculation. Note that it rises with temperature: the 75 °C table values are about 20% higher than room-temperature measurements, so cold circuits measure slightly better than calculated.
Assumptions
- Uncoated conductors at 75 °C operating temperature.
- DC resistance — AC reactance is separate (see the cable reactance calculator).
Limitations
- Coated conductors and tinned copper differ slightly.
- Terminations and connections add resistance not included here.
Frequently asked questions
What is the resistance of 12 AWG copper wire?
About 1.98 Ω per 1000 ft at 75 °C (NEC Chapter 9 Table 8). At room temperature it measures roughly 1.6 Ω/kft — resistance rises with temperature.
Why is aluminum wire resistance higher?
Aluminum's resistivity is about 64% higher than copper's. For the same ampacity, aluminum conductors are roughly two sizes larger, which offsets the cost advantage.