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.
Open calculatorLast 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 temperature field corrects the 75 °C table value to your conductor temperature using the linear temperature coefficient (≈0.32%/°C near 75 °C for both materials) — cold conductors measure lower, hot conductors higher.
- The round-trip figure doubles the one-way length since both conductors carry current.
Worked example
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.
Conductor resistance by size at 75 °C (ohms per 1000 ft)
Values come from the calculator's conductor table for copper and aluminum.
| Size | Copper (Ω/kft) | Aluminum (Ω/kft) | Cross-section (mm²) |
|---|---|---|---|
| 14 AWG | 3.1400 | 5.1496 | 2.08 |
| 12 AWG | 1.9800 | 3.2472 | 3.31 |
| 10 AWG | 1.2400 | 2.0336 | 5.26 |
| 8 AWG | 0.7780 | 1.2759 | 8.37 |
| 6 AWG | 0.4910 | 0.8052 | 13.30 |
| 4 AWG | 0.3080 | 0.5051 | 21.20 |
| 3 AWG | 0.2450 | 0.4018 | 26.70 |
| 2 AWG | 0.1940 | 0.3182 | 33.60 |
| 1 AWG | 0.1540 | 0.2526 | 42.40 |
| 1/0 AWG | 0.1220 | 0.2001 | 53.50 |
| 2/0 AWG | 0.0967 | 0.1586 | 67.40 |
| 3/0 AWG | 0.0766 | 0.1256 | 85.00 |
| 4/0 AWG | 0.0608 | 0.0997 | 107.00 |
| 250 kcmil | 0.0515 | 0.0845 | 127.00 |
| 350 kcmil | 0.0367 | 0.0602 | 177.00 |
| 500 kcmil | 0.0257 | 0.0421 | 253.00 |
A circuit's resistance is the one-way length times the per-length value, doubled for the round trip.
From resistance to voltage drop
A 12 AWG copper circuit that runs 100 ft one way has 0.396 Ω of round-trip resistance at 75 °C. Carrying 15 A, it drops 5.94 V (Ohm's law, V = I × R), which is 5.0 % of a 120 V supply. Many designers aim to keep the drop of a branch circuit near 3 %, so this run would need a larger conductor.
Temperature changes the answer
Metal resistance rises with temperature. For 12 AWG copper the calculator gives 1.628 Ω/kft at 20 °C and 1.980 Ω/kft at 75 °C, about 22 % higher. Use the temperature your conductors actually run at, not the room temperature.
Aluminum compared with copper
At the same size, 4/0 aluminum has 0.0997 Ω/kft against 0.0608 Ω/kft for copper, a ratio of 1.64. That is why an aluminum feeder is usually one or two sizes larger than a copper one carrying the same current.
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.
What is the resistance of 4/0 AWG aluminum?
4/0 AWG copper is 0.0608 Ω/1000 ft at 75 °C; aluminum is about 1.64× that, ≈0.10 Ω/1000 ft. Set the size to 4/0 and material to aluminum in the calculator for the exact figure at your temperature.
How does temperature affect wire resistance?
Resistance rises roughly linearly with temperature: about 0.32–0.39% per °C for copper and aluminum 1350. The 75 °C table values are ~18–20% above a 20 °C room-temperature measurement.
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.