| Size | Copper | Aluminium | Al ÷ Cu | Drop per 100 ft at 20 A (1φ) |
|---|---|---|---|---|
| 14 AWG | 3.07 Ω | — | — | 12.28 V |
| 12 AWG | 1.93 Ω | 3.18 Ω | 1.65 | 7.72 V |
| 10 AWG | 1.21 Ω | 2 Ω | 1.65 | 4.84 V |
| 8 AWG | 0.778 Ω | 1.26 Ω | 1.62 | 3.11 V |
| 6 AWG | 0.491 Ω | 0.808 Ω | 1.65 | 1.96 V |
| 4 AWG | 0.308 Ω | 0.508 Ω | 1.65 | 1.23 V |
| 3 AWG | 0.245 Ω | 0.403 Ω | 1.64 | 0.98 V |
| 2 AWG | 0.194 Ω | 0.319 Ω | 1.64 | 0.78 V |
| 1 AWG | 0.154 Ω | 0.253 Ω | 1.64 | 0.62 V |
| 1/0 AWG | 0.122 Ω | 0.201 Ω | 1.65 | 0.49 V |
| 2/0 AWG | 0.0967 Ω | 0.159 Ω | 1.64 | 0.39 V |
| 3/0 AWG | 0.0766 Ω | 0.126 Ω | 1.64 | 0.31 V |
| 4/0 AWG | 0.0608 Ω | 0.1 Ω | 1.64 | 0.24 V |
| 250 kcmil | 0.0515 Ω | 0.0847 Ω | 1.64 | 0.21 V |
| 300 kcmil | 0.0429 Ω | 0.0707 Ω | 1.65 | 0.17 V |
| 350 kcmil | 0.0367 Ω | 0.0605 Ω | 1.65 | 0.15 V |
| 500 kcmil | 0.0258 Ω | 0.0424 Ω | 1.64 | 0.10 V |
Using these figures
Resistance is quoted per 1,000 feet, so divide by 1,000 and multiply by your run length. For voltage drop, remember to account for both legs of the circuit:
1-phase: Vdrop = 2 × L × I × R ÷ 1,000
3-phase: Vdrop = √3 × L × I × R ÷ 1,000
Where L is the one-way distance. The 2 accounts for current going out on one conductor and back on the other; three phase uses √3 because the phase currents are 120° apart and their drops add vectorially rather than arithmetically.
The conditions attached to these numbers
- DC resistance at 75°C. Copper's resistance rises about 0.393% per °C, so a conductor running at 90°C has roughly 6% more resistance than the table value.
- Uncoated copper. Tin-coated copper is slightly higher.
- Solid for 14-10 AWG, stranded above. Stranded is 2-4% higher than solid at the same gauge because the individual strands spiral and are therefore longer than the cable.
- For large conductors at high current, use Table 9 instead. AC impedance includes reactance, which becomes significant above about 1/0.
Why resistance halves every three sizes
AWG is a logarithmic scale, and it is worth internalising the two shortcuts it gives you:
- Three sizes larger, half the resistance. 12 AWG is 1.93 Ω/kft, 9 AWG would be about 0.96, and 6 AWG is 0.491.
- Six sizes larger, a quarter the resistance and four times the area. 12 AWG to 6 AWG is 1.93 down to 0.491.
Numbers run backwards — smaller gauge number means bigger wire — and after 1 AWG the scale continues 1/0, 2/0, 3/0, 4/0 (spoken "one aught" and so on), then switches to kcmil, thousands of circular mils, where bigger numbers finally mean bigger wire.
Aluminium runs about 1.6 times copper's resistance at the same size, which is the other half of why aluminium goes two sizes up.
Common questions
What is the resistance of 12 AWG copper wire?
1.93 ohms per 1,000 feet at 75C, per NEC Chapter 9 Table 8. So 100 feet is 0.193 ohms, and a 100 ft circuit run and back is 0.386 ohms.
How much more resistance does aluminium have than copper?
About 1.6 times at the same size. That is why aluminium conductors are typically two sizes larger than copper for the same job.
Does wire resistance change with temperature?
Yes. Copper rises about 0.393% per degree C. A conductor at 90C has roughly 6% more resistance than the 75C table value, which means correspondingly more voltage drop.
Why does resistance halve every three AWG sizes?
AWG is a logarithmic scale where each three-size step roughly doubles the cross-sectional area, and resistance is inversely proportional to area.