Electrical Resistance Guide: Ohms, Wire Tables & Calculations
What Is Electrical Resistance?
Resistance (measured in ohms, Ω) is the opposition to current flow in a conductor. Every material has some resistance — even copper wire. Higher resistance means more voltage drop and more heat generation.
Ohm's Law: R = V / I
Resistivity formula: R = ρ × L / A
- ρ (rho) = resistivity of the material (Ω·cmil/ft)
- L = length (ft)
- A = cross-sectional area (cmils)
Wire Resistance Table (NEC Chapter 9, Table 8)
| AWG | Area (cmils) | Cu Ω/1000ft @ 20°C | Al Ω/1000ft @ 20°C | Cu Ω/1000ft @ 75°C |
|---|---|---|---|---|
| 14 | 4,110 | 2.525 | 4.148 | 3.075 |
| 12 | 6,530 | 1.588 | 2.609 | 1.935 |
| 10 | 10,380 | 0.999 | 1.642 | 1.217 |
| 8 | 16,510 | 0.628 | 1.032 | 0.765 |
| 6 | 26,250 | 0.395 | 0.649 | 0.481 |
| 4 | 41,740 | 0.249 | 0.410 | 0.303 |
| 2 | 66,360 | 0.156 | 0.258 | 0.191 |
| 1/0 | 105,600 | 0.0982 | 0.162 | 0.120 |
| 2/0 | 133,100 | 0.0779 | 0.128 | 0.0949 |
| 4/0 | 211,600 | 0.0490 | 0.0806 | 0.0597 |
Temperature Effect on Resistance
Copper resistance increases linearly with temperature:
RT = R20 × (1 + 0.00393 × (T − 20))
| Temperature | Multiplier (Cu) | 14 AWG Ω/1000ft | 12 AWG Ω/1000ft |
|---|---|---|---|
| 20°C (68°F) | 1.000 | 2.525 | 1.588 |
| 30°C (86°F) | 1.039 | 2.624 | 1.650 |
| 40°C (104°F) | 1.079 | 2.724 | 1.713 |
| 50°C (122°F) | 1.118 | 2.823 | 1.776 |
| 60°C (140°F) | 1.158 | 2.924 | 1.839 |
| 75°C (167°F) | 1.216 | 3.070 | 1.931 |
Note: Wire operating at rated ampacity reaches 60–90°C. Use the 75°C resistance values for voltage drop calculations under load.
Series and Parallel Resistance
Series: Rtotal = R1 + R2 + R3 + ...
Parallel: 1/Rtotal = 1/R1 + 1/R2 + 1/R3 + ...
For two resistors in parallel: Rtotal = (R1 × R2) / (R1 + R2)
Common Material Resistivities
| Material | Resistivity (Ω·cmil/ft) | % of Copper |
|---|---|---|
| Silver | 9.8 | 95% |
| Copper | 10.37 | 100% |
| Aluminum | 17.0 | 164% |
| Steel | 63.0 | 608% |
| Nichrome | 675 | 6,510% |
Common Mistakes
- Using 20°C resistance for voltage drop: Wire under load operates at 50–75°C. Use the 75°C column for realistic voltage drop calculations.
- Forgetting round trip: Current travels out AND back. Double the one-way wire length for total resistance.
- Ignoring parallel paths: Parallel wires (paralleled conductors, parallel feeders) halve the resistance. Two 2 AWG in parallel = same resistance as one 2/0 AWG.
Standards Reference
- NEC Chapter 9, Table 8 — Conductor Properties
- NEC Table 310.16 — Ampacities
- ASTM B3 — Soft Annealed Copper Wire
- IEC 60228 — Conductors of Insulated Cables
Frequently Asked Questions
What is the resistance of 12 AWG copper wire?
12 AWG copper wire has a resistance of 1.588 Ω per 1,000 feet at 20°C (68°F). At 75°C operating temperature, this increases to 1.931 Ω/1000ft. For a 100-ft circuit (200 ft round trip), the total wire resistance is about 0.387 Ω.
How does temperature affect wire resistance?
Copper resistance increases 0.393% per °C. A wire at 75°C has 21.6% more resistance than at 20°C. This means more voltage drop and more heat generation under load. Always use the operating temperature resistance for voltage drop and power loss calculations.
Why does aluminum wire have more resistance?
Aluminum has 64% higher resistivity than copper (17.0 vs 10.37 Ω·cmil/ft). This means aluminum must be 2 AWG sizes larger than copper to achieve the same resistance and ampacity. For example, 2 AWG aluminum ≈ 4 AWG copper in resistance.
How do I calculate wire resistance for a long run?
R = (Ω/1000ft) × (total length in ft) / 1000. For a 150-ft run of 12 AWG copper (round trip = 300 ft): R = 1.588 × 300 / 1000 = 0.476 Ω. Voltage drop = I × R. At 15A: V_drop = 15 × 0.476 = 7.14V.
What's the difference between resistance and impedance?
Resistance (R) opposes DC current. Impedance (Z) opposes AC current and includes resistance plus reactance (from inductors and capacitors). Z = √(R² + (X_L − X_C)²). For purely resistive loads (heaters), Z = R. For motors and transformers, Z > R.