Wire Resistance Calculator
Calculate the total resistance of a wire run based on gauge, length, and material per NEC Table 9.
Results
How to Use This Calculator
Select the conductor material: copper or aluminum. Choose the wire gauge from 14 AWG through 2000 kcmil. Enter the total length of the conductor in feet. Select the temperature, as resistance changes with conductor temperature. The standard reference temperature is 75 degrees Celsius for most NEC calculations. The calculator provides the total DC resistance based on NEC Chapter 9 Table 8, the AC resistance based on NEC Chapter 9 Table 9 (which includes skin effect), and the total resistance for the entered length. For example, 500 feet of 10 AWG copper at 75C has a DC resistance of 500 x 1.24 / 1000 = 0.62 ohms. Using Ohm's Law, you can then calculate the voltage drop for any current flowing through that conductor.
Understanding the Concept
Every electrical conductor has resistance that opposes the flow of current and converts electrical energy into heat. Wire resistance depends on four factors: material (copper has lower resistance than aluminum), cross sectional area (larger gauges have lower resistance), length (longer runs have proportionally more resistance), and temperature (resistance increases as the conductor heats up). Understanding wire resistance is essential for voltage drop calculations, power loss analysis, and troubleshooting. The NEC provides two sets of resistance values: DC resistance in Chapter 9 Table 8 and AC resistance in Chapter 9 Table 9. AC resistance is slightly higher than DC resistance for larger conductors due to skin effect, where alternating current tends to flow near the surface of the conductor rather than uniformly through the cross section. For conductors smaller than 4/0 AWG, the difference between AC and DC resistance is negligible. For larger conductors, particularly those above 500 kcmil, skin effect becomes significant and AC resistance values from Table 9 must be used for accurate calculations.
The Formula Explained
Total wire resistance is calculated as R_total = R_per_1000ft x Length / 1000, where R_per_1000ft is the resistance per 1000 feet from NEC Chapter 9 Table 8 (DC) or Table 9 (AC). For copper at 75C, common values include 14 AWG at 3.14 ohms, 12 AWG at 1.98 ohms, 10 AWG at 1.24 ohms, 8 AWG at 0.778 ohms, 6 AWG at 0.491 ohms, and 4 AWG at 0.308 ohms per 1000 feet. Temperature correction uses the formula R_T = R_ref x (1 + alpha x (T minus T_ref)), where alpha is the temperature coefficient (0.00323 per degree Celsius for copper, 0.00330 for aluminum) and T_ref is the reference temperature. Power dissipated as heat in the conductor is P_loss = I squared x R_total, per NEC Chapter 9 Table 9 notes. This power loss represents wasted energy and contributes to conductor heating.
Frequently Asked Questions
What is the resistance of 12 AWG copper wire per foot?
The resistance of 12 AWG solid copper wire is approximately 1.98 ohms per 1000 feet at 75 degrees Celsius, per NEC Chapter 9 Table 8. That equals 0.00198 ohms per foot. For a 100 foot run, the total resistance is 0.198 ohms. For a complete circuit with 100 feet of outgoing and 100 feet of return conductor, the total circuit resistance is 0.396 ohms. Stranded wire has slightly higher resistance than solid due to the air gaps between strands.
Why is aluminum wire resistance higher than copper?
Aluminum has a higher resistivity than copper at the atomic level. Copper's resistivity is approximately 1.68 micro ohm centimeters, while aluminum is 2.65 micro ohm centimeters, making aluminum about 1.6 times more resistive than copper for the same cross sectional area. This is why aluminum conductors must be sized approximately two AWG sizes larger than copper to achieve comparable ampacity and resistance. For example, 6 AWG copper (0.491 ohms per 1000 feet) is comparable to 4 AWG aluminum (0.508 ohms per 1000 feet).
How does temperature affect wire resistance?
Wire resistance increases with temperature. Copper has a temperature coefficient of approximately 0.00323 per degree Celsius, meaning resistance increases by about 0.323% for each degree above the reference temperature. A conductor operating at 90C has approximately 5% more resistance than the same conductor at 75C. This is why NEC resistance tables specify the temperature at which the values apply. In practice, conductors carrying load are warmer than ambient temperature, so actual resistance is higher than values calculated at room temperature.
What is skin effect and when does it matter?
Skin effect is the tendency of alternating current to concentrate near the outer surface of a conductor, effectively reducing the usable cross sectional area and increasing the AC resistance compared to DC resistance. Skin effect becomes significant in conductors larger than about 250 kcmil. For example, a 500 kcmil copper conductor has a DC resistance of 0.0258 ohms per 1000 feet but an AC resistance of 0.0295 ohms per 1000 feet, about 14% higher. For 12 AWG and smaller conductors, skin effect is negligible and AC and DC resistance are essentially equal.
How do I calculate power loss in a wire?
Power loss is calculated using P = I squared x R, where I is the current in amps and R is the total wire resistance in ohms. For a 20A circuit with 0.4 ohms of total conductor resistance, the power loss is 20 squared times 0.4 = 160 watts. This wasted energy heats the conductor and reduces efficiency. On long, heavily loaded circuits, power loss can be significant. For a circuit operating 8 hours a day at 160W loss, that represents 1.28 kWh per day or about 467 kWh per year of wasted energy, costing roughly $50 to $70 annually at typical utility rates.