Wire Resistance Calculator — Ohms by AWG and Length

Find the DC resistance of a copper or aluminum conductor from its AWG size and length, one-way and round-trip, using the NEC Chapter 9 Table 8 ohms per 1,000 ft.

Results are estimates for planning and education, based on your inputs and standard engineering values (AWG resistance, NEC ampacity, resistivity). Electrical work can be dangerous and is governed by the NEC and your local code — verify all sizing with a licensed electrician and your authority having jurisdiction (AHJ). Not a substitute for professional design.

Calculator

Resistance (one-way)0.1930 Ω
Resistance (round-trip)0.3860 Ω

Formula

Conductor resistance scales with length: R = (Ω per 1,000 ft) × length_ft / 1,000 for one direction, and the round-trip (out and back) is twice that. The ohms per 1,000 ft come from NEC Chapter 9, Table 8 for copper; aluminum is higher resistance, scaled by the ratio of resistivities K_al / K_cu = 21.2 / 12.9 ≈ 1.643. Larger conductors (lower AWG number, more circular mils) have lower resistance, because resistance is inversely proportional to cross-sectional area.

Worked example

100 ft of 12 AWG copper, which is 1.93 Ω per 1,000 ft: one-way R = 1.93 × 100 / 1,000 = 0.193 Ω, and the round-trip (supply plus return, 200 ft of wire) is 0.386 Ω. The same 100 ft in aluminum is 0.193 × 1.643 = 0.317 Ω one-way. Use the round-trip value with Ohm’s law to find voltage drop: at 20 A, 0.386 × 20 = 7.7 V — close to the dedicated voltage-drop formula result.

A conductor's resistance is what drives both the voltage it drops and the heat it produces, and this tool computes it from the metal, gauge and length using the standard resistivity constants. Resistance rises with length and falls as the wire gets thicker, and aluminum runs about 1.6 times higher than copper for the same gauge — the same relationship that makes aluminum drop more voltage. It is a useful building block: knowing resistance lets you predict drop under any current with Ohm's law, or estimate the heating in a run. The figure is quoted at a reference temperature, so a conductor running hot has somewhat higher resistance than the nominal value, one more reason to leave a design margin.

Reference table: Wire Resistivity: Copper & Aluminum K Factor

Frequently asked questions

How do I calculate the resistance of a wire?
Multiply the conductor’s ohms per 1,000 ft by the length in feet and divide by 1,000. For 12 AWG copper at 1.93 Ω/1,000 ft, a 100 ft run is 1.93 × 100 / 1,000 = 0.193 Ω one-way. Double it for the round-trip (out-and-back) resistance of a circuit.
What is the difference between one-way and round-trip resistance?
One-way resistance is for a single conductor over the run length. A circuit needs a supply and a return conductor, so the current travels twice the run length; round-trip resistance is double the one-way value and is what you use to compute total voltage drop.
Why does aluminum have more resistance than copper?
Aluminum is a poorer conductor — its resistivity constant K is 21.2 versus 12.9 for copper, a ratio of about 1.643. So an aluminum conductor of the same AWG has roughly 64% more resistance, which is why aluminum feeders are usually sized one or two AWG larger than copper for the same load.
How does wire size affect resistance?
Resistance is inversely proportional to cross-sectional area (circular mils). Each three steps down in AWG number roughly doubles the area and halves the resistance, so 6 AWG has about a quarter of the resistance per foot of 12 AWG copper.
How is resistance related to voltage drop?
By Ohm’s law, voltage drop = current × round-trip resistance. That is the same physics as the voltage-drop formula VD = 2 × K × I × L / CM, just expressed through the tabulated ohms per 1,000 ft. Use the wire-resistance value with the load current to estimate the drop on a run.
Does temperature change conductor resistance?
Yes, slightly — copper resistance rises about 0.4% per °C. The NEC Table 8 values are referenced at 75°C, a reasonable operating assumption. For an estimate the temperature effect is small, so this tool uses the standard table values.

Source: NEC Chapter 9, Table 8 conductor DC resistance; K_al / K_cu ratio for aluminum · All sources

Related calculators