Wire Resistance Calculator — Free Online Calculator
Calculate the DC resistance of copper or aluminum wire per foot or meter for any AWG gauge size.
How to Use This Calculator
Select wire gauge and material, enter the total wire length. The calculator shows resistance per foot, per meter, and total resistance for the entire run.
The Formula Explained
Wire resistance depends on material resistivity, cross-sectional area, and length. R = ρL/A, where ρ is resistivity, L is length, and A is area. Copper has about 61% the resistance of aluminum for the same gauge.
Why Wire Resistance Matters in Practice
Wire resistance is the invisible tax on every electrical circuit. Every watt of power the wire dissipates as heat is a watt that didn't reach the load, and a watt that's heating up the insulation and the building. On short residential runs the tax is negligible — maybe 1-2% of total power — but on long industrial feeders or undersized circuits it can easily hit 5-10%, which adds up fast on 24/7 loads.
The resistance of copper and aluminum is well-characterized and surprisingly consistent across manufacturers (standards require tight tolerances). What varies is the effective resistance under real conditions: temperature raises resistance, AC current at higher frequencies concentrates on the surface (skin effect), parallel conductors influence each other (proximity effect), and mechanical damage to the conductor strands creates hot spots. The resistance values you calculate are baseline minimums — real-world installations typically see 5-15% higher effective resistance.
Worked Example: Power Loss in a 100-Amp Feeder
A 100-amp feeder runs 200 feet using 1 AWG copper. From tables, 1 AWG copper has DC resistance of about 0.1563 ohms per 1,000 feet at 75°C. For 200 feet of wire carrying current both ways (hot + neutral in single-phase), the total conductor length is 400 feet. Total resistance: 0.1563 × (400/1000) = 0.0625 ohms.
At 100 amps: voltage drop = I × R = 100 × 0.0625 = 6.25 volts. On 240V: 6.25/240 = 2.6% drop. Well within limits. Power dissipated as heat in the wire: P = I² × R = 100² × 0.0625 = 625 watts. That's 625 watts of pure heat production along the conduit run, continuously, while the feeder is at full load. Annual energy wasted (assuming 2,000 hours of operation at full load): 625 × 2,000 = 1,250 kWh, about $200 per year at $0.16/kWh. Over 30 years that's $6,000 in wasted electricity, easily more than the cost of upsizing to 1/0 AWG at install time.
Worked Example: Copper vs Aluminum Cost Analysis
A 200-foot 200-amp feeder: should you use copper or aluminum? Copper 2/0 AWG handles 175A at 75°C (not quite enough), so you need 3/0 AWG (200A). 3/0 copper resistance is 0.0766 Ω/1000ft, so 200 ft × 2 (both conductors) = 400 ft × 0.0766/1000 = 0.0306 Ω. Voltage drop at 200A: 200 × 0.0306 = 6.12V on 240V = 2.55%. Good.
Aluminum equivalent: you need 4/0 aluminum (205A at 75°C). 4/0 aluminum resistance is about 0.1253 Ω/1000ft. 400 ft × 0.1253/1000 = 0.0501 Ω. Voltage drop at 200A: 200 × 0.0501 = 10.02V on 240V = 4.2%. Still within the 5% total limit, but uses more of the drop budget.
Cost: 3/0 copper at 2026 prices runs about $8/ft, so 400 ft = $3,200. 4/0 aluminum runs about $2.50/ft, so 400 ft = $1,000. Aluminum saves $2,200 upfront. But aluminum wastes about 2x the power: 200² × 0.0501 = 2,004W vs 200² × 0.0306 = 1,224W. The 780-watt difference, at 2,000 hours/year, costs about $250 per year in waste. Payback on the copper premium: about 9 years. For a 40-year service life, copper is cheaper long-term — but aluminum wins on short-term cash flow.
Common Wire Resistance Gotchas
1. Using 20°C resistance values. Manufacturer data sheets often list resistance at 20°C for marketing comparisons. NEC design values (Chapter 9 Table 8) are at 75°C, which is more realistic for a loaded conductor. The difference is about 20% — non-trivial.
2. Forgetting to double for round-trip. In a simple two-wire circuit, current flows out on the hot and back on the neutral. Total wire length in the current path is 2x the one-way distance. Three-phase balanced loads don't have this factor-of-2 because the neutral carries zero (ideally).
3. Ignoring skin effect on large wire. At 60 Hz, skin effect starts mattering around 1/0 AWG and gets significant by 500 kcmil. The effective AC resistance can be 10-20% higher than DC resistance for large conductors.
4. Assuming parallel wires split current equally. Two 250 kcmil conductors in parallel only carry equal current if they're the same length, same temperature, and same material. Unequal splitting means one conductor runs hotter than the other, reducing total ampacity.
5. Not accounting for connection resistance. Every splice, lug, and terminal adds microhms of resistance. In a properly torqued connection, this is negligible. In a loose connection, it can dominate the entire circuit — and generate enough heat to start a fire. Loose connections are the leading cause of electrical fires in older homes.
Reducing Wire Losses in the Real World
Upsize conductors on continuous high-load circuits. If your feeder runs at 80% load 24/7, the resistive losses are significant money. Going up one wire gauge typically cuts I²R losses by 20-25%, and the payback is often under a decade at current electricity prices.
Prefer higher voltage for power transmission. At double the voltage, you can carry the same power with half the current, which quarters the I²R losses for the same wire size. This is the same principle that drives utility transmission at 345 kV.
Keep conductors cool. Routing through cool spaces, separating bundled conductors, and using larger conduit all reduce operating temperature, which reduces resistance, which reduces losses, which reduces temperature. Positive feedback in the right direction.
Torque every connection to spec and recheck after first heating cycle. Loose connections are the number one electrical fire cause. Torque screwdrivers cost $40 and are cheap insurance. Recheck after the first few hours of operation because thermal cycling can loosen connections that were initially tight.
NEC Resistance References
Chapter 9 Table 8 gives DC resistance values for conductors at 75°C, both copper and aluminum, for AWG 18 through 2,000 kcmil. This is the definitive source for calculations. Chapter 9 Table 9 gives AC resistance and reactance values for conductors 1/0 AWG and larger, in different conduit materials (PVC, aluminum, steel). The steel conduit values are highest because steel is magnetic and increases reactance significantly.
For high-precision work, manufacturer data sheets supplement the NEC values with specific resistance, reactance, and temperature coefficients. IEEE 141 (Red Book) and IEEE 241 (Gray Book) provide extended resistance and impedance data for industrial and commercial calculations, including harmonic effects on effective resistance.
Wire resistance: ohms per length, why it matters in real circuits
Every conductor has resistance. It is small, but it is not zero, and on long runs or high currents it shows up as voltage drop, heat, and energy waste. Resistance per unit length is the fundamental number that drives the wire-size and voltage-drop calculations you do elsewhere on this site.
The calculator above uses the conductor resistivity, length, and cross-sectional area to give you the DC resistance in ohms. For AC at 60 Hz, the result is within a fraction of a percent for small conductors and increasingly conservative for very large ones, where skin effect and proximity effect bump impedance slightly above DC.
The formula and what it does
In US units with circular mils: R = (K x L) / CM where K is 12.9 ohm-cmil/ft for copper at 20 C and 21.2 for aluminum. So 100 ft of 12 AWG copper (6,530 CM): R = (12.9 x 100) / 6,530 = 0.198 ohms. NEC Chapter 9 Table 8 lists these values to four decimal places.
Worked example
Scenario: 150 ft of 10 AWG copper carrying 25 A. Find the resistance and the resulting power dissipated as heat in the wire.
10 AWG copper has 10,380 CM. R = (12.9 x 150) / 10,380 = 0.186 ohms. Power dissipated = I squared x R = 25 squared x 0.186 = 116 watts. That is 116 watts of heat in your wall, every minute the load runs. At 16 cents per kWh and 8 hours per day, the wasted energy alone is $54 a year, plus the wire runs warm.
Common mistakes to avoid
Forgetting temperature. NEC table values are at 75 C, not room temperature. A wire at 20 C has 22 percent lower resistance than at 75 C. The calculator can let you pick a temperature; in most installs assume 75 C unless you know otherwise.
Confusing one-way and round-trip. Resistance is per conductor. For voltage-drop calculations on single-phase circuits you double the length because current flows through both the hot and neutral.
Mixing CM and AWG units. 12 AWG is not 12 CM. AWG is a logarithmic gauge; CM is the actual cross-section area. Always convert via NEC Chapter 9 Table 8.
Frequently asked questions
Does the resistance change with temperature?
Yes, about 0.4 percent per degree C for copper. Run a wire hot (in conduit on a roof, for example) and resistance climbs measurably. The NEC tables are at 75 C, which is the worst case for normal operation.
What is the difference between DC and AC resistance?
For small conductors at 60 Hz, almost none. For very large conductors (above about 1/0 AWG), AC resistance is slightly higher because of skin effect (current crowds toward the surface) and proximity effect (current redistributes in nearby parallel conductors). NEC Chapter 9 Table 9 gives the AC-corrected effective impedance.
How does resistance affect a motor?
Wire resistance creates voltage drop, which lowers the voltage at the motor terminals. A motor under-voltage by 10 percent draws roughly 11 percent more current to maintain torque, raising winding temperature 15-20 percent and accelerating insulation failure.
Why is aluminum resistance higher than copper?
Aluminum has about 61 percent of copper conductivity. For the same gauge, it has 1.64 times the resistance. That is why aluminum feeders are typically sized one to two AWG larger than the copper equivalent to deliver the same ampacity and voltage drop.
Do crimps and splices add resistance?
A properly made compression splice adds almost nothing (under 0.001 ohm). A loose mechanical connection can add 0.1 ohm or more, which on a 30 A circuit dissipates 90 watts at that single point. This is how fires start.
How precise are the NEC resistance values?
Very. The table 8 values are tied to ASTM standards (B 33 for copper, B 800 for aluminum) and accurate to within fractions of a percent for new conductors at the stated temperature.