Same cross-section does not mean same resistance—copper and aluminum use different resistivity. This guide walks through the copper vs aluminum wire resistance calculator: identical length and mm², material toggle, and operating temperature.
Benefits
- Aluminum ρ ≈ 61% higher than copper at the same mm² and length.
- Fair compare: same L, A, and T—only material changes.
- Outputs Ω, R at 20°C, and Ω/m for each material run.
How it works
- Enter cross-section (mm²), one-way length (m), and operating °C.
- Run once with copper, once with aluminum—or compare mental ρ ratio.
- Read resistance gap; upsize Al or shorten run if drop budget is tight.
FAQ
How much higher is aluminum resistance than copper?
At the same mm² and length: R_Al ÷ R_Cu ≈ ρ_Al ÷ ρ_Cu ≈ 0.0282 ÷ 0.0175 ≈ 1.61. Example: 10 mm², 20 m at 25°C → Cu ~0.035 Ω, Al ~0.056 Ω (~61% more). Temperature correction applies equally to both.
Why do codes upsize aluminum conductors?
Higher ρ means more drop and I²R loss at equal ampacity tables. NEC often requires aluminum one standard size larger than copper for the same breaker rating. Calculate both materials at your actual length before assuming equal mm² is equal performance.
When is aluminum still economical?
Lower $/kg and lighter weight can win on long feeders when upsized Al still beats copper cost. Run this calculator at upsized Al mm² vs baseline Cu—if resistance matches and material savings hold, the trade is rational; if not, copper may win on drop alone.
Technical specifications
- R = ρ × L ÷ A; ρ_Cu ≈ 0.0175, ρ_Al ≈ 0.0282 Ω·mm²/m at 20°C.
- R_Al / R_Cu ≈ 1.61 for equal geometry.
- R(T) uses same α form per material in the tool.
- Related: dc-cable-size, residential-voltage-drop, conductor-resistance-calculator.
mm² on the label hides material physics
Ten mm² copper and ten mm² aluminum have the same area but different resistivity. Installers comparing quotes on price per meter without a resistance check often discover extra drop after pull-in. Toggle material with length and temperature held constant—the ~1.61× gap is the first filter before ampacity tables and termination practices enter the decision.
Upsized aluminum is the fair fight
Code practice may put 12 AWG copper against 10 AWG aluminum for the same circuit. Run the calculator at both mm² pairs on your actual homerun length. If upsized Al still exceeds drop budget, copper or a shorter route wins on engineering—not brand preference. Document Ω for each scenario in the job folder.
Weight and cost follow resistance math
Aluminum wins weight per amp when sized correctly; copper wins Ω per mm². Resistance drives inverter clipping, battery sag, and nuisance trips. After comparing Cu vs Al Ω at field temperature, roll forward to voltage-drop and loss dollars—cheaper wire with 30% more R can cost more in energy over a decade than copper upfront.