What it compares
Enter a starting copper or aluminium cross-sectional area and select the comparison material. The result is the area of the comparison material that gives the same calculated resistance for an equal conductor length using the source's 20°C material resistivity values.
The result is a physical resistance comparison. It is not a wire-size lookup and does not select a conductor for a particular circuit.
Method and example
The source gives R = K × L ÷ CMA, where K is material resistivity, L is length and CMA is conductor cross-sectional area. For equal length and resistance, Acomparison = Areference × Kcomparison ÷ Kreference. The common area and length-unit conversions cancel in this ratio, so the entered area can be in mm².
Using the source's values, Kcopper = 10.4 and Kaluminium = 17 at 20°C. A 10 mm² copper area corresponds to about 16.3 mm² aluminium for the same calculated resistance at the same length. At the same 10 mm² area, the aluminium resistance ratio is about 1.63 times the copper value.
The source resistivity values are rounded reference figures, so outputs display three significant digits or scientific notation rather than implying product-level precision.
Limits and interpretation
The comparison uses only the source's generic copper and aluminium resistivity values at 20°C. It does not account for conductor construction, product-specific resistance, operating temperature, joints, terminations, AC reactance, skin effect or harmonic currents.
An equal-resistance area is not an approved substitute conductor size. Ampacity, insulation, termination ratings, overcurrent protection, installation method and adopted requirements must be assessed separately.
No AWG, kcmil or trade-size mapping, conductor recommendation, voltage-drop limit or code result is provided.
Common questions
Does the result tell me what aluminium wire to install?
No. It reports a theoretical equal-resistance area only. It does not map to a listed size or check ampacity, terminals, installation conditions or Canadian requirements.
Why does length not need to be entered?
The comparison assumes the same length for both conductors. Length appears in both resistance equations and cancels when solving for equal resistance.
Can I use the comparison for AC circuits?
Not as a full AC impedance comparison. This model compares only the source's 20°C resistivity values; it excludes reactance, frequency and conductor construction.
Related tools
Source
British Columbia Industry Training Authority, Construction Electrician Apprenticeship Program, Level 1, Learning Guide D-1: Use Electrical Circuit Concepts, Version 1 (January 2011), Learning Task 18 “Factors affecting resistance,” printed pages 130–132. Page 131 gives copper and aluminium resistivity values at 20°C; pages 130–132 give the resistance relation and cross-section examples. This Canadian trade-learning source supports only the physical comparison here, not a provincial code rule or conductor-substitution approval.
Source and units checked November 10, 2026. Report a problem or suggest a source; inputs are not attached automatically.