Copper vs Aluminium Conductors

The one number that matters

Conductivity is measured against a standard: 100% IACS is defined as the conductivity of annealed copper, 1.7241 × 10⁻⁸ Ω·m at 20 °C. Conductor-grade aluminium is 61% IACS, which works out to 2.8264 × 10⁻⁸ Ω·m.

Since resistance is inversely proportional to conductivity, aluminium has 1 ÷ 0.61 = 1.64 times the resistance of copper at the same cross-sectional area. And because voltage drop follows resistance directly, aluminium produces about 64% more drop, size for size.

Resistance, size for size

The ratio column sits close to 1.64 all the way down, which is what you would expect from a difference that is purely material rather than geometric.

Equivalent sizes

To match a copper conductor on voltage drop you need enough extra aluminium area to cancel the conductivity difference. Two AWG sizes is the working rule; here is what it looks like against the actual table.

The area increase column hovers around 1.6, which is the conductivity ratio showing up exactly where you would expect it. The match is not always precisely two AWG sizes because the AWG series steps in fixed ratios that do not line up perfectly with 1.64, which is why it is worth checking the table rather than applying the rule blindly on a close call.

Ampacity

Higher resistance means more heat for the same current, so aluminium’s allowable ampacity is lower size for size. The ratio is not the same 1.64, because ampacity is set by heat dissipation, which depends on surface area and insulation rating as well as on resistance.

In practice the ampacity substitution lands in roughly the same place as the voltage drop substitution: about two AWG sizes. But they are separate constraints and it is worth confirming both, which is what the wire size calculator does.

Weight and handling

Aluminium’s density is about 2.70 g/cm³ against copper’s 8.96, roughly 30%. Even after going up two sizes to compensate for conductivity, an aluminium conductor of equivalent capacity weighs around half what the copper one does.

On a 400 ft feeder of large conductor, pulled by hand, that is the difference between a routine pull and a difficult one. It is a genuine engineering consideration, not a footnote, and combined with the cost difference it is why large feeders and service entrances are so often aluminium.

Terminations

This is where aluminium demands more care, and where its reputation came from.

Oxide layer

Aluminium forms an oxide film on contact with air. Unlike copper oxide, aluminium oxide is an insulator and it re-forms almost immediately. A termination has to break through that film and keep it from re-forming, which is what antioxidant compounds and the mechanical action of a properly torqued connector are for.

Creep

Aluminium deforms slowly under sustained pressure. A joint torqued correctly on installation day can loosen over months as the metal flows away from the pressure point. A loose joint has resistance, resistance means heat, and heat accelerates the creep. Connectors listed for aluminium are designed around this.

Galvanic corrosion

Aluminium and copper in direct contact with moisture present form a galvanic couple, and the aluminium corrodes. A dissimilar-metal joint needs a connector listed for the combination, marked AL/CU or equivalent.

Thermal expansion

Aluminium expands and contracts more than copper with temperature. A joint that cycles daily works itself loose faster if the connector was not designed for the movement.

Which to use

The decision usually resolves itself by scale. On a branch circuit the cost saving is trivial and the extra size is a nuisance, so copper wins. On a 400 A service entrance the cost saving is substantial and the weight saving is the difference between a manageable pull and a miserable one, so aluminium wins. Between those extremes, run both through the calculator and compare.

Questions

Switch material on the voltage drop calculator to see the difference on your own circuit, or let the wire size calculator pick the size for each.