Copper vs Aluminum

Ask which metal conducts electricity better and the answer is copper, comfortably. Ask which metal conducts electricity better per kilogram and the answer flips to aluminum, by about a factor of two. Almost every argument about these two is really an argument about which of those two questions the job is asking.

If the conductor is hanging in the air, aluminum. If it is buried in a wall, terminated in a screw and forgotten for forty years, copper.

Two conductivities, and which one your problem cares about

Aluminum carries roughly 61% of the current that copper carries through the same cross-section. To match a copper cable you therefore need an aluminum one of about one and a half times the area — a noticeably fatter cable, which is a problem in a conduit and no problem at all on a pylon.

But aluminum is a third of copper's density, so that fatter cable still comes out at roughly half the mass. Hang a hundred kilometres of it between towers and the mass is the entire engineering problem: heavier conductors need taller towers, closer spacing and deeper foundations, and those costs dwarf the price of the metal.

Aluminum's weakness on a transmission line is that it is mechanically feeble and it creeps under sustained tension. The standard answer is to build the tension out of a different metal entirely: ACSR conductor is a core of galvanised steel strands carrying the mechanical load, wrapped in aluminum strands carrying the current. Newer variants swap the steel core for a carbon-composite one to run hotter with less sag, but the principle is the same — let each material do only the job it is good at.

What went wrong in American houses

Copper prices spiked in the mid-1960s and a great many United States homes were wired with solid aluminum branch circuits until around 1972. The consequence was not that the wire failed. It was that the ends failed.

Three things gang up at a screw terminal. Aluminum expands about 40% more than copper for the same temperature rise, so a joint that warms and cools daily works itself loose. Aluminum also cold-flows under sustained clamping pressure, so the screw slowly loses its grip even without thermal cycling. And where copper's tarnish is a middling conductor that a contact can live with, aluminum's surface oxide is a genuine insulator that reforms the instant fresh metal is exposed. A loosening joint heats, oxidises, heats more, and the runaway ends at a scorched outlet.

The United States Consumer Product Safety Commission's field work found older homes with aluminum branch wiring were dramatically more likely to have connections that had reached fire-hazard temperatures than comparable copper-wired homes, and the accepted repairs are mechanical — a permanent crimped copper pigtail, or a rated connector designed for the dissimilar joint.

None of this made aluminum unusable. Alloys in the AA-8000 series were formulated specifically to resist creep, and aluminum service-entrance conductors and large feeders are ordinary, code-compliant practice today. It is the small solid branch conductor terminating in a general-purpose screw that earned the reputation.

Where the two metals touch, they corrode

Copper and aluminum sit far enough apart electrochemically that a direct joint between them in any damp environment is a galvanic cell, and aluminum is the side that dissolves. Utilities and electricians deal with this with bimetallic transition fittings, plated lugs and joint compounds rather than by pretending the couple is benign. It is a real constraint on retrofits: you cannot simply splice new aluminum onto old copper and close the box.

Fabrication splits the same way. Copper solders and brazes without ceremony. Aluminum's oxide skin makes soldering awkward enough that aluminum assemblies are more often welded, crimped or mechanically clamped.

Heat sinks split the difference

Copper moves heat about 1.7 times as well as aluminum, and yet the majority of heat sinks you will ever see are aluminum. That is a manufacturing verdict rather than a thermal one: aluminum extrudes into fin profiles cheaply and weighs little enough to hang off a circuit board without extra bracing. High-performance coolers hybridise — a copper base plate or copper heat pipes to pull heat out of a small hot spot fast, aluminum fins to dump it into the air over a large area.

Price, energy and the electrification squeeze

Both prices move too much to quote, but the shape of the difference is stable. Copper trades at several times aluminum's price per tonne, and because aluminum is also far less dense, the cost of the metal needed to carry a given current is a small fraction of copper's. That gap is why aluminum keeps winning the substitution argument in busbars, transformer windings and vehicle harnesses whenever copper spikes.

The two metals get expensive for opposite reasons. Primary aluminum is electricity in solid form — smelting consumes on the order of 13 to 15 megawatt-hours per tonne, so aluminum's cost floor follows power prices and its production migrates to wherever electricity is cheap. Copper's cost is a geology story: ore grades at the big porphyry mines have been falling for decades, and every new tonne comes from moving more rock. Recycled aluminum needs only a few per cent of the energy of primary metal, which is why the recycling loop matters more here than in almost any other commodity.

Copper has one commercial argument aluminum cannot answer at all: registered antimicrobial touch surfaces. Copper alloys kill bacteria on contact; aluminum does nothing of the kind.

Per amp, per kilogram, per terminal

• Overhead transmission and distribution — aluminum, with a steel or composite core.
• Branch circuits inside a building — copper, and no serious debate remains.
• Large feeders and service entrances — aluminum in a creep-resistant alloy, properly terminated, saves real money.
• Motor and transformer windings where efficiency is the product — copper, because losses are paid for over the machine's whole life.
• Anything where mass is the design constraint — aluminum, including car harnesses and aircraft.
• A joint between the two — a transition fitting, not a twist of wire.
• A door handle in a hospital — copper alloy, for a reason that has nothing to do with electricity.