Copper and aluminum conductor compared

The only fair comparison is conductance

Annealed copper defines the scale: 100% IACS, the international annealed copper standard. Electrical-grade aluminum sits at about 61% IACS. Read that as resistance rather than as a ranking. For the same cross-section, aluminum carries roughly three fifths of the current copper carries, and presents roughly 1.6 times the resistance.

Designers work in resistivity, which says the same thing per unit length. The figures below are the ones that turn up in cable tables, and they are standard properties of the two metals rather than measurements of any particular reel.

Why aluminum needs a larger conductor

Resistance falls as cross-section rises, so an aluminum conductor has to be bigger than a copper one to do the same work. The trade rule is about two AWG sizes larger. Aluminum 4/0 comes close to copper 2/0 for resistance, which is why 4/0 aluminum is a standard service and feeder size in buildings where 2/0 copper would otherwise have been used.

Weight runs the other way, and it runs further. Aluminum is about a third the density of copper, so at equal conductance aluminum weighs roughly half as much. That single fact explains aluminum on long overhead spans, on large feeders, and anywhere the cost of supporting and handling the conductor matters more than the cost of the metal.

The consequence for a seller is that size and metal cannot be valued separately. A 500 kcmil aluminum feeder is not a smaller 500 kcmil copper feeder. It is a different product that happens to share a number.

Thermal expansion belongs in the same conversation. Aluminum expands more than copper over the same temperature rise, so a joint that is merely snug works loose as the load cycles. That is why aluminum terminations specify a torque and often a re-torque, and why a reused aluminum joint is treated with more suspicion than a reused copper one.

Terminations and galvanic contact

Aluminum oxidises the moment it meets air. The oxide film is hard, self-healing and an insulator, which is why aluminum terminations are made by abrasion under an oxide-inhibiting compound rather than by simple tightening. Aluminum also cold-flows under sustained pressure, so a joint that was correct on the day it was made can relax over time. Torque, hardware and the connector listing all matter.

The connector carries the answer, and it is worth a photograph. A dual-rated lug is marked for both metals, and the marking reads clearly in a close frame of the barrel. A copper-only lug fitted to aluminum cable tells a reader that the installation was wrong, which says something about the whole lot rather than about one termination.

Put copper and aluminum in contact and add moisture and an electrolyte, and the joint becomes a galvanic cell. Aluminum is the anode and corrodes. The joint resistance climbs, the joint heats, and heat accelerates the corrosion. Listed bimetallic connectors, splice bars and dual-rated lugs exist for exactly this reason, and a copper-only lug on aluminum is a defect, not a shortcut.

Aluminum has its own trouble without copper in the picture. In contact with concrete, mortar or wet steel it corrodes readily; copper does not. On the coast, airborne chloride makes every one of these mechanisms faster. Old solid aluminum branch-circuit wire has a poor record at devices, which is why modern aluminum building wire is made from AA-8000 series alloy rather than the older EC-grade metal.

Telling them apart in the yard

Colour and weight settle nearly all of it in under a minute. Where they do not, the cut end and the jacket print will.

• Cut end. Copper is red-brown. Aluminum is silver-white and dulls to grey quickly in damp air.
• Weight for the same size. Aluminum runs about 30% of the copper weight. Hold a coil of each and the difference is obvious.
• Surface. Aluminum leaves a grey smear on paper or glove. Copper does not.
• Stiffness. Aluminum is springier at the same size and fights a bend. Copper bends and stays bent.
• Jacket print. Read along the legend for COPPER, CU, ALUMINUM, AL or ALUMINUM ALLOY. It is printed at intervals and usually survives a lost tag.
• Magnet. Neither metal responds, but the magnet finds a steel core, steel armour or a steel interlock in seconds.
• Tinned copper. Silver on the outside, copper at the cut. A file or a fresh cut answers it.
• Copper-clad aluminum. Copper skin, white core, and much lighter than it looks in a photograph.

Why a lot is never described as one metal

A reel of copper and a reel of aluminum on the same pallet are two lots, even when they came off the same job. So are two reels in one photograph with one description. The two metals are reviewed on different bases, and a description that merges them forces the reader to assume the worse of the two for everything on the list.

Where a reel cannot be identified, say that and photograph the cut end. An honest unknown is workable. A wrong certainty is not, because it casts doubt on every other line in the same manifest.

Copper-clad aluminum deserves its own line rather than either heading. It photographs like copper, weighs like aluminum, and is a third product with its own handling. Name it as CCA and let the reader decide.