Telling copper cable from aluminum

Start at the cut end

Everything else is inference; a cut end is evidence. Cut a few inches off with a proper cable knife or a fine-tooth saw, square it up, and look at it in daylight. Copper is a warm red-brown, dulling to a darker brown as it oxidises. Aluminum is a cool silver-gray that goes chalky white with oxide. Stranded conductors show the color on every strand, which is a useful check against a painted or sleeved outer surface.

Tinned copper is the common trap. A tin coating makes a copper conductor look silver, and on an old cable the coating may be patchy, with copper showing through where the strands rub. Scrape a strand with a knife blade: tin comes away as a thin bright layer over red metal, aluminum stays silver all the way through. Copper-clad aluminum is the reverse trap — it looks convincingly copper on the outside and shows a white core when cut, which is why the cut, not the surface, is the test.

If the end has been sealed with a heat-shrink cap or a putty compound, cut behind the seal only on cable you are certain is dead and released. That certainty belongs to the owner's qualified crew, not to a seller with a knife, and the safety guide on this site explains why the two questions are not the same.

Color, weight and the magnet

The three quick tests answer different questions, and it is worth knowing which one you are actually asking.

• A magnet sticking to a conductor means steel core or steel armour, not aluminum.
• A magnet sliding off tells you only that no steel is present.
• Weighing a known length is the only field test that quantifies the metal.
• When two tests disagree, trust the cut end.

Where each metal turns up

Copper dominates anything where current density, termination reliability or a small cross-section matters. Medium-voltage and high-voltage underground cable in New York is very often copper, particularly in the dense downstate network where ducts are full and conductor sizes are pushed hard. Building wire, control cable, motor leads, grounding conductors, fire alarm and instrumentation are copper almost without exception. Copper is also the default for anything that will be spliced in a wet location, because the metal tolerates it better.

Aluminum dominates where weight and cost per amp matter more than size: overhead conductors of every kind, large feeder and service entrance cable, utility service laterals, and the big 600 V feeders in industrial plants. Aluminum is entirely serviceable in those roles when it is installed with the right connectors and the right inhibitor. What it does not tolerate is improvisation — a mismatch between conductor and lug is where most aluminum failures begin.

The practical consequence for a seller is that a lot's copper content is usually concentrated in a minority of the reels. A yard full of large aluminum feeder and a handful of copper control and medium-voltage reels will carry most of its value in the small pile. Sorting by metal before a buyer arrives is the single most effective thing a seller can do, and it costs a knife and an afternoon.

Termination and the bimetal problem

Aluminum forms an oxide layer within seconds of being exposed to air, and that layer is an insulator. Terminations therefore depend on abrading the surface, applying an oxide-inhibiting compound, and using a connector listed for the metal — the AL7 and CU9 markings on a lug exist for this reason. Copper does not need the same ceremony, although it also benefits from a clean surface and a correctly torqued connection.

The harder issue is where the two metals meet. Copper and aluminum in contact, with moisture present, form a galvanic couple: the aluminum becomes the anode, corrodes, and the joint heats. Bimetal connectors and transition splices exist precisely to separate the metals, and twisting a copper conductor onto an aluminum one is not an acceptable substitute. Aluminum also creeps under sustained compression and expands more with temperature, so a termination that was tight when made can loosen over years of load cycling — which is why aluminum terminations are inspected and re-torqued, and why a seller's honest note that a cable came out of a failed joint is useful information.

For valuation, this mostly matters as a source of doubt. Cable cut out of service with a scorched lug, a melted splice body or a heat-discolored conductor end has a history, and that history is visible. Photograph it and say what it is, rather than cutting the damaged section away and hoping nobody notices that the length has shrunk.

Why copper content is a floor, not a price

A copper conductor has a value that can be realised by melting it, minus the cost of getting it to a furnace and the loss in the process. That is the floor, and it is remarkably reliable: it does not care about the voltage class, the insulation type, the standard, or whether the reel tag survived. It is the number a buyer can defend without knowing anything else about the lot.

The actual price is set by what the cable can still do as cable, which is usually more than the metal is worth. Reuse value depends on identity, footage, condition and documentation — the four things that turn a heavy object into a specified product. So copper content tells you the worst case and almost never the offer. Sellers who lead with the metal weight are negotiating on the floor, and should not be surprised when the offer sits near it.

Aluminum has no comparable floor in the same sense: the metal is worth far less per pound, so a lot of unidentified aluminum cable is priced on handling and freight, and a distant yard can be worth nothing at all. That asymmetry is why a mixed pallet of copper and aluminum is worth more separated than together, even though the combined weight is unchanged.