Copper or aluminum: telling them apart on a reel

Copper and aluminum are both used as conductors in Tennessee plants and on utility lines, they are stored in the same yards, and in poor light they can look similar enough to confuse. The differences that matter to a buyer are conductivity, weight, how each metal is terminated, and how much documentation is needed before a lot can be placed with a new owner.

For a seller the practical question is not which metal is better. It is which metal you have, in what alloy and size, and whether the reel says so. Copper lots are usually simple because the material is familiar and its recovery value is well understood. Aluminum lots need their paperwork in better order, because resale turns on alloy and on terminations rated for it.

Copper sets the reference. Annealed copper is treated as 100 percent IACS, the international standard for conductivity, and every other conductor metal is compared against it. Aluminum reaches roughly 61 percent of that figure, so for the same cross section it carries less current and runs warmer at the same load. To match the ampacity of a copper conductor, an aluminum one has to be about two AWG sizes larger.

Weight reverses the comparison. Copper is roughly 3.3 times denser than aluminum, so the aluminum conductor that matches a copper one in ampacity weighs far less even though it is physically larger. That single ratio explains most of where each metal is used in practice. Aluminum wins wherever weight and span matter, and copper wins wherever space is tight or the run is short enough that the size penalty does not matter.

Both figures are averages for the metals in their usual conductor tempers, and the practical consequence is the same either way. You cannot swap a size for a size between the two metals and keep the circuit unchanged. Ampacity tables, voltage drop, conduit fill and termination hardware all key off the metal as well as the size, which is why the tag matters more than the look of the conductor.

• Copper is the conductivity reference at 100 percent IACS
• Aluminum is roughly 61 percent, requiring a larger size
• Copper is about 3.3 times the density of aluminum
• Equal ampacity favors aluminum on weight
• Equal space favors copper on size

Aluminum trades weight for size. Copper trades size for weight, and pays for it.

Start with the tag and the jacket print. Aluminum cable is usually marked AL, sometimes ALUM, and copper CU, and the type string often carries the metal: USE-2 RHH RHW-2 AL, for instance. Where the tag has gone, the stencil on the flange may still carry a part number, and that number will decode through the manufacturer's catalog into metal, size and insulation in a single step. A part number is worth more than a photograph of a faded label, because it can be looked up exactly.

Do not rely on color. Bright copper is red-brown and aluminum is silver, but tinned copper is silver as well, and weathered aluminum can take a dull brown cast from a gravel yard. A scratch shows bright metal under either surface, so the test that feels decisive tells you very little about which metal you are holding. Light is the problem: in a shed at dusk, silver is silver. Take the reel into daylight before you decide.

Weight is a better check and the cut end is better still. Lift two reels of the same nominal size and the aluminum one is obviously lighter, though a full reel against a half reel will fool you badly. At the cut end, aluminum strands are softer and mark more easily with a thumbnail, and a magnet will find a steel core or steel armor, though it says nothing about copper against aluminum.

• Read AL, ALUM or CU on the tag and jacket print
• Check the type string, which often names the metal
• Expect tinned copper to look silver like aluminum
• Compare two reels of the same size by weight, not by eye
• Photograph the cut end where the tag has gone