Is Aluminum Electrical Cable an OK Substitute for Copper?

Q:

I want to upgrade the relatively low-amp electrical cable running to my garage so that I can convert the space to a woodworking shop. There’s a significant difference in price between 60-amp copper cable and 60-amp aluminum cable. What trade-offs come along with saving a few bucks?

Justin Fink, Plainville, CT

A:

Electrician Brian Walo writes: You need to consider two things with aluminum cable: first, whether you’re allowed to use it in your area; and second, how much bigger the wire would have to be compared to copper.

Aluminum was outlawed in branch-circuit wiring due to its tendency to expand and contract with temperature changes, as well as its susceptibility to oxidation over time, both of which can result in poor connections and a fire hazard.

However, in larger-capacity circuits, such as a main service entrance for a house, or in a situation such as yours—a feeder from the main electrical panel to a subpanel—aluminum cable may still be permitted. Aluminum is certainly approved for those uses by the National Electrical Code (NEC), but the final word is always left to your local building officials.

I have no problem with using aluminum in a situation such as yours. Compared to copper, it’s less expensive and a lot lighter. There are a few things to know about choosing and using it, though.

Differences in the loadcarrying capacities of aluminum and copper are where you need to do your homework. For example, a 2/0 (pronounced two-aught) copper service-entrance cable is good up to 200 amps, but you would need 4/0 aluminum—a much thicker cable—to carry the same load. These differences in size are less of a concern in your 60-amp setup, but they can complicate high-amp installations.

For example, your situation likely will call for #4 and #6 aluminum conductors (see photo), which won’t feel much bigger or stiffer than the slightly smaller-gauge copper conductors you would need to carry the equivalent load. If you’re lucky, you may even be able to run the new conductors through your existing conduit (minimum 1-in.-dia. PVC or EMT, in this case). As you get into higher-amperage applications, however, the added thickness and stiffness of aluminum cable can make retrofit applications trickier. It’s more difficult to bend for changes in direction, and it’s harder to snake through holes and to pull through conduit.

You also may hear that aluminum conductors will corrode, which is true, but not the whole story. Both copper and aluminum will oxidize when exposed to the atmosphere, but the difference is that the corrosion on copper is conductive, so it doesn’t cause problems with the flow of electricity. With aluminum, the corrosion is preventive and reduces the flow of electricity. When running aluminum wiring, you should always apply a liberal coating of antioxidant compound to all connections to eliminate potential problems.

As usual, your best source for information is the current version of the National Electrical Code, your local building department, or a licensed electrician familiar with the rules in your jurisdiction.

From Fine Homebuilding #240