Older electrical wire insulation was made from natural gum rubber that was “vulcanized” by the addition of sulfur or other curing agents to harden it and make it more durable. These curing agents were found to corrode the copper, so manufacturers coated copper wires with a tin coating as a corrosion preventative. Then, as a final step, cotton fabric was wrapped around the rubber insulation for additional protection from damage. 

   The tinning makes the wires look like aluminum, but there are several ways to tell the difference between the two silver-color wire materials:

Tinned copper was manufactured up to the late-1950s and we typically see it in older homes as the multi-strand wiring to a kitchen range breaker in the panel, but silver-color wire in homes built from about 1960 onward is aluminum.

The fabric outer wrap, like in the photo above, indicates tinned copper, and it is typically slightly frayed behind any wire connections. Smooth thermoplastic insulation means aluminum.

If you look closely at the area of exposed wire at the connection to a breaker or lug, you can usually find tiny nick marks in the metal with the underlying copper peeking through. Also, if you are able to view end of the cables at the service lugs, you will get a good view of the cross-section with copper in the center.

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Here’s links to a collection of our other blog posts about ELECTRICAL WIRING:

• Which house appliances need a dedicated electrical circuit?

• Can a short circuit cause a high electric bill?

• What is the maximum spacing requirement for securing NM-cable (nonmetallic-sheathed cable)?

• Is it alright to just put wire nuts on the end of unused or abandoned NM-cable or wiring?

• What causes copper wires to turn green or black in an electric panel?  

• What are typical aluminum service entrance wire/cable sizes for the electrical service to a house?

• Why is it unsafe to bond neutral and ground wiring at subpanels?

• Should I get a lightning rod system to protect my house?

• Why is a strain relief clamp necessary for the cord connection to some electric appliances?  

• Does a wire nut connection need to be wrapped with electrical tape?

• What is the minimum clearance of overhead electric service drop wires above a house roof?

• What are the requirements for NM-cables entering an electric panel box? 

• What is the color code for NM cable (Romex®) sheathing?

• Why is undersize electric wiring in a house dangerous? 

• What causes flickering or blinking lights in a house?

   Visit our ELECTRICAL page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.