How To Look At A House
McGarry and Madsen's home inspection blog for buyers of
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What is tinned copper wiring?
Wednesday, July 11, 2018
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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