Aluminum or tinned copper?

I looked and the silver color immediately told me aluminum, but the insulation looks older than the 60’s or 70’s. This house was built in 1903, originally wired with K&T, 60 amp service (so probably in the 20’s or 30’s). It had an addition put on in the late 50’s.

It is also the only suspect wire in the entire panel - everything else is obviously copper.

I scratched it and it stayed the silver color.

I’m thinking this may be a leftover KT wire based on the insulation. Your thoughts please?

(For those that just say “Report what you see & move on” or something about reaching into a panel, this is in my house & I opened the main…I’m trying to learn something.)


Tinned Copper, looks too old to be aluminum. Just loosen it to get a look at the cut end or find the corresponding ungrounded conductor(hot). It will be aluminum too if this one is.

Tinned copper to facilitate soldered connections for K&T. Don’t need to mess with it. You know what it is.

I agree tinned copper. Copper had a coating of tin to keep it from reacting with the old rubber insulation.

Aluminum will have a plastic insulation over the conductor, not fabric and rubber.

Exactly as others above have already stated.

Thank you all - great information

I like how you gently corrected my soldering comment.

From Wikipedia

On reflection, tinning would be unnecessary to facilitate soldering connections with copper.

Thanks Robert


Tin (cloth/rubber)

AL (Plastic)

TY Billy

BTW Chuck - Roy is in your camp. 7 YO thread

This board needs some stickies or FAQ’s or something. There’s a lot of valuable information, but it’s difficult to find.

Thanks but I am no longer in that camp. Roberts’ gentle correction got me thinking: I’ve soldered a lot of copper in my days, it needs flux, but doesn’t need to be pre-tinned to solder well. When I get to thinking, I research. When I research, I learn.

I am now the wiser for Roberts’ comment above.