Why outlet makers warn against aluminum wires?

overhang - when was your house built?

>>"I have copper wires going from the box into the wall but the ends coming out of the wall are for some reason dull silver colored.That is why i assumed they're aluminum.Can it be some other material?How can i find out?Thanks."<<
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If your home was built in the 50s, or ealier, it is possible the conductors are tinned copper.

With the circuit turned off you can take a knife and scrape the tinned coating off exposing the copper conductor.

Some homes built in the middle to late 70s were wired with AL romex.
Jim

Daddy0
I believe the plastic cone wire nuts you reffering to are the MARRETTES as they called in the trade.
George

Aluminum wire will vibrate with the frequency of the signwave. In this case, 60hz. This causes it to loosen from any connections other than solid crips. When it loosens, you get arcing at the connection. It's also is soft so it will loosen up over time. You need to retighten at least every year.
Al wire is not compatible with copper and cannot be connected together without special al/cu connectors. They do make a grease for this also.
Al wire must be sized larger than copper for current carrying capacity (usually one size bigger eg; #10 copper you would use #8 al.)
A lot of trailers use al and older homes. They used a special al/cu outlet for this purpose. Inserting copper blades from the plug has no effect. You can plug most any connector wire into an outlet with no problem. Just because the conductors are al doesn't mean the plug connector is. It is usually copper(or an alloy.)

When multiple dedicated circuits are used for audio/home theater is it necessary to run all of the 110V circuits off the same side of the 220V feed?