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Aluminum wires can't be connected directly to copper wires - otherwise the contact spot overheats badly, such connections should only go through steel connectors. I was rather surprised when I found that.

What other similar limitations to wire materials are there for wire connections? Can I connect anything to anything with the only exception of not connecting copper to aluminum?

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Other than tinned copper, I'm not familiar with any other kinds of common wire. Copper and alu can only be connected in an AL/CU approved (CSA/UL) compression connector. NOLOX on this type of connection is recommended as well. What kinds of wire are you thinking of? – shirlock homes Nov 25 '11 at 10:41
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@shirlock homes: Steel wire, maybe some special purpose wire like one used in heating elements. – sharptooth Nov 25 '11 at 10:50

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There are aluminum to copper wire nuts available, like Ideals 30-16. Basically it is a wire nut filled with NOLOX. It is made mostly for mobile homes that have aluminum wire and updating devices that might have failed or if you want to change the color, etc... They are not cheap, about over a couple of dollars each. You can but them on the internet or at mobile home repair shops.

Aluminum to copper wire nuts

Never use wire nuts that are rated copper only. If you do you are looking at a failure in the connection for sure.

EDIT - For larger cable (I just saw mains in your question) you can used al/cu rated split bolts. Use plenty of NOLOX then wrap in rubber tape and then a good electricians tape, not the cheap black tape.

Split Bolts

Do not use the copper split bolts, they are rated copper only. Make sure the connector shows al/cu ratings on it.

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Note: cu/al is obsolete, your new stuff should all show the CO/ALR rating. – gregmac Nov 28 '11 at 3:19
From your link, "CO/ALR applies only to standard light switches and receptacles; CU/AL is the standard marking for circuit breakers and larger equipment." Either way, describing split bolts or wire nuts with al/cu works. – lqlarry Nov 28 '11 at 5:45

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