Why does the aluminum alloy mechanical connector (screw type) also work well with copper wire?
Copper wire has a lower rate of expansion and does not over compress itself under the screw of an aluminum connector. High tensile aluminum alloy connector bodies maintain enough elastic (spring) compression load on the copper wire to maintain contact pressure on the copper wire even when the aluminum connector itself may expand. Soft copper wire work hardens when compressed (unlike aluminum wire which does not harden) and so copper can resist creep better than soft aluminum wire. The aluminum alloy lug or connector provides for the “gas tight” joint needs of both copper and aluminum wire and are therefore UL/CSA “dual rated” meaning they can be used with either wire material, while maximizing cost savings.