: : Power transmission wiring (that's 'transmission' in the electric power : industry generation and distribution sense) is invariably aluminium, : not copper. When you're carrying a gazillion volts per wire between : transmission towers a quarter of a mile apart, it makes a lot of sense : weight-wise, and you lose a negligible conductivity benefit over : copper.
Actually, you lose a lot of conductivity: the resistivity of copper is about 16.8 nOhm m, pure aluminium is 27 and dural (I imagine that transmission lines would not be pure - too many fatigue problems) is around 50. However, since aluminium is around a third of the density of copper, you can simply use more of it: twice the area of pure Al will have 33% less weight and 25% more conductance than Cu.
In other words, it's not conductivity that matters but conductance.
Yours electropedantically,
Ian