Copper Efficiency

Copper does not have efficiency. It may have less line losses.

I think by copper efficiency one means the ratio between used copper (current flowing through the conductor) over the total amount of copper in the machine at any moment in time.

In most machines not all conductors are loaded at any point in time. There still are some "dead spots" on the stator or rotor, dependant on the orientation of the rotor and how the machine is winded. So when a conductor has no current (f.i. while inversing current orientation), the copper is not used, thus lowering the copper efficiency.

Manufacturers try to raise copper efficiency as high as possible while defining the spec of the machine (high voltage - low current or low voltage - high current).

You Are looking for Copper conductivity?

In AC transmission there is a phenominon with a frequency called SKIN EFFECT, which will reduce the effective conductor cross section area ( i.e, the area which the electrons are drifting ) but it is not in the case for DC transmission.So the Cu was used efficently in DC Lines.

But this saving in copper may be offset by its switchgear.