In big AC transmission lines, there's a complex optimization involving
wire weight, tensile strength, ohmic losses, skin effect, corona
losses, wire cost, and tower spacing/cost.
No shit.
For one thing, they are primarily designed for high tensile strength
as they have to stay mounted through all weather and environmental
conditions.
After that, their resistance is an issue as the primary material has
to be steel for the tensile forces involved. They usually get clad in
Aluminum as copper is too costly for such long runs, and the losses in
using aluminum are little in comparison. This is also the reason that
high voltages are used in long haul transmission lines. The loss
over 2000 feet of line with 120 volts on it is significantly different
than the loss over 2000 feet of line with 20,000 volts on it.
Corona will become a problem as that line voltage is raised. At
that time line spacing becomes an issue.
Tower spacing is a function of the terrain being traversed. Line
spacing ON a given tower design is a function only of the voltage that
is proposed to be carried, and the total number of conductors.
Skin effect, in these high tension line realms is only an issue if
the idiots that made the wire didn't know how deep to make the
cladding. If the wire is clad to thinly, there will be more loss as
the steel is more resistive, and the wire will heat more as well.
If it is clad too thickly, an unnecessary cost is introduced.
This is specifically because the skin depth is so deep at this
frequency, NOT due to it being a thin depth! So in power line cases,
the effect is an issue of how deep the cladding is, not how thin.
In RF transmission lines, which are typically nickel or silver
plated, it becomes a cost issue, and claddings are made as thin as
possible for a given application frequency. These cases are where one
will see hollow conductors, or plated tube or solids. This is where a
Litz configuration or plated conductor will assist one in design of a
circuit.
At 60Hz, a high voltage step up transformer will have some transfer
efficiency number. At switching frequencies, the same transformer
design (wire turn count wise) will operate better if the primary, and
or secondary have litz wire used in them as the effective resistance
of the winding will be reduced at the higher frequencies.