There is an earlier thread entitled "help with transmission line losses" (if I remembered the title correctly)... You should be able to find it with the advanced search of this site if you look for the phrase "dielectric losses." A number of other people posted to that thread, so it might be worth taking a quick look.
The choice of insulation material can make some difference
due to the dielectric constant of the insulation material. The diameter affects the average voltage stress on the insulation, so increasing that is another way to control dielectric losses, but that has a trade-off with the cost of insulation materials.
The equation for dielectric loss from the Neher McGrath paper on
calculation of temperature rise of cables (IEEE Transactions Oct. 1957 -
there have been some updates to this, too) is:
Wd = 0.00276(E^2)(SIC)(insulation PF) / log[(2T+Dc)/Dc]
Where:
Wd is dielectric loss in watts per conductor foot,
E is phase to neutral voltage in kV,
SIC is relative dielectric constant of the insulation material,
Insulation PF is the power factor of the insulation material (also called
tangent delta),
T is the insulation thickness,
Dc is the diameter of the conductor (over the semicon shield).
This equation only calculates dielectric losses for a single conductor cable, so if you had in mind the comparison of other factors like inductive losses of three phase cables, it gets a little more involved, but this should give you a start.