This function computes the overall capacity of a cable.
When the voltage level is medium or high, the cables have geometric parameters that can be used to compute the capacity as
C = lCABLE 2 π ε0/log(GMD/r),
where lCABLE is the length of the cable, ε0 is the dielectric constant of the air, GMD is the geometric mean distance, and r = d/2 where d is the inner diameter of the cable.