Difference between overhead cables and under ground cables?

Answer 1

The principle difference between underground and overhead cable is thermal dissipation.

With overhead cable, heat can dissipate into the air, and wind can improve the rate of thermal dissipation. On the other hand, overhead cable is exposed to solar gain that will result in some heating.

With underground cable, heat dissipation is governed by the thermal conductivity of the soil. The Neher-McGrath formulas can be used to calculate thermal conductivity and the required derating of conductors.

Comparing the two, an overhead cable generally has a higher thermal ampacity than an underground cable of the same material, dimensions and insulation.