What is the difference in current carrying capacity of underground and overhead cables of same specs?

Question

The mathematical relationship and factors which affect the current carrying capacity of overhead and underground cables of same specifications for same application?

Answer 1

Underground cables of the same diameter as overhead lines are de-rated for current, because of increased heat dissipation in bundled cables.  

Underground cables cannot be placed several feet apart from each other (phase) -- they must be tightly packed and insulated from each other.  Electrical insulation is generally a good heat insulator, so the wires tend to get hotter for a given amount of current.

Most of the mathematical relationships are empirical in nature from years of trial-and-error.  Cable ampacities for all cables are given in tables and charts in reference materials on the subject.

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Answer 2

It will be the same in current capacity.