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