Ampacity of Electric Power Cables in Vertical Protective Risers

A method is presented to compute the ampacity of an electric power cable located in both vented and nonvented circular protective risers. While techniques for calculating the ampacity of underground cables and overhead lines are well established, techniques for rating the riser portion of a cable circuit have not been as thoroughly studied. Usually cable ampacities are based on the assumption that the underground portion of the circuit will be cooler than the portion located in the riser. Therefore, approximate derating factors are used to reduce the underground cable circuit ampacity when risers are present.