Ampacity Derating Factors for Cables Buried in Short Segments of Conduit

Buried cables are often routed through short segments of conduit, and when this situation occurs, the ampacity must be reduced or the cable will overheat as a result of the high thermal resistance created at the location of the conduit. This problem is examined for extruded cables by using a finite element heat transfer software program to determine the derating in ampacity that cables in conduits must experience in order to remain below a maximum conductor temperature. The derating factors are provided as a function of conduit length, soil resistivity, burial depth and number of cables in the conduit. The results show that once the length of conduit exceeds about 20 times its outer diameter, then the ampacity of the circuit must be reduced to the value that it would have if the entire length were buried in the conduit. Factors that result in lower cable ampacities, such as high soil thermal resistivity and deeper burial depths lead to larger derating factors.