Ampacity derating factors for cables buried in short segments of conduit

560 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 20, NO. 2, APRIL 2005 Ampacity Derating Factors for Cables Buried in Short Segments of Conduit Pascal Vaucheret, R. A. Hartlein, Senior Member, IEEE, and W. Z. Black, Fellow, IEEE Abstract—Buried cables are often routed through short seg- equiv- conductor property of a single cable that is ther- ments of conduit, and when this situation occurs, the ampacity mally equivalent to the conductor property of a must be reduced or the cable will overheat as a result of the triplexed cable high thermal resistance created at the location of the conduit. equiv-insul insulation property of a single cable that is ther-This problem is examined for extruded cables by using a finite mally equivalent to the insulation property of aelement heat transfer software program to determine the derating in ampacity that cables in conduits must experience in order to triplexed cable remain below a maximum conductor temperature. The derating insul value for cable insulation material factors are provided as a function of conduit length, soil resistivity, soil value for surrounding soil. burial depth and number of cables in the conduit. The results show that once the length of conduit exceeds about 20 times its I. INTRODUCTIONouter diameter, then the ampacity of the circuit must be reduced to the value that it would have if the entire length were buried in INCE the mid 1900’s the accepted calculation of under- the conduit.