Enhanced Thermal Model of Power Cables Installed in Ducts for Ampacity Calculations

One of the most widely used installation types of power cables is inside nonmetallic ducts. Ampacity calculations for this type of installations were summarized back in 1957 by Neher and McGrath. The method has been standardized in the IEC 60287 family of standards. It is shown in this paper that there is a noticeable mismatch between the IEC standard calculations for steady-state conditions with experimental tests and finite-element method simulations. After an in-depth investigation, it was found that the source of the discrepancy between reality and the standard is an inaccuracy in the calculation of the heat transfer of the air between the cable surface and the internal duct surface. The standard calculations simplify the problem too much, resulting in relative large errors. A more precise formulation, yet simple enough for computer implementation, is proposed in this paper. The accurate calculation of the steady-state temperature is obtained from first heat transfer principles. Numerous laboratory experiments and finite-element simulations are used to evaluate the accuracy of the proposed method.