Extended Values for Geometric Factor of External Thermal Resistance of Cables in Duct Banks

In the Neher-McGrath analysis for calculating the external thermal resistance between cables in duct banks and ambient earth, approximate formulas for the geometric factor of the duct bank are employed which are valid only for a limited range of the height/width ratio of the cable bank. This limitation poses difficulties for power cable engineers when calculating external thermal resistances of some cable systems. This paper describes an efficient finite-element-based technique for calculating geometric factors for extended ranges of the height/width ratio and presents results for a wide range of values in a direct tabular and graphical format suitable for conventional Neher-McGrath calculations.