Sizing of cables in randomly-filled trays with consideration for load diversity

Method for demonstrating increased ampacity of cables in trays with loading diversity. Ampacity tables for sizing cables in randomly-filled cable trays are provided in NEMA WC 51-1986 [l] based on a model developed by J. Stolpe which ensures that the maximum cable teimperature does not exceed the insulation rating (typically 9OoC) under worst-case conditions [2]. The Stolpe model intentionally disregards the reduced heating effect of deenergized or lightly-loaded cables to ensure that all possible hot spot conditions are enveloped. In recent years other methods have been proposed to credit loading diversity in order to justify increased ampacity. However, since they involve certain assumptions about the heat distribution within the cable mass, these methods may fail to identify individual overloaded conductors. This paper describes a isimple method which considers the performance of individual conductors while providing a means of increasing ampacity as a result of loading diversity.