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

A method for demonstrating increased ampacity of cables in trays with loading diversity is given. Ampacity tables for sizing cables in randomly-filled cable trays are provided in NEMA WC 51-1986 based on a model developed by J. Stolpe which ensures that the maximum cable temperature does not exceed the insulation rating (typically 90°C) under worst-case conditions. 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. 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 simple method which considers the performance of individual conductors while providing a means of increasing ampacity as a result of loading diversity