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Ampacity of Cables in Single Covered Trays

A mathematical thermal model is developed to predict the operating temperatures of cables in a single covered tray when there is load diversity in the power cable bundle. The model accommodates two different loading scenarios: one in which the heat is distributed evenly across the cable tray cross section; and one which concentrates the heavily loaded cables along the centerline, while surrounding them with more lightly loaded cables. The temperature predictions provided by the model are compared to data found in other IEEE papers, data collected in laboratory measurements, and new data from a four-year study of cable trays in an operating nuclear plant. Reasons for differences between the field data and the computer results are discussed. The model is used to evaluate the conservatism in the available Codes and Standards. A derating factor is introduced that is defined in terms of the ampacity of power cables in open-top trays. The derating factor accounts for the added thermal resistance present when a cover is placed over the cables, trapping a layer of stagnant air on top of the cable mass. The computer model is then used to predict values for the derating factor as a function of cable depth. The derating factor is shown to be independent of the composition of cables in the tray. The presence of a cover is shown to reduce the ampacity based on an uncovered tray by up to 25 percent depending on the depth of the cables in the tray.