Ampacity of diversely loaded cables in covered and uncovered trays

摘要

The existing code for ampacity of cables in a tray does notnaccount for load diversity among the cables and it does not consider thenpresence of tray covers. This paper provides two factors that can benused to determine ampacity values for these two cases. These factors cannbe used in conjunction with existing code ampacity values so that cablenampacities can be calculated for diversely loaded, covered and uncoveredntrays. Also, the case of a few heavily loaded cables in an otherwisenlightly loaded tray is addressed. This particular situation can producennonconservative ampacity values if treated with a one-dimensional heatntransfer model. The problem of both load diversity, and the presence ofna cover are addressed with a computer code that has been described innprevious papers. The computer model is designed to provide conservativenampacity values by assuming that the more highly-loaded cables arenplaced along the tray centerline and the lightly-loaded cables arenpositioned on the outer surfaces of the cable bundle. In this way thenheavily-loaded cables are insulated from the environment and thus thenprogram calculates a conservative cable temperature. The factors thatnaccount for load diversity show that a small percentage of cables in thentray can be loaded significantly beyond the code allowable ampacitynvalue if the remainder of the cables are lightly loaded or unenergized.nOn the other hand, the code-allowable ampacity must be reduced by up ton25% when a solid cover is placed over the tray