Thermal characteristics of shuttle car cable reels

The present method of calculating the ampacities of power cables that are wound on a cable reel involves the use of derating factors that are published in the Insulated Cable Engineers Association/National Electrical Manufacturers Association Standards (ICEA/NEMA) Publication WC8. These factors consider only one to four layers of cables and it is recommended that the derating values be multiplied by the ampacities of single cables suspended in air. No guidance is provided for installations that have greater than four layers of cables. This paper presents a mathematical model that is capable of calculating the ampacity of a wide variety of cable designs consisting of an arbitrary number of layers on a cable reel. The model considers round cables with copper conductors. The validity and accuracy of the ampacity model were verified by comparing the predicted temperature distribution within the reel with measured temperatures collected during an extensive testing program conducted at the US Bureau of Mines (USBM). The mathematical model predicted a temperature distribution within the cable layers that was very close to the measured variation in temperature. The value of the program is illustrated by calculating ampacities for several copper conductor sizes. Those ampacities suggest that the ICEA/NEMA derating factors do not lead to conservative temperatures for modern designs when they are wound on reels. This conclusion can be very significant for those applications, such as trailing cables used in mobile mining equipment, in which the overheating of a cable reel can result in a hazardous situation