CALCULATION OF THE INTERNAL THERMAL-RESISTANCE AND AMPACITY OF 3-COREUNSCREENED CABLES WITH FILLERS

The equations given in IEC 287 and in the Neher-McGrath paper for the internal thermal resistance of 3-core cables were developed for paper insulated cables. For such cables, it was assumed that the insulation and filler materials have the same thermal resistivities. In reality, in 3-core cables a variety of materials are used as fillers. The major ity of these will have a higher thermal resistivity than the insulatio n. In this paper, a new formula is developed to compute the value of t his thermal resistance taking into account the thermal resistivity of the filler. The effect of filler resistivity on cable ampacity is also discussed.