The equations given in IEC 287 and in the Neher-McGarth 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 majority of these will have a higher thermal resistivity than the insulation. In this paper, a new formula is developed to compute the value of this thermal resistance taking into account the thermal resistivity of the filler. The effect of filler resistivity on cable ampacity is also discussed.
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