Rating of cables in a nonuniform thermal environment

Abstract

When cables cross external heat sources or cross the areas of high thermal resistivity, the conductor temperature will be higher than the values attained outside of the unfavourable area. For perpendicular and oblique crossings as well as street crossings, these effects are usually ignored for distribution circuits, whereas for transmission cables, corrective actions in physical installation condition are sometimes taken. Almost never are analytical solutions used to determine the effect of external heat sources and high thermal resistivity regions on the ampacity of the rated cable. The main reason no computations are performed is an absence of either derating formulae or derating tables (curves) and nor the lack of a need. To fill this gap, analytical solutions for the computation of the derating factors have been developed and sample computational results are presented in this paper. An alternative approach to address the issue of higher operating temperatures is to design and install dynamic feeder rating systems. This topic is also discussed in the paper.