Cable crossings-derating considerations. I. Derivation of deratingequations

摘要

Dangerously high interference temperatures can occur at pointsnwhere cables cross external heat sources even when the crossing occursnat 90°. For perpendicular and oblique crossings, these interferencentemperatures are usually ignored for distribution circuits, whereas forntransmission cables, corrective actions in physical installationncondition are sometimes taken. Analytical solutions are almost nevernused to determine the effect of external heat source on the ampacity ofnthe rated cable. The main reason no computations are performed is annabsence of either derating formulas or derating tables (curves) and notnthe lack of a need. To fill this gap, an analytical solution for thencomputation of the derating factors has been developed and is presentednin this paper. The solution is simple and accurate enough to be suitablenfor standardization purposes. A numerical example involving thenintersection of a pipe-type cable by a distribution circuit is presentednto show the effect of perpendicular and oblique crossings on thenampacity of both circuits. In this practical example, the ampacity ofnthe pipe-type cable is significantly affected for a range of crossingnangles. A conservative practice, used by many utilities in cases likenthis, would be to assume that the cables are parallel. However, in ournexample for a 90° crossing, such an approach would unnecessarilyndecreases the ampacity of the pipe-type cable by almost 20%