Ampacity reduction factors for cables crossing thermally unfavorable regions

When power cables cross regions with unfavorable thermal conditions, temperatures higher than the design value can occur. If the region is wide enough, the rating of the cable will usually be based on the assumption that the entire route is characterized by the same conditions. In a majority of cases, the unfavorable thermal environment will be very short, usually a few meters (e.g., street crossing). In these cases, the effect of the crossing is usually ignored. However, the conductor temperature in such cases may be much higher than in the remainder of the route and cable derating is required. Only rarely are analytical solutions used to determine the effect of unfavorable short sections of the route on the ampacity of the rated cable. The main reason no computations are performed is an absence of either derating formulas or derating tables (curves) and not the lack of a need. To fill this gap, an analytical solution for the computation of the derating factors has been developed and is presented in this paper. The solution is simple and accurate enough to be suitable for standardization purposes. A numerical example involving a pipe-type cable crossing a street is presented to show the effect of street crossings on the ampacity of the cable circuit. In this practical example, the ampacity of the pipe-type cable has to be derated considerably.