Ampacity reduction factors for cables crossing thermally unfavorable regions

When power cables cross regions with unfavorable thermal conditions, temper atures 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 case s, the unfavorable thermal environment will be very short, usually a few me ters (e.g., street crossing). In these cases, the effect of the crossing is usually ignored. However, the conductor temperature in such cases may be m uch higher than in the remainder of the route and cable derating is require d. Almost never analytical solutions are used to determine the effect of un favorable short sections of the route on the ampacity of the rated cable. T he main reason no computations are performed is an absence of either derati ng. formulas or derating tables (curves) and not the lack of a need. To fil l this gap, an analytical solution for the computation of the derating fact ors has been developed and is presented in this paper. The solution is simp le and accurate enough to be suitable for standardization purposes. A numerical example involving a pipe-type cable crossing a street is presen ted to show the effect of street crossings on the ampacity of the cable cir cuit. In this practical example, the ampacity of the pipe-type cable has to be derated considerably.