How to include soil thermal instability in underground cable ampacity calculations

Abstract

Previous papers have described a method to calculate and model the extent of drying that will occur in soil due to the heat produced by underground cables. Up until now, the techniques presented to include soil thermal instability in cable ampacity calculations have been limited to single isolated conductors with 100% load factors. This paper provides more generalized techniques that permit the inclusion of soil thermal instability in any calculation that can be performed using the Neher-McGrath method. Computer programs designed to perform Neher-McGrath calculations are often limited in their versatility and restrict the user in the type of inputs that may be entered. This paper includes suggestions that will permit the computer user to include soil thermal instability information in calculations done using commercially available computer programs.