Introduction 

This article addresses key design considerations in the sizing of MV and LV cables. It begins with a definition of soil thermal resistivity and discusses methods such as Fluidized Thermal Backfill (FTB) for improving thermal performance. All relevant derating factors required for ampacity evaluation under actual site conditions are presented and illustrated through examples. Practical guidelines for selecting derating factors during the planning stage are provided. Next, the article examines uprating factor of cables that carry cyclic loads. The limitation in extrapolating standard formula for cyclic loading for solar plant application is brought out. Finally, the article concludes with field observations of unequal current distribution in Multiple Runs of parallel cables. 2.0 Soil Thermal Resistivity Soil thermal resistivity (RTH) is defined as “the difference in degrees centigrade between opposite faces of a one-meter cube of soil caused by the transference of one watt of heat and is expressed in °C- Meter / Watt.

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