One of the most important factors affecting underground cable ampacity is the thermal resistivity of the soil. It is well known that thermal resistivity of the soil will vary with moisture content. It is also well known that the heat generated by cables can cause soil drying thus affecting the soil thermal resistivity. The ability of the soil to maintain its thermal resistivity in the presence of a heat source is known as thermal stability. Soil will increase in resistivity due to drying caused by heating from underground sources. This phenomenon makes it challenging for the design engineer to decide how to account for the drying effect in cable ampacity calculations. This paper will examine the information available from standard soil tests and the information these tests may provide relating to the migration of moisture in soil and the resulting changes in soil resistivity. Furthermore a method is suggested for including this information in underground cable ampacity calculations.

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