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Soil thermal stability is one of the most important values that an engineer must know to calculate the amount of current any particular cable can be allowed to carry. When a cable is buried in soil, whether direct buried or in an underground pipe, the heat generated by the I2R losses in cable must be carried away through the soil surrounding the cable. The rate at which this heat can be carried away determines the temperature the cable will reach during any loading condition.
If this temperature becomes too great the cable can be damaged. The thermal stability of the soil surrounding the cable is the main factor in determining the rate at which heat can be conducted away from the cable, and therefore, the ultimate amount of current the cable can carry. Soil thermal resistivity is one of the most important values that an engineer must know to calculate the amount of current any particular cable can be allowed to carry. Once the thermal resistivity of the surrounding soil is known the Neher-McGrath method is commonly used to determine the amount of current a cable can carry without exceeding its allowable temperature.
Thermal resistivity is a measure of the ability of a material to resist the flow of heat. In the case of soil this property is commonly measured using either laboratory or field tests. Several soil tests are commonly performed to characterize a soil’s properties. A soil sample is taken from the field and the in-place soil unit weight test gives the overall soil unit weight in lb/ft3 as given in Equation 1. The water content of the soil sample is also tested and the results of this test gives the weight of water contained in the soil sample divided by the weight of dried soil and is given in percent as shown in Equation
The soil thermal resistivity is measured by inserting a heat generating thermal probe into the soil or soil sample (if done in a lab) and soil resistivity is measured as described in IEEE Std. 442 “IEEE Guide for Soil Thermal Resistivity”  . A known heat rate in W/cm is injected into the probe and a plot is made of the temperature of the probe/soil interface versus time.
|Measurement of Soil Thermal Stability, Thermal Resistivity, and Underground Cable Ampacity|