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The thermal resistivity of the soil surrounding a power cable is a very important factor in evaluating the load-carrying capability of underground cable. This paper presents a method for measurements of soil thermal properties with a spherical probe along the cable installation routes. On the basis of two-port network analysis, equations for the temperature response of the hollow spherical probe are obtained. The paper offers a mathematical model for calculation of the soil thermal parameters, including the related standard deviations, using the least square method. The propagation time of thermal flux is calculated, and it is shown that the transient stage of the spherical probe can be used for measurements in infinite as well as limited samples of soil. Thermal self-capacity and contact thermal resistance between probe and surroundings are taken into the calculations. The realization of the probe with electrical heater and temperature detector, as well as additional measuring devices, is described.