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The performance of an underground transmission and distribution system is critically influenced by the thermal properties of the surrounding medium, as well as the thermal properties of the cable itself. The thermal behaviour of the cable is strongly dependent on the loading conditions and thermal parameters of the cable materials as well as the thermal characteristics of the surrounding soil, ambient environment and boundary conditions. A combined experimental- computational investigation is performed to examine the thermal parameters which may influence the performance of the underground cable. First, the thermal specification of the soil was tested by simulating a high temperature gradient along the body of the tested sample enclosed by a heat source-heat sink pair facing each other. In the second part, the 15 kV XLPE underground power cable is energised as a heat source as in the actual case. The thermal field at different spots and loadings was investigated using a developed full-size experimental setup to monitor the thermal behaviour of the underground cables, surrounding soil and boundaries phenomena (heat coefficient losses at the convective boundaries and the heat losses at the isolated boundaries). The proposed combined finite-element-gradient optimisation method is used to estimate the cable thermal parameters. This is based on matching the computational simulation of the experimental model based on finite element to that obtained from the experimental measurements.