Endurance of an insulation material to high temperatures determines the maximum current-carrying capacity (ampacity) of an underground power cable. Cable ampacity is calculated conventionally using the installation conditions and maximum steady-state operation temperature according to IEC-60287 standard. In this article, finite element method results are compared with IEC-60287. A further verification has also been made with experiments. In this work, ampacity analyzes of 154 kV high voltage XLPE underground power cable are made using ANSYS 5.61 finite element analysis software. An experimental set-up is developed to measure conductor and surface temperature of the cable in underground conditions. The results of experiments and numerical calculations are compared to the IEC-60287 standard. Additionally, the thermal regions of three cables in flat installation are investigated to show the versatility of the finite element method. The effects of insulation thickness and external thermal source on ampacity are also analyzed using the finite element method.