In urban areas, underground cables are commonly used for bulk power transmission. The utilization of electricity in factories, domestic premises and other locations is typically performed by cables as they present the most practical means of conveying electrical power to equipment, tools and other different applications. Estimation of cable current carrying capacity (ampacity) gains higher potential in recent times due to the continuous increase of energy utilization in modern electric power systems. This paper presents a theoretical study based on relevant IEC standards to calculate the ampacity of underground cables under steady state conditions. The ampacity formula stated in IEC standards are coded using Matlab software. Further, an untraditional experimental ampacity test of a 38/66 kV- XLPE/CU- 1 X 630 mm2 cable sample is performed in the extra high voltage research center. This paper proposes a new approach that uses the complementary laboratory measurements in cable ampacity data preparation. The modified approach gives more accurate estimation of cable parameters. The level of improvement is assessed through comparisons with the traditional ampacity calculation techniques. Main factors that affect cable ampacity, such as the insulation condition, soil thermal resistivity, bonding type, and depth of laying are examined. Based on paper results, cable ampacity is greatly affected by the installation conditions and material properties.