The underground power cables are generally installed in the urban areas and cities where in the thermal problems along the cable path are fixed rarely. In the case of poor thermal circumstances for a small portion of the cable there are no defined guidelines for calculating the derating in the ampacity in the IEC and the IEEE standards. Hence, a very important situation involving a variation in the thermal conditions along the whole path of the cable occurs when the path of the cable is under the road crossing or is closer to other pipelines or cables. When this occurs the ampacity must decrease to prevent the rise in the cable temperature due to the higher thermal resistance which occurs at the conduit position. The finite element method using the ANSYS Software is used for studying this problem of determining the ampacity correction and studying the issues faced by the conduits for operating and maintaining their temperature below the maximal cable temperature. The derating factors for conduit length, depth of burial and soil resistivity are suggested.