This paper presents a study of the factors that determine cable ampacity adjustments and corrections in the electrical distribution network. Ampacity is defined as the maximum current carrying capacity of any cable. This characteristic parameter depends on many of the cable properties, installation conditions and surrounding environment. In long-distance cable-based power transmission (e.g., HVDC) and distribution network (e.g., in Singapore and many other cities), there are circumstances where the power cables are laid or pass through areas of unfavorable conditions, e.g., regions having temperatures higher than the allowable values. Depending on the extent of such regions and the criticality of its effect on the cable, the rating of the cable will be adjusted based on the assumption that the entire route is characterized by the same conditions. The configuration and application of underground electric cables demand careful consideration of temperature effects on the cable materials. These pessimistic calculations will reduce the cable ampacity to greater extent. An analysis of the methodologies used for the derating factors is presented in this paper.