The use of geotextiles and geomembranes for the lining of feeder trenches is becoming common practice in wind and solar farms. In the presence or rocky soils, the liner prevents the migration of the bedding out of the trench, which would create air pockets around the cables. These air pockets act as thermal insulators, reducing the ampacity of the feeders. Commercially available software tools are not suitable for modelling the thin liner layer around the trench bedding. It is important, however, to estimate the reduction of the feeder ampacity in the presence of these liners. This paper presents a simplified analytical method to estimate the reduction of the feeder ampacity when a lining is used in underground cable trenches. The analytical approach is derived from the calculations presented by Neher-McGrath. Validation of the method is carried out by modeling the cable trench using finite elements analysis.