Thermal management of an electric machine plays a significant role in the improvement of machine performance. Material selection of different components is one of the key stages in electric machine design for high efficiency and high torque density applications. In the stator, the slot liner between the copper winding and laminate is typically good as electrical insulation but poor in thermal conductivity. This affects the heat transfer from the winding to the stator significantly. This paper identifies the key parameters in slot liner selection and the effect of these parameters in terms of heat distribution in the stator of a 150kW motor by using a lumped parameters network. The simulation provides an insight and comparison between various air gaps (liner-laminate) conditions, different liner and impregnation materials and the impact of manufacturing processes evaluated under the same speed and load operating conditions.