This paper presents a selection of design temperatures for thermal loads in buried plastic piping. Gas companies often assume that the CFR requirement for design of mechanical couplings for an instantaneous change in temperature of ΔT = 55°C applies to all aspects of the PE system. This assumption leads to very conservative designs, and does not represent the range of temperature changes due to pipe installation and seasonal temperature variations in the ground. This paper presents data from measured ground temperatures in the northeastern United States to develop seasonal design temperature ranges. Measurements of PE pipe temperature when exposed to direct sunlight then inserted into cast iron piping are used to assess temperature changes that might occur during pipe installation. Design scenarios are identified that represent reasonable situations for the range of temperatures PE piping would be subject to, and stresses are evaluated for these conditions. Stress superposition methods are used to develop design charts for evaluating thermal loads.