Activation energies at different endpoints representing a specific extent of thermal-oxidative aging of nuclear electrical cable insulation were calculated to compare degradation kinetics at various aging temperatures (110 °C, 121 °C, 136 °C, and 150 °C). Density and total color difference were measured at each aging time point for an ethylene-propylene rubber (EPR) cable insulation to calculate activation energy. Endpoints were chosen as absolute values that were in the range of property change (density or total color difference) during degradation. The time to a defined endpoint for each temperature was used via classical Arrhenius analysis to estimate the activation energy (Ea), similar to the process historically used by cable manufacturers to qualify electrical cables for use in nuclear power plants. The estimated Ea with standard deviation was reported by extrapolating time to endpoint for temperatures not directly tested.