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This paper describes thermal modeling, using computational fluid dynamics (CFD), to predict the temperature rise of a high-voltage SF6 gas circuit breaker during a heat-run test. In electrical equipment, maximum temperature rise in various components is limited by their material properties and, hence, the heat-run test forms part of type test. A methodology to predict the temperature rise at the design stage is developed using ANSYS-CFX and found to be beneficial to reduce the number of heat-run tests during the product development process. The developed methodology has been implemented on a 72.5-kV SF6 gas circuit breaker and validated with the experimental temperature rise results of the existing design at two current ratings: 1000 A and 2000 A. The methodology was further used to predict thermal performance of the breaker at a higher current rating of 3150 A. The prediction helped to understand temperature rise in various components and hot-spot locations. Various design modifications were then tested virtually to evaluate the temperature rise. Thereafter, the final modified design where the temperature rise of all the components was within the permissible limits and which also satisfied other constraints of manufacturing feasibility and cost was fabricated and physically tested. Good agreement is found between the CFD and experimental results.