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Bubble evolution from overloading of transformers is a concern due to possible dielectric failure. Since overloading is inevitable for short periods during peak demand periods, it is necessary to specify the limits of overloading with respect to winding hot spot temperature which determines bubble evolution. Through extensive coil model testing in the early 1990s, a mathematical equation was formulated to compute bubble evolution temperatures under a variety of conditions: moisture in insulation, gas content of oil, and the pressure in the system. This paper describes the coil models, test results and the mathematical formula. Also presented is a tentative mechanism of bubble formation.