Some electromechanical systems involve totally enclosed electrical machines. In these cases, forced internal air cooling, for instance by the use of fans, is nearly impossible. However, in the same time, permanent magnet machines have to expel their internal rotor losses. In this context, an accurate estimate of heat exchange transfers and heat sources are important, for instance for the determination of the temperature distribution. This can be used for future designs of the same type of machine. This paper details the thermal modeling of a totally enclosed permanent magnet machine, such as an integrated starter generator (ISG). For this purpose, a lumped parameter network is built. Thermal conduction, convection and radiation heat transfer modes are considered. Losses are calculated using both analytical expressions and (coupled) electromagnetic finite element analysis (FEA). Simulation results are compared and validated with experimental data.