This paper describes the electro-thermal design of a medium voltage modular multilevel converter prototype, from the submodule power semiconductor thermal requirements to the overall system level integration. The high number of semiconductors and capacitors involved are stressed differently depending on the actual operating conditions, as discussed in the paper. The presented design considers air cooling concept at the submodule level, with its enclosures used as air guide towards the chimney like structure. Forced air cooling is applied at the cabinet level. The numerical design is verified by means of 3-D finite element method simulations of different complexities.