Enclosure manufactures typically determine the size of enclosure needed based on entering some data into a computer program they can access for the purpose. They need to know the ambient temperature that the drive can operate in (50C for NEMA 1, 40C for NEMA 4/12) and how much heat is dissipated by the drive. Use the following formula for a guideline only:Watts Dissipated = (HP x 750 x .04) climate space /EFF OF MOTOR. Consult the manual for further information (section 5.0).