To determine if ventilation is needed for a particular enclosure, add the total heat dissipation of all heat sources within the cabinet, determine the ambient temperature, determine maximum internal temperature, choose a cabinet size, and then contact your cabinet distributor.
They generally have a software program, much like Compumotor Sizing Software, that determines if external cooling is required. If it is required, you may use the following equation to determine how much cooling is recommended.
cfm = (3160)(kW)/Delta T
kW = heat dissipated in the enclosure by a power source, kW
Delta T = temperature difference of exhaust air and incoming ambient