A sealed enclosure containing electrical apparatus will have an internal temperature rise that is dependent on the thermal resistance of the enclosure and the total power dissipated within that enclosure.
The thermal resistance of the enclosure is a function of the size of the enclosure, the shape of the enclosure and the exposed surface area of the enclosure. If the enclosure is free standing with air able to freely circulate over all vertical surfaces, then it will have a lower thermal resistance than an enclosure where the air flow over one or more surfaces is restricted. Likewise, increasing width is far more effective in reducing the thermal resistance of an enclosure than increasing height.
Adding ventilation louvers without fans will only slightly reduce the thermal resistance of the enclosure. (usually less than 10% reduction in thermal resistance or temperature rise!)
The thermal resistance can be calculated for a given height, width and depth of enclosure provided the installation conditions are specified. From the thermal resistance and total power dissipated within the enclosure, the temperature rise can be calculated.
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