When to use: Use when sizing the thermal management of a DDC control panel or BAS enclosure. Sum the heat dissipated by controllers, I/O modules, and the power supply/transformer, then compare it against the enclosure's passive free-convection capacity. A sealed metal box sheds roughly 0.5 W per ft² per °F of internal-to-ambient temperature difference. If the heat load exceeds what the surface can passively shed, a filtered fan or panel A/C is required to keep internal temperatures within the rating of the electronics.