Plenum Enclosure for Chief Suspended Ceiling Kits The new plenum-rated enclosure for Chief's suspended ceiling kits (CMS440, CMA440, CMA450, CMA455 - new units only) is designed to provide convenient storage for your non-plenum rated cables and equipment. Hinged door for easy access; multiple knockouts (.75", 1" and 1.5") for ease in routing cables. Clean installation. Enclosure covers the column location on the suspended ceiling kit for routing cables through the ceiling.