Chicago plenum, 3-1/2" deep, 4" square junction boxes with ceiling grid span bars and (4) 1/2” & (2) 3/4” side knockouts and (3) 1/2” & (2) 3/4” bottom knockouts are installed in plenum ceilings where extra depth is needed for dimmers, cameras, speakers, strobes, signals and other electrical and electronic devices. They slide on mounting bars for perfect location on ceiling tiles. They work with both standard and revealed ceiling tiles. Their adjustable screws allow box depths to be changed for a perfect flush mount on ceiling tiles. Garvin Industries’ labor saving design eliminates the need to awkwardly install boxes and extension rings to accommodate deep devices.