Chicago Plenum, 2-1/8" deep, 4" square drawn boxes with air tight (4) 1" & (4) 3/4" side knockouts and (3) 1/2" & (2) 3/4" bottom knockouts are installed in air plenum walls or ceilings spaces for lighting fixtures, switches, receptacles or other outlet devices. Square boxes are used to bring power to a number of electrical devices where multiple conductor runs are split in two or more directions.