Chicago plenum, 1-1/2" deep, 4-11/16" junction boxes with air tight (8) 1/2" & (4) 3/4" side and (3) 1/2” & (2) 3/4” bottom knockouts are larger boxes that are installed when larger sized conductors are required in plenum air spaces. These boxes are great for industrial applications. Garvin Industries’ shallow 4-11/16” boxes can be used where limited wall depth imposes limitations.