Chicago plenum, 2-1/8" deep, 4-11/16" junction boxes with air tight (6) 3/4" & (4) 1" side and (3) 1/2" & (2) 3/4" bottom knockouts are larger boxes that are installed when larger sized conductors are required in plenum spaces. These boxes are great for industrial applications. Garvin Industries’ 2-1/8” deep boxes provide extra volume for wiring and devices.