Chicago Plenum, 2-1/8" deep, 4-11/16" extension rings with air tight (6) 1/2" and (6) 1/2"-3/4" knockouts are added to Chicago plenum junction boxes to increase wiring capacity and provide access for additional conduit runs. Garvin Industries’ extension rings prevent having to remove the old conduit boxes and wiring when additional wiring is needed. Their mounting slots allow a quick installation in existing boxes.