Chicago Plenum, 1-1/2" deep, 4" square drawn extension rings with air tight (8) 1/2" & (4) 3/4" side knockouts have threaded tabs on both sides of the boxes for dual side wiring access on front or back of the box. They are added to junction boxes to increase wiring capacity and provide access for additional conduit runs and electrical wire. Garvin Industries’ extension rings prevent the need to remove old conduit boxes and wiring from circuits when additional wiring is needed. Their bottom mounting slots allow a quick installation onto existing boxes. Red painted extensions rings are also available for fire alarm circuit identification.