Steel City 4" Square Extension Ring, 1/2" & 3/4" KO, 21 cu in

An extension ring sits on top of an existing 4 inch square box to add depth and cubic-inch capacity when the box is too crowded for the conductors and devices in it. The Steel City 4 inch square ring adds 1-1/2 inches of depth and 21 cubic inches of volume, so it is the usual fix when a remodel adds wiring to a box that is already full, or when a device needs to sit forward of the original box face.

21 cubic inches of added volume. That matters when you are counting conductors under the box fill rules: the ring's capacity adds to the original box's marked volume, which is what keeps a crowded junction or device box legal instead of forcing you to open the wall and replace the box.

It is steel, finished in a silver (galvanized-look) coating. A steel ring bolted to a steel box carries ground continuity through the metal path the way the box does, which is why metal rings are what you use on EMT and MC systems rather than a nonmetallic extension.

The ring itself is the same part in every case; the suffix is the carton and packaging code. Orderable versions are RD (53151 1234 RD), UB30 (531511/23/4UB-30), 30L (531511234-30L), 30 (53151123430), UB (531511234UB), and UB25R (531511234UB-25R). Pick whichever packaging your job or stockroom standard calls for, since the ring, depth, and knockout pattern do not change between them.