Atkore Allied Tube & Conduit CK300RKON 3" Compression Coupling

It's a 3 inch compression coupling, so it joins two lengths of 3 inch conduit by tightening a compression nut over a ferrule on each end. No threading and no tools beyond a wrench are needed on the raceway itself, which is why compression fittings are the normal choice for EMT runs where the tubing ends are cut square in the field.

The body is steel with a zinc plating applied specifically to resist corrosion, which suits indoor and damp-location work like mechanical rooms, parking structures above grade, and slab penetrations. Zinc plated steel is not the same as hot dip galvanized or stainless, so for corrosive or chemically aggressive environments step up to a fitting specified for that service.

Steel, with heavy walls. For an MRO or institutional spec that calls out steel set screw or compression fittings rather than die cast zinc, a steel bodied coupling like the CK300RKON is what satisfies it. The heavier wall also holds up better to being struck during a pour or a pull.

Compression, like the CK300RKON, uses a nut and ferrule that grip the tube wall and give a concrete tight joint, so it is what you want in a slab pour or any place the inspector wants a tighter mechanical joint. Set screw couplings are quicker and cheaper for dry, exposed indoor work but are generally only concrete tight when specifically listed as such. If the 3 inch run is being buried in concrete, go compression.