Rigid Metal Conduit (RMC) is an extremely stout form of conduit used in environments where severe damage to conductors is possible. Damage can come from many sources including temperature, moisture, oxidation, physical damage from vehicle traffic, and galvanic action. IMC is considered a “lighter” version of RMC that is used for similar environments, however when you need the biggest, baddest conduit out there…RMC is the way to go.
RMC is a heavy-duty galvanized steel conduit that, because of its rigidity, is allowed to be a means of structural support for many fixtures, raceways, and enclosures. Brand new RMC will have both ends threaded, usually one end will come with a Rigid coupling. When cutting Rigid, you lose one of the threaded ends, so there are companies that make threading machines which allow you to rethread the ends of the conduit.
Bending RMC can be quite a challenge without using a machine bender of some sort, whether hydraulic or electric. Trying to hand bend any RMC over 1/2 inch will give you an embarrassing workout. It’s ok, just make sure you get a video for TikTok.
Rigid Metal Conduit has a little less burial depth requirements, similar to IMC, since it can handle a little more abuse if dug up with a mini-excavator on accident. It can also handle extreme moisture and corrosion, so direct burial in soil is no problem. (Read more on RMC in Article 344 of the 2017 NEC)