RMC, short for Rigid Metal Conduit, has the thickest-wall of the steel raceways. RMC conduit is permitted in all atmospheric conditions and occupancies, as it is hot-dip galvanized for excellent corrosion resistance. The rigid conduit is smooth inside for fast wire pulling.
Galvanized rigid conduit is available in trade sizes 1/2" through 6", and nominally 10′ length. Rigid steel conduit is threaded on both ends, with a steel conduit coupling applied to one and a thread protector on the other.
The standards for safety for conduit rigid could be UL 6 and ANSI C80.1.