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How much do you know about rigid metal conduits?

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How much do you know about rigid metal conduits?

Steel conduit has been in use as a “raceway system” for electrical conductors since the early 1900s. The wall thickness and strength of steel make metal conduit the wiring method recognized as providing the most mechanical protection to the enclosed conductors.


Rigid metal conduit is a listed threaded metal raceway of circular cross section with a coupling, which can be either a standard straight tapped conduit coupling or the integral type. Threads on the uncoupled end are covered by industry color-coded thread protectors, which protect the threads, keep them clean and sharp, and aid in trade size recognition.


RMC is available in trade sizes ½ through 6.  Thread protectors for trade sizes 1, 2, 3, 4, 5, and 6 are color-coded blue; trade sizes ½, 1½, 2½, and 3½ are black; and trade sizes ¾ and 1¼ are red. The standard finished length of RMC with coupling is 10 feet.


RMC can have a primary coating of zinc, a combination of zinc and organic coatings, or a nonmetallic coating, such as PVC. Supplementary coatings can be applied to all three where additional corrosion protection is needed.


RMC is the heaviest-weight and thickest-wall steel conduit. Where galvanized by the hot-dip process, it has a coating of zinc on both the inside and outside. Electro-galvanized RMC has a coating of zinc on the exterior only, with approved corrosion resistant organic coatings on the interior. RMC with alternate corrosion protection generally has organic coatings on both the exterior and the interior surfaces.


Galvanized RMC is noncombustible and can be used indoors, outdoors, underground, concealed or exposed. RMC with non-zinc-based coatings may have temperature limitations that will be noted on the manufacturer's product label and may not be listed for use in environmental air spaces; consult the manufacturer's listings and markings. Rigid metal conduit (RMC) is the thickest-wall and heaviest-weight steel conduit.

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