What's the difference of electrical conduits?

With several varieties of conduit available at most home centers and supply houses, it's essential to know the merits of each and what you can and cannot do with it.

Most metal conduits are made of galvanized steel, but some types also can be found in aluminum. The most common steel conduits seen in a residential application, and the ones most commonly available from local home centers, are rigid metal conduit (RMC), electrical metallic tubing (EMT), flexible metal conduit (FMC), and intermediate metal conduit (IMC).

RMC is the thickest, stiffest, and heaviest of the four and requires special tooling to bend and thread. Most homes in my area have only one run of RMC, if any, and it's used for the service-entrance mast that attaches to the electric-meter base. Given the amount of tension on the mast from the overhead service wiring, and the importance of protecting the serviceentrance conductors from damage, RMC is clearly the pipe of choice for this application. However, all other applications within a typical residence can be done with another type of metal or plastic conduit, so RMC's further use is limited more to commercial and industrial settings. Most RMC is made of galvanized steel, but it is also available in aluminum.

FMC is a helically wound, flexible tubing most often made from aluminum. Because it cannot be used outside or in other wet locations, FMC is somewhat limited in its usefulness, but there are a few applications where it is the clear choice. For instance, most new direct-wire cooktops and ovens require a section of FMC for connection to a power source that can remain flexible during the actual installation of the appliance, and many furnaces and heat pumps are coupled to their disconnect with a short whip of FMC. For that reason, many newer ovens and cooktops come wired with FMC attached, and most home centers and supply houses stock at least 1⁄2-in.-dia. and 3⁄4-in.-dia. FMC and associated fittings. It can be cut with a hacksaw, but specialized cutting tools are available.

EMT is a thin-walled metal conduit most often found in galvanized steel, but it's also available in aluminum. EMT is relatively inexpensive; can serve as protection from physical damage; and can easily be bent, cut, and installed with a minimal amount of special tools. EMT can be installed outdoors, provided all fittings, supports, and fasteners are made of corrosion-resistant materials. I often see EMT in older homes to protect the ground wire coming out of the meter base, but it can satisfy several other needs in residential construction and renovation. I have used EMT quite a bit as a means for protecting exposed wiring, such as on garage walls for new receptacles or light fixtures.

IMC is thinner than RMC, but thicker and far more rigid than EMT. It's rarely used in my area, but some electricians may choose it over RMC for its lower cost and lighter weight. It's code-approved for the same uses as RMC.