The term "electrical conduit" refers to durable tubing or other types of enclosure used to protect and provide a route for individual electrical wiring conductors. Conduit is typically required where wiring is exposed or where it might be subject to damage. A conduit can be made of metal or plastic and may be rigid or flexible. All conduit is installed with compatible fittings (couplings, elbows, connectors) and electrical boxes, usually made of the same or similar material. Conduit must be installed in accordance with the National Electrical Code (NEC) and all applicable local code rules.
There are seven different types of conduit used commonly in residential and light commercial wiring.
Rigid Metal Conduit—RMC and IMC
Rigid metal conduit, or RMC, is heavy-duty galvanized steel tubing that is installed with threaded fittings. It is typically used outdoors to provide protection from damage and can also provide structural support for electrical cables, panels, and other equipment. RMC is sold in 10- and 20-foot lengths and has threads on both ends.
Intermediate metal conduit, or IMC, is a thinner, lighter-weight version of rigid metal conduit and is approved for use in all of the same applications as RMC. Because IMC is lighter and easier to work with than RMC, it is more common in new construction.
Electrical Metallic Tubing—EMT
Another example of a rigid electrical conduit is EMT (electrical metal tubing), which is most commonly made of galvanized steel but can also be aluminum. EMT is also called "thin-wall" conduit because it is thin and lightweight, especially compared to RMC. EMT is rigid but can be bent with a simple tool called a conduit bender.
EMT is installed with couplings and fittings that are secured with setscrew or compression-type fasteners. The tubing itself is not threaded like RMC and IMC. Common sizes of EMT include 1/2-inch, 3/4-inch, and 1-inch. It is commonly used for exposed indoor wiring runs in residential and light commercial construction. If installed outdoors in exposed locations, it must be assembled with special watertight fittings.
Electrical Non-Metallic Tubing—ENT
Electrical nonmetallic tubing (ENT) is flexible corrugated plastic tubing that is moisture-resistant and flame-retardant. It is easy to bend and installs with snap-lock or glued plastic fittings. Unlike EMT, non-metallic tubing cannot installed in exposed locations, so it is commonly used inside walls. In addition to installation in standard wood- or metal-frame walls, ENT can be installed inside concrete block structures and can be covered with concrete. Due to the blue color of one common brand of this conduit, ENT is nicknamed "smurf tube," after the Smurfs cartoon characters.
Flexible Metal Conduit—FMC and LFMC
Flexible metal conduit (FMC) is also called "Greenfield," after the name of its inventor. It has a spiral construction that makes it flexible so it can snake through walls and other structures. Standard FMC is used in dry indoor locations, often for short runs between a wall box and a motor or fixed appliance, such as a garbage disposer.
Liquid-tight flexible metal conduit (LFMC) is a special type of flexible metal conduit that has a plastic coating and is used with sealed fittings to make it watertight. It is commonly used with outdoor equipment, such as air conditioner units.
Rigid PVC Conduit
Rigid polyvinyl chloride (PVC) is similar to plastic plumbing pipe and is installed with plastic fittings that are glued in place. It can be bent after being heated in a portable heater box. Because the conduit tubing and fittings are glued together, the conduit assemblies can be watertight, making PVC suitable for direct burial in the ground for many applications. It is also allowed in corrosive environments.