Electrical conduit is metal or plastic pipe through which electrical wires are run. Either rigid or flexible, conduit protects the wires and is used in exposed locations, such as along the exterior surface of a wall. It is also used in unfinished areas, like basements, crawlspaces, and attics, and for surface-mounted installations outdoors.
Types of Conduit
Conduit used for residential wiring include several types of metal and plastic materials designed for different applications. Here are the most common types:
- EMT: Electrical metallic conduit is rigid, thin-walled metal conduit typically made of galvanized steel. Because it is thin-walled and lightweight, it is easy to bend with a special tool called a conduit bender, and it cannot be threaded like a heavier conduit.
- FMT: Flexible metal conduit has a spiral construction that allows the conduit to bend easily, simplifying installation. It is commonly used for short runs in exposed locations, such as wiring for garbage disposers and hot water heaters.
- RMC and IMC: Rigid metal conduit and intermediate metal conduit are heavy-duty conduits made of galvanized steel. They are used for the structural piping that houses the wiring for the home's connection to the utility service lines. IMT has largely replaced RMC in new construction. Both RMC and IMC are joined with threaded connections.
- PVC: Rigid PVC is a plastic pipe similar in size to EMT. It can be heated and bent and is joined with glued or threaded connections.
- LFMC: Liquidtight flexible metal conduit is a flexible metal conduit covered with plastic sheathing to make it watertight. It is used for outdoor wiring serving air conditioning units and other outdoor equipment.
Conduit Is Not the Same as BX
At first glance, electrical conduit may seem similar to BX (or AC, for "armored cable") wiring. Like conduit, loose but insulated individual wires are encased in BX's metal shell. Also like conduit, BX can be used in exposed locations.
The difference is that, according to electrical code (NEC 320.12), armored cable may not be used in damp or wet locations, in places where exposed to corrosive conditions, or places where damage might occur. By contrast, certain types of conduit may be installed in such locations.
Boxes and Connectors Used With Conduit
Conduit always terminates at some type of electrical box or at fixed equipment that also serves as a box. Special connectors are used to secure conduit to boxes and to join lengths of conduit together to form long straight runs or bends. The boxes, connectors, and conduit form a wire enclosure system called a raceway.
In most cases, the conduit material matches the box material. Plastic conduit typically is used with plastic boxes, but it can also be used with metal boxes and special connectors. Metal conduit must be used only with metal boxes.
Wiring Used With Conduit
Conduit is a hollow tube that wires are pulled, or "fished," through during installation. This differs from cable, which is a group of wires encased inside a flexible protective sheathing. The most common type of cable used in home wiring is non-metallic (NM), or Romex, cable. While NM cable can be run inside conduit, this is seldom done.
The types of wire most commonly installed inside of conduit are THHN and THWN. THHN/THWN wires are individual, insulated, and color-coded wires. In fact, they are similar to the wires you see when you strip the outer sheathing off of NM cable.
Grounding With Conduit
In some installations using metal conduit, the raceway—the conduit, boxes, and connectors—serve as the ground for the circuit. In this way, the metal raceway acts in place of the grounding wire that is found in NM cable (this is also the main reason why only metal boxes may be used with metal conduit).
This type of grounding system was more common in older construction than it is today, and many electricians today include an insulated ground wire in metal conduit as preferred means of grounding the circuit.
For a metal raceway to serve as a ground, all parts of the raceway must be electrically connected, with no interruptions. If a remodeler unwittingly installs a plastic box in the raceway, or if any connection comes loose, the ground path will be broken, leaving the circuit ungrounded.