A raceway is an enclosed electrical conduit that forms a pathway for electrical wiring. Raceways protect the wires and cables from physical threats. In this article, you will learn about the different types of raceways, code requirements, construction considerations, and installation techniques.
Types of Raceways
Different types of raceways specified depending on the environment, cost factors, project specifications, and code requirements. Raceways can be categorized into four categories: conduits, tubing, wireway, and speciality.
Rigid Metal Conduit
Rigid metal conduit (RMC), sometimes described as rigid steel conduit (RSC), are circular raceways made of heavy-duty steel. It is typically specified for outdoor use and is also suitable for providing structural support for electrical cables, panels, and other equipment.
Intermediate Metal Conduit
Intermediate Metal Conduit (IMC) is a circular metal raceway. They are designed for outdoor exposure and strong connections. IMCs are made from a strong steel alloy. The steel alloy allows IMCs to be stronger, lighter, and house more conductors than the standard rigid metal conduit. This makes IMCs a popular choice for raceway construction. However, IMC requires a bushing wherever the cable or wire enters a box, fitting or other enclosure to ensure protection at the joints. This step can be avoided if the enclosure offers protection to the wire that is equivalent to that of a bushing.
Flexible Metal Conduits
Flexible Metal Conduits (FMC) is a raceway of an interlocked spiral metal strip. FMCs are primarily used in the last 6 ft or less of a raceway between the more rigid raceway system and equipment that moves, shakes or rotates. FMCs operate best in enclosed indoor environments. Outdoor equipment often use liquid-tight flexible metal conduits (LFMC). LFMC has a similar construction to FMC but adds an outer thermoplastic layer for protection against moisture and corrosive materials.
Rigid PVC Conduit
Rigid polyvinyl chloride (PVC) can be bent via a controlled heat application to its desired shape. PVC conduit is glued together, making for watertight assemblies that are suitable for underground applications. PVC conduit is also permissible in corrosive environments.
Electrical Metallic Tubing
Electrical Metallic Tubing (EMT) is a unthreaded steel raceway. EMTs are also called “thin wall” conduit because they are a thin and lightweight raceway. They are rigid and require a conduit bender to bend. EMTs are threadless conduits and therefore require threadless connections. This allows EMTs to have quick, easy and inexpensive installations compared to threaded metallic conduits. EMTs are commonly used for exposed indoor wiring in light industrial operations. Outdoor applications for EMTs are possible but require additional components to ensure the conduit is waterproof.
Electrical Nonmetallic Tubing
Electrical Nonmetallic Tubing (ENT) is a flexible corrugated plastic tubing that is moisture resistant and flame retardant. It bends easily and uses snap-lock or glued plastic fittings for a secure connection. ENTs can not be installed in exposed locations like EMTs since the sunlight will make the raceway brittle. Instead, ENTs are installed inside walls. Installation of ENT can occur in concrete block structures and covered in concrete if a metal or wood frame helps support the ENT, making it a common for buried wiring.
A wireway is a trough with hinged and removable covers for housing electrical wiring. Compared to other types of raceways, wireways are thinner and made of aluminum. This allows the wireway to be lightweight and easily affixed to a wall or structure. The hinged cover allows for quick maintenance of the cables. Wireways typically contain only wires and not the other electrical components seen in raceways.
Speciality raceway is designated for a specific purpose and is uncommon for utility use. At times, pipe fittings may qualify as a raceway. Bus duct is a sheet metal duct with electricity conducting busbars. Bus duct is frequently used at the inlet to major pieces of equipment that require energization. Cable bus frequently competes with bus duct and is an assembly of insulated cable conductors, typically used in the inlet of equipment. Other types of raceway include floor and underground troughs and ducts, as well as gutters. It should be noted that cable tray, a common support structure for cabling, is not considered to be a raceway. This is because cable tray does not meet the core raceway requirements of being fully enclosed.
Code & Regulatory Requirements
When constructing raceways, they must follow the National Electrical Code (NEC). The NEC is a regionally adoptable standard code for the safe installation of wiring and equipment in the United States. It was first published in 1897 as part of the National Fire Code series by the National Fire Protection Association. The NEC makes amendments to the code every three years with the most recent draft made in 2020. Contrary to popular belief, the NEC is not federal law.
The NEC states the number and size of conductors must not exceed a certain amount so that heat does not damage the wire. The fill percentage is dependent on how many wires are in the raceway, the type of conductor, the type of raceway, and the application of the raceway. Wireways are limited to 20% fill. For EMT, according to Table 1 in Chapter 9 of the NEC, one wire can fill 53% of the cross-sectional area, two wires can fill 32%, and over two conductors can fill 40%. Other circumstances can cause this number to decrease so NEC must be consulted to determine the proper fill requirement. Punchlist Zero provides an easy to use raceway fill calculator to determine what is permissible by code.
The NEC defines the minimum cable bend radius as the smallest radius a cable or wire can bend without causing damage to the wiring. If the cable exceeds the bend radius it is prone to kinking and other sheath damage. It can lead to an exposed wire and cause a short. The bend radius is determined by the conductor type, the size of the wire, and the type of insulation used by the wire. The NEC represents the bend radius as a factor of the diameter of the cable. The cable manufacturer often gives the bend radius of a cable.
Raceway materials are often dependent on the type of raceway and the application of the raceway. The most common material of raceways is galvanized steel. Galvanized steel offers strength and protects the cables from corrosive materials. This makes it a popular material among EMTs and IMCs. However, other materials perform better than galvanized steel in certain situations. For example, ENTs prefer to use PVC over galvanized steel in order to reduce costs. PVC also outperforms galvanized steel for wiring in walls and in cement.
The installation of raceways requires careful planning and designing. The path of the wiring must be clearly marked beforehand and supported properly. The raceway is also subject to inspection to ensure it is up to NEC requirements. It is essential to confirm wires or outlets are de-energized prior to raceway installation.