Electrical Conduit Types: RMC, EMT, and More Explained
An electrical conduit is a crucial component in any electrical system, as it provides a protective cover for the wires and cables that run through it. There are a variety of different types of electrical conduit available, each with its own unique set of advantages and disadvantages. Whether you’re a professional electrician or a DIY enthusiast, understanding the different types of electrical conduit can help you make informed decisions when it comes to your electrical projects.
Electrical Conduit Types
The most common types of electrical conduits are:
1. Rigid metal conduit (RMC)
In this method, all wires are enclosed in a pipe called a conduit. The pipe is usually steel, sometimes aluminum, or brass (especially for swimming pool lighting). A conduit differs from a water pipe in that the interior surface is carefully prepared so wires can be pulled into it with a minimum of effort and without damage to their insulation. Also, the chemical composition of the steel in the conduit is carefully controlled so it will bend easily. Steel conduit usually has a galvanized or similar finish inside and outside. Some galvanized steel conduits and some aluminum conduits have an additional enamel or plastic coating for further protection against corrosion.
2. Intermediate metal conduit (IMC)
This is similar to ordinary rigid metal conduit and, size for size has the same outside diameter but thinner walls. That makes the internal area in square inches a little more than in ordinary conduit, and that size difference often makes a difference in allowable wire fill. IMC is installed in the same way as ordinary RMC. The threads are identical and RMC fittings can be used without modification. It is more likely to kink due to the reduced wall thickness, making it less tolerant of conduit hickeys. Conduit benders work very well. The only real application difference between the two products is that RMC is made in all trade sizes up to 6 in., but IMC goes only up to 4 in.
Both RMC and IMC can support boxes that have threaded hubs or conventional sheet metal boxes to which hubs identified for this purpose were added.
3. Electrical metallic tubing (EMT)
A raceway that is similar to rigid metal conduit but of thin wall construction is called “electrical metallic tubing” (abbreviated EMT).
It is made of galvanized steel or occasionally of aluminum, either of which may have additional plastic or other protective coatings. The rules that apply to rigid metal conduits—such as the number of wires permitted, bending, and supports—apply also to EMTs. It is made only in sizes through 4 in. It must be supported within 3 ft of every box, cabinet, or fitting, and additionally at intervals not exceeding 10 ft, regardless of size. If there isn’t any structural element available for support, the first point of support can be as far as 5 ft from the termination, provided the intervening tubing consists of an uncut length of EMT. In addition, an unbroken length of EMT can be fished. This may seem odd, but there are cases where you may be able to poke a length of tubing down into a wall cavity.
The internal diameter in the smaller sizes is the same as in rigid conduit, but in the larger sizes, it is a little larger. However, because the walls are so thin, EMT must never be threaded in the field (there is a patent in existence for a factory-threaded product, but it isn’t presently in production). Instead, all joints and connections are made with threadless fittings that hold the material through pressure.
4. Flexible metal conduit (FMC)
This material is often called “flex” or “greenfield.” It is similar in appearance to armored cable but is larger in diameter since it is an empty raceway and wires must be pulled into it after it is installed. It is available in steel or aluminum in both normal and reduced wall thicknesses. The aluminum and reduced wall steel products require care to prevent separating the convolutions, and they require the use of appropriate terminal fittings evaluated with these forms. Try to use connectors that clamp the entire circumference of the raceway in order to distribute pull-out forces, particularly in areas where the flex may be disturbed after installation.
5. Liquidtight flexible metal conduit (LFMC)
This material is similar to an ordinary flexible metal conduit, plus it has an outer liquid-tight nonmetallic sunlight-resistant jacket. It is commonly called “Sealtite,” which is the trade name of one manufacturer. In order to properly seat the connector, take care to cut the conduit squarely. Part of the connector goes inside the conduit, making a good connection for grounding continuity; part of it goes over the outside, forming a watertight seal. The plastic content in the outer jacket limits the amount of heat it will stand. In general, size your wires so the current they will carry does not exceed the 60°C ampacity column limits because higher temperatures will soften the jacket. You can exceed those limits if the product is marked accordingly. The product is made and can be used in the smaller 3⁄8-in. trade size for applications similar to those where 3⁄8-in. flexible metal conduit can be used (and with the same wire fill).
6. Liquidtight flexible nonmetallic conduit (LFNC)
This material has the same function as its metallic precursor but has a completely nonmetallic wall. It is available in three forms, two of which are relatively uncommon—the “A” type with reinforcement between the core and cover, and the “C” type having a corrugated wall without additional reinforcement. The “B” type, with integral reinforcement within the conduit wall, has become a very popular wiring method. This type does not have the restriction to 6 ft (unless a longer length is required for flexibility, a relatively unusual circumstance) that the other types share. As in the case of the metallic version, you have to be sure the enclosed wires don’t run above 60°C unless the product is marked accordingly. Both this product and the metallic version can be used outdoors, and even directly buried if listed and marked for this duty. Where exposed to sunlight, it must be marked for this duty as well.
7. Rigid nonmetallic conduits (formerly, Type RNC)
A rigid nonmetallic conduit is immune to conventional corrosive influences, although some industrial chemicals will attack it. There are several forms of this product for use underground. One form of underground conduit, a black, high-density polyethylene, can be supplied in continuous lengths on a reel; this form can also be shipped with preinstalled conductors. This form will burn and can’t be used in exposed locations or in buildings. However, the polyvinyl chloride (PVC) version can be used above and below grade and is the predominant type used. Recently a form of fiberglass has been successfully listed for use above grade, but it is comparatively difficult to terminate and its use is only a tiny fraction of PVC.
8. Electrical nonmetallic tubing (ENT)
As RNC is the nonmetallic counterpart to RMC, electrical nonmetallic tubing is the counterpart to EMT. It is made of identical PVC in trade sizes up to 2 in., but in corrugated wall construction that allows it to be bent by hand without the application of heat. It can be supplied in continuous lengths from a reel. It is even available as a prewired assembly with specified conductor combinations already pulled in place. However, it is not a cable, and it is subject to all the normal restrictions for raceways, including the 360-deg bend rule. It must be supported every 3 ft and within 3 ft of terminations. It cannot be used outdoors or for direct burial; however, it can be used in cases where it runs in concrete, even if the concrete is below grade.