Electrical conduits are metal, plastic or fiber pipes designed to protect electrical cables and wires. They’re used in just about every residential and commercial establishment, especially for wiring that is either exposed, or fitted outdoors. Since they are essentially a safety feature, you should select the material, size and fittings based on the environment of the installation and the type of wiring it’ll be housing.
Types of Conduits
When it comes to options, there are almost as many types of conduits as there are wires and cables, and they can be broadly classified into metallic and nonmetallic variants. Here are some of the most common types under these categories:
- Rigid Metal Conduit (RMC) – RMCs, or ‘rigids’ are one of the most commonly used conduits in commercial establishments. They usually have the toughest and thickest walls made from coated stainless steel or aluminum, so they offer excellent protection against impacts, punctures and cuts. Additionally, they are available with different coatings to prevent corrosion and can be used indoors and outdoors in most conditions, even as equipment grounding conductors. However, they are heavy, difficult to bend, more expensive and you will need to use compression fittings or a rethreader if you cut them.
- Electrical Metallic Tubing (EMT) – These are by far the most commonly used conduits, even though they’re not technically conduits at all (they’re actually classified as tubings). EMTs are usually called ‘thinwalls’, since they have a much thinner wall than RMCs, which is why they should not be used in places where they’re likely to encounter stresses. These are also available in galvanized steel or with a rust-resistant coating, and can be used in most indoor and outdoor applications, as ground conductors, embedded in concrete and for direct burial too. They are much lighter and bend easily, and fittings for EMTs are both readily available and inexpensive.
- Flexible Metallic Conduit (FMC) – Commonly known as ‘Greenfield’ or ‘flex’ conduits, FMCs are made from spirally wound metal strips that interlock. They’re most often used for the last few feet of wiring, where conventional conduit systems are difficult to maneuver and terminate. Another great advantage of FMCs is their ability to absorb vibrations and allow movement, so they’re often used to house wiring for pumps, motors and manufacturing equipment. The downside is that they don’t offer much protection against impacts and corrosion, cannot be used outdoors, buried or embedded.
- Liquidtight Flexible Metallic Conduit (LFMC) – Also called ‘liquidtight’ and ‘sealtight’, they are manufactured the same way FMCs are, except LFMCs have an overall non-metallic outer covering that is waterproof and resistant to cracking from sunlight exposure. The additional protection from the covering overcomes most of the limitations of FMCs, so they can be used outdoors and in wet conditions, provided water is not allowed to enter from joints and end connectors. While they can be also installed with direct burial, they are still vulnerable to physical damage and should not be exposed to stresses, or encased in concrete.
- Aluminum Conduits – These are a type of rigid conduit and are also common in commercial and industrial applications. Since aluminum resists corrosion extremely well, aluminum conduits are preferred in locations where they would be exposed to large amounts of water, or corrosive substances. They are, however, particularly vulnerable to concrete, since the metal reacts to it, but can be treated with specific coatings if they need to be embedded in concrete.
- Polyvinyl Chloride Conduits (PVC) – PVC conduits are available in both Schedule 40 and 80, which are basically ratios of the thickness of the wall to its outer diameter. Both have the same outer diameter, which means a Schedule 40 conduit will have a thinner wall, so it is not permitted for installations where the conduit will be exposed to physical stresses. Most commonly, the sections have one belled end and can be joined together, or to fittings using solvent welding. PVC conduits are rated for almost any application, especially since they are flame retardant, resistant to sunlight and very affordable. However, they cannot be used in hazardous locations, as ground conductors or in locations where they are exposed to ambient temperatures over 50°C.
- Liquidtight Flexible Nonmetallic Conduit (LFNC-B) – Like the flexible metallic conduits, LFNC-B conduits are also generally used in short sections towards the end of a conduit. They have a seamless, smooth inner wall surface with integral reinforcement and the exterior is waterproof, resistant to sunlight and dust. Since they can be buried, encased in concrete and used in wet locations, LFNC-B conduits are an excellent choice for indoor and outdoor applications, both exposed and concealed. There are a few drawbacks though, one of which is they are not suitable for housing conductors carrying over 600 volts. Additionally, they are susceptible to physical damage and a separate equipment grounding conductor will need to be installed if needed.
- Electrical Nonmetallic Tubing (ENT) – Though there is no code requiring it, ENTs are usually light blue in color. They are pliable corrugated raceways made from non-metallic material that is resistant to chemicals, moisture and flames. Certain fittings are designed specifically to be used with ENTs, but regular PVC fittings can also be used with solvent welding. They are easy to install, without the need for any special equipment, though the environments they are rated for are limited. ENTs can only be used indoors, in areas where they will not be exposed to physical damage, hazardous conditions, temperatures over 50°C and the conductors do not carry over 600 volts.
To ensure the conduit system meets the requirements of regulatory authorities, safety standards and local building codes, you need to take into account the conditions at the installation location, as well as the method of installation. Some installations may also require different types of conduits to be used in the same system.
Conduit fittings are available in a huge variety of sizes, shapes and materials, and they’re normally used for connecting runs of conduit together, and for connecting conduit ends to boxes, enclosures or electrical devices. Fittings are needed to connect conduits to boxes or enclosures of different sizes and when the direction of most metallic conduits has to be changed. There are also straps and clamps, which are used to provide additional support to conduits and to keep them secured. You may need to use special types of fittings if a conduit run is likely to be exposed to moisture, vapors, or hazardous conditions.
Types of Fittings
Section 110-3 of the NEC requires all the components to be listed and labelled, so all listed parts meet certain construction and performance requirements. However, the Code does not specify the material used for the fittings, of which there are several. This will be determined by availability, design considerations or personal preference. If you’re unsure about how the conditions will affect the fittings, you can contact a manufacturer with details of your application, consult a local distributor, or get an engineering recommendation.
Conduit fittings can be listed as follows, based on the function they serve and how they are installed:
- Conduit Bodies – These are tubular units with openings at each end for admitting conduits, and providing access to the wires. There are quite a few designs and you’ll find conduit bodies that connect two conduits in a straight line, create 90° bends and join two different types or sizes of conduit. Even the access point for the wires can either be exposed, or have a cover with screws. Since conduit bodies can perform such a wide range of functions, and some are also intended to be used as pull-boxes, they are marked with the purposes they are rated to serve, as well as the internal volume.
- Bends – To save time, equipment and labor costs, you can tackle changes in the direction of a conduit with pre-fabricated bends. Commonly called ‘factory bends’ or ‘elbows’, they are available in a variety of lengths and curvatures, and according to NEC requirements, you can bend certain tubings by hand, using a mechanical bender, or a hydraulic bender for larger ones. However, an installation may require a lot of bends, and bending conduits on location might damage them or reduce the internal diameter. Even with the use of factory bends, the NEC does restrict the number of bends you can have between pull boxes to a maximum of 360°, including offsets at the box or enclosure.
- Coupling – Conduit couplings are essential for almost any coupling system, for securely linking together lengths of conduit and attaching site-fabricated bends. Even though PVC conduits with a belled side can be linked without couplings, they would still be needed for sections where the conduit has been cut to size. Couplings are available in a variety of sizes and some of them, like rigid conduit couplings, are threaded on the inside. However, when the conduit is passing through a wall or making some other transition, consider using a conduit body instead. Even for the first sections of conduit entering or leaving the box or enclosure, use pullboxes or bodies so the conductors can easily be retracted if the enclosure needs to be removed.
- Drains – In areas where the temperature varies significantly, or the conduit runs from a warm area to a cooler one, moisture in the air starts condensing. To prevent drips and water-logging in the conduit runs, install drains at the lowest points of each run that might be affected, or at locations where water might get trapped and accumulate. For embedded and buried conduits, installing a run within the encased part may be difficult. You can create a low-point or a dip just before the conduit goes underground and install a drain there.
- Bushings and Locknuts – Bushings create a smooth entry point to conduits without any sharp edges, protecting the conductors from damage during wire pulls. They are also extremely important when the conduit system enters an enclosure or bus box. A bushing is installed on the inside of the box opening and threaded into the conduit end, separating the conductors from the edges of both the opening and the conduit end. Locknuts are threaded on the inside, with teeth on one surface or both, which grip the surface. They installed on both sides of the opening to ensure that both the conduit and bushing are held firmly in place. If the locknut has teeth on only one side, that side should face the box.
- Unions – Conduits often run along areas where space is constrained and couplings are difficult to install. Apart from that, sections might at some point get damaged, which is where unions are very useful. They have two separate heads and a locking mechanism which can connect two pieces of conduit together, even if they can’t be physically turned. Unions are available in different configurations with male and female heads, or a combination of both. The heads can be fit on each individual conduit end and secured together, usually with a surface nut that slips over the two parts and locks them together.
- Nipples and Hubs – Nipples can save time and money when a conduit is needed between enclosures that are placed closed to each other, or for short runs where field-threading the ends of a conduit is difficult. Like prefabricated bends, nipples are threaded on both ends and can be installed quickly and easily. As per the NEC, a nipple cannot exceed 24 inches in length, beyond which it is considered a raceway and can only be used for three current carrying conductors. Hubs are used to connect conduits to bus boxes or enclosures that don’t have a factory-threaded entry.
- Reducers and Reducing Washers – A reducer is a special kind of combination coupling that allows you to connect raceways of one trade size to larger or smaller ones. Although it’s generally better to use a pull box, reducers are handy for areas that have limited space. Reducing washers, or threaded reducers, are similar to hubs, except both the inner and outer surfaces are threaded. They are used to connect conduits to enclosure or box openings that have a larger diameter, with locknuts on both sides to fasten them in place.
- Straps and Clamps – Long runs of conduit and tubing require additional support to keep them from sagging or breaking under their own weight. Straps are used to mount the conduit to ceilings or walls using a screws or bolts. To fasten conduit runs to fixed structures, like beams, hangers or unistrut channels, the conduit is supported using clamps. For most installations, the NEC specifies the maximum length of unsupported RMC and EMT running in a straight line as 10 feet, and within three or five feet of a box. For liquidtight flexible conduits, this reduces to every 4.5 feet of conduit length and within 12 inches of a box or enclosure.