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Conduit is enclosed raceway used for physical protection of wire and cables in distribution runs. There are several types of metallic, coated metallic, and non-metallic conduit, with variations in strength, weight, inside diameter, rigidity or flexibility, and resistance to environment.
Metallic conduit—typically aluminum or stainless steel—offers high tensile strength and competitive lifecycle costs. When installed properly, by code, the raceway also serves as an equipment grounding conductor. Aluminum conduit offers high reflectivity and resistance to oxidation, while providing good electrical and thermal conductivity. Stainless steel conduit is resistant to corrosion and carries high pressure ratings.
Nonmetallic conduit, frequently made from polyethylene (PE) or polyvinyl chloride (PVC), is much lighter weight and easier to cut and fabricate on the job than comparable metallic conduit. Nonconductive and resistant to sunlight and impact, it is well suited for many wet and corrosive environments.
The three basic types of metallic conduit, distinguished by weight and wall thickness, are: RMC, rigid metallic conduit, the thickest walled and heaviest in weight; EMT, electrical metallic tubing, often referred to as thin-wall, the lightest in weight and easy to bend, cut and form; and IMC, intermediate metal conduit, which falls in between. Building codes treat all metallic conduit as noncombustible—an important characteristic—especially in applications such as installation of raceways in plenums. Metallic conduit can be galvanized with zinc to improve resistance to corrosion and abrasion or otherwise coated, with PVC or urethane, for example, to improve other characteristics.
Rigid non-metallic conduit (RNC) and fittings are made of extruded rigid plastic (polyvinyl chloride). Schedule 40 conduit and (the heavier-duty) Schedule 80 conduit are suitable for use above ground and underground.
High density polyethylene conduit: Type HDPE is suitable for installation outside a building, encased in concrete or direct buried, to carry electrical conductors or cables for voice/data/video. It is available smooth-walled and ribbed wall, which reduces surface contact with the cables during a pull.
Liquidtight flexible metallic conduit (LFMC) and liquidtight flexible nonmetallic conduit (LFNC) each feature a plastic jacket over the flexible core. The jacket seals the core from liquid, making the conduit suitable for use in corrosive environments.
Indeed, while conduit is pretty much a commodity product, manufacturers often exceed standard specifications to advance products beyond code requirements, improve handling, installation, weathering, reliability and other qualities.
For example, metallic conduit manufacturers have developed better paints and better methods of application in order to provide longer protection against corrosion than was the norm five or ten years ago.
Pulling and pushing wires through conduit is always hindered by friction. Electricians often need to apply lubricants as the wire is fed. Lubricants work well, but frequently they are messy. Metallic conduit manufacturers have developed organic coatings that make the conduit slippery, reducing friction so it is easier to pull wires through, often without the aid of any lubricant. The coatings also provide an additional layer of protection on the inside of the conduit.
Availability of long lengths of conduit—e.g. 20 foot pieces—can reduce the number of connections considerably on long runs.
Other innovations include color-coated conduit, which can enable differentiation among several systems—e.g. fire safety, datacom, high voltage, and security areas—after installation without need for spray paint, powder coating or taping in the field.
By William & Patti Feldman