Electrical conduit
An electrical conduit is a tube used to protect and route electrical wiring in a building or structure. Electrical conduit may be made of metal, plastic, fiber, or fired clay. Most conduit is rigid, but flexible conduit is used for some purposes. Conduit is generally installed by electricians at the site of installation of electrical equipment. Its use, form, and installation details are often specified by wiring regulations, such as the US National Electrical Code and other building codes.
Comparison with other wiring methods
Electrical conduit provides very good protection to enclosed conductors from impact, moisture, and chemical vapors. Varying numbers, sizes, and types of conductors can be pulled into a conduit, which simplifies design and construction compared to multiple runs of cables or the expense of customized composite cable. Wiring systems in buildings may be subject to frequent alterations. Frequent wiring changes are made simpler and safer through the use of electrical conduit, as existing conductors can be withdrawn and new conductors installed, with little disruption along the path of the conduit.A conduit system can be made waterproof or submersible. Metal conduit can be used to shield sensitive circuits from electromagnetic interference, and also can prevent emission of such interference from enclosed power cables. Non-metallic conduits resist corrosion and are light-weight, reducing installation labor cost.
When installed with proper sealing fittings, a conduit will not permit the flow of flammable gases and vapors, which provides protection from fire and explosion hazard in areas handling volatile substances.
Some types of conduit are approved for direct encasement in concrete. This is commonly used in commercial buildings to allow electrical and communication outlets to be installed in the middle of large open areas. For example, retail display cases and open-office areas use floor-mounted conduit boxes to connect power and communications cables.
Both metal and plastic conduit can be bent at the job site to allow a neat installation without excessive numbers of manufactured fittings. This is particularly advantageous when following irregular or curved building profiles. Special tube bending equipment is used to bend the conduit without kinking or denting it.
The cost of conduit installation is higher than other wiring methods due to the cost of materials and labor. In applications such as residential construction, the high degree of physical damage protection may not be required, so the expense of conduit is not warranted. Conductors installed within conduit cannot dissipate heat as readily as those installed in open wiring, so the current capacity of each conductor must be reduced if many are installed in one conduit. It is impractical, and prohibited by wiring regulations, to have more than 360 degrees of total bends in a run of conduit, so special outlet fittings must be provided to allow conductors to be installed without damage in such runs.
Some types of metal conduit may also serve as a useful bonding conductor for grounding, but wiring regulations may also dictate workmanship standards or supplemental means of grounding for certain types. While metal conduit may sometimes be used as a grounding conductor, the circuit length is limited. For example, a long run of conduit as grounding conductor may have too high an electrical resistance, and not allow proper operation of overcurrent devices on a fault.
Types
Conduit systems are classified by the wall thickness, mechanical stiffness, and material used to make the tubing. Materials may be chosen for mechanical protection, corrosion resistance, and overall cost of the installation. Wiring regulations for electrical equipment in hazardous areas may require particular types of conduit to be used to provide an approved installation.Metal
Rigid metal conduit is a thick-walled threaded tubing, usually made of coated steel, stainless steel or aluminum.Galvanized rigid conduit is galvanized steel tubing, with a tubing wall that is thick enough to allow it to be threaded. Its common applications are in commercial and industrial construction. It is designed to protect wire and connectors.
Intermediate metal conduit is a steel tubing heavier than EMT but lighter than RMC. It may be threaded.
Electrical metallic tubing, sometimes called thin-wall, is commonly used instead of galvanized rigid conduit, as it is less costly and lighter than GRC. EMT itself is not threaded, but can be used with threaded fittings that clamp to it. Lengths of conduit are connected to each other and to equipment with clamp-type fittings. Like GRC, EMT is more common in commercial and industrial buildings than in residential applications. EMT is generally made of coated steel, though it may be aluminum.
Aluminum conduit, similar to galvanized steel conduit, is a rigid tube, generally used in commercial and industrial applications where a higher resistance to corrosion is needed. Such locations would include food processing plants, where large amounts of water and cleaning chemicals would make galvanized conduit unsuitable. Aluminum cannot be directly embedded in concrete, since the metal reacts with the alkalis in cement. The conduit may be coated to prevent corrosion by incidental contact with concrete. Aluminum conduit is generally lower cost than steel in addition to having a lower labor cost to install, since a length of aluminum conduit will have about one-third the weight of an equally-sized rigid steel conduit.
Non-metal
conduit has long been considered the lightest in weight compared to steel conduit materials, and usually lower in cost than other forms of conduit. In North American electrical practice, it is available in thirteen different size and wall thicknesses, with the thin-wall variety only suitable for embedded use in concrete, and heavier grades suitable for direct burial and exposed work. Most of the various fittings made for metal conduit are also available in PVC form. The plastic material resists moisture and many corrosive substances, but since the tubing is non-conductive an extra bonding conductor must be pulled into each conduit. PVC conduit may be heated and bent in the field, by using special heating tools designed for the purpose.Joints to fittings are made with slip-on solvent-welded connections, which set up rapidly after assembly and attain full strength in about one day. Since slip-fit sections do not need to be rotated during assembly, the special union fittings used with threaded conduit are not required. Since PVC conduit has a higher coefficient of thermal expansion than other types, it must be mounted to allow for expansion and contraction of each run. Care should be taken when installing PVC underground in multiple or parallel run configurations due to mutual heating effect of densely packed cables, because the conduit will deform when heated.
LSZH conduit : This new kind of electrical conduit is generally made of plastics such as PP or PE.
In the industry, it has many names, summarized in the following table:
It is a new type of plastic wire conduit in the industry. Compared with PVC electrical conduit, it has three advantages.
First: low smoke. Due to the unique material and formula, LSZH conduit only produces a small amount of black smoke when burning, and most compounds will absorb heat energy and release steam when burning. Compared with the large amount of smoke produced by PVC conduits, it reduces the interference to the visual field during the combustion process by reducing the amount and density of smoke;
Second: halogen-free. Unlike PVC, LSZH conduit does not release hydrogen chloride when burning, thereby reducing the possibility of being inhaled by people during combustion.
Third: environmental protection. In addition to being halogen-free, when the compound reaches a specific temperature, it absorbs heat energy, releases steam and does not release corrosive gases. This can make its application more extensive; for example, in new nuclear power plants, the use of LSZH cables and conduits will increase.
Fourth: flame retardant. Due to the chemical properties mentioned in the first point, the LSZH conduit absorbs heat energy and releases steam when burning, thus achieving a flame-retardant effect. The latest products on the market and UL test results can reach UL94 V-0 flame retardant with excellent performance.
Reinforced thermosetting resin conduit or fiberglass conduit is light in weight compared to metallic conduits, which contributes to lower labor costs. It is sometimes referred to as FRE which stands for "fiberglass reinforced epoxy", however this term is a legally registered trademark of FRE Composites. It may also provide lower material cost. RTRC conduit can be used in a variety of indoor and outdoor applications. Fiberglass conduit is available in multiple wall thicknesses to suit various applications and has a support distance very similar to steel. High temperature, low smoke, no flame, classified area, and zero halogen versions are also manufactured for specialty applications such as subway tunnels and stations and in the US can meet National Fire Protection Association 130 requirements. Like other non-metallic conduits, a bonding conductor may be required for grounding. Joints are epoxy-glued, which requires some installation labor and time for joints to set. RTRC conduit may not be bent in the field and appropriate fittings must be used to change directions, nor is RTRC conduit approved to support luminaires.
Rigid nonmetallic conduit is a non-metallic unthreaded smooth-walled tubing.
Electrical nonmetallic tubing is a thin-walled corrugated tubing that is moisture-resistant and flame retardant. It is pliable such that it can be bent by hand, and is often flexible although the fittings are not. It is not threaded due to its corrugated shape, although some fittings might be.