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  • 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.
    Aluminium 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. Aluminium 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. Aluminium conduit is generally lower cost than steel in addition to having a lower labour cost to install, since a length of aluminium conduit will have about one-third the weight of an equally-sized rigid steel conduit.

     

    PVC conduit has long been considered the lightest in weight compared to steel conduit materials, and usually lower in cost than other forms of conduit. 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.

     

    Flexible conduits are used to connect to motors or other devices where isolation from vibration is useful, or where an excess number of fittings would be needed to use rigid connections. Electrical codes may restrict the length of a run of some types of flexible conduit.
    Flexible metallic conduit is made by the coiling of a self-interlocked ribbed strip of aluminium or steel, forming a hollow tube through which wires can be pulled. FMC is used primarily in dry areas where it would be impractical to install EMT or other non-flexible conduit, yet where metallic strength to protect conductors is still required. The flexible tubing does not maintain any permanent bend, and can flex freely.
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