Conduits

In most commercial construction and large multifam-ily residential construction, individual plastic-insulated conductors are placed in metal conduits. The NEC generally requires all wiring to be enclosed in a rigid metal corrosion-resistant conduit, which protects wiring from injury and corrosion and serves as a system ground. The conduit also protects against fire hazards due to overheating or arcing of conductors. In addition, it provides a corrosion-resistant support for the conductors. Wires are installed in the conduits after the conduit system has been inspected and approved.

Steel conduit (Fig. 30-6) protects the enclosed wiring from mechanical injury and damage from the surrounding atmosphere. It provides a grounded metal enclosure for wiring to avoid shock hazards, as well as a system ground path. Steel conduit protects its surroundings from fire hazard from overheating or acing of the enclosed conductors.

There are three types of steel conduit, differentiated by wall thickness. Heavy-wall steel conduit, also called rigid steel conduit, is made of heavy-walled steel tubing joined by screwing directly into a threaded hub with locknuts and bushings. Intermediate metal conduit (IMC) and electric metal tubing (EMT or thin-wall conduit) are made of thin-walled steel tubing joined by compression or setscrew couplings. The inside diameters of IMC and EMT are larger than that of heavy-wall steel conduit, due to the thinner walls, which makes pulling wires through easier. They are also lighter and easier to bend in the field, but the NEC restricts their uses.

Steel conduit is fastened to the structure of the building with pipe straps and clamps. Banks of conduits hung from the ceiling use trapeze mountings. Conduit may also be installed in a concrete slab. The slab is covered with a concrete topping, which holds the conduit. The top of the conduit must be a minimum of 19 mm (4 in.) below the finished floor surface to prevent cracking.

Flexible metal conduit uses a helically wound metal conduit for connection to motors and other vibrating equipment. Flexible metal conduit is also known by the trade name "Greenfield." It consists of an empty, spirally wound, interlocked armored steel or aluminum raceway. Flexible metal conduit is used for motor connections and other locations for acoustical and vibration isolation of motors, ballasts, and transformers. It is also used for wiring inside metal partitions. Flexible metal conduit with a liquid-impervious plastic jacket for use in wet locations is known by the name "Sealtite."

Aluminum conduit is lighter than steel conduit, and resists corrosion better. Aluminum conduit loses less voltage over long distances, and will not spark. Aluminum conduit generally doesn't require painting. However, aluminum has several disadvantages that limit its use. When embedded in concrete, it may cause spalling and cracking, and it corrodes in earth. The threaded joints between sections freeze and the threads conduit conduit

Steel conduit Figure 30-6 Conduit.

Steel conduit Figure 30-6 Conduit.

deform. In addition, it is difficult to make electrical contact with ground straps from aluminum conduit.

Nonmetallic conduit may be made of fiber, asbestos-cement, soapstone, rigid PVC, or high-density polyethylene. Nonmetallic conduit is used in nonhazardous areas, and must be flame-retardant, tough, and resistant to heat distortion, sunlight, and low temperatures. Many of the materials used have limitations. For example, PVC may be used for indoor exposed locations, but is limited to nonhazardous areas. Asbestos-cement, PVC, and fiber can be used outdoors or underground. Nonmetallic conduit requires a separate ground wire.

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