EC&M’s Electrical Calculations Handbook - Chapter 11 pptx

Raceway Types and Their Characteristics Although a very few types of conductors are rated for use without a raceway such as for direct burial or for aerial installation, most conductors

Raceway Types and Their Characteristics

Although a very few types of conductors are rated for use without a raceway (such as for direct burial or for aerial installation), most conductors require protection in the form

of a raceway The most common raceways are conduit (both the metallic and nonmetallic) and sheet-metal wireways of various configurations For the convenience of the engineer

or designer selecting a raceway type for a particular instal-lation, a brief summary of the types of raceways normally used, along with their trade names, is as follows:

Rigid metal conduit (RGS) Can be made of heavy-wall

steel or aluminum

Intermediate metal conduit (IMC) Normally made of

steel conduit

Electrometallic tubing (EMT) Thin-walled steel conduit Electric nonmetallic tubing (ENT) Corrugated plastic

flexible raceway

Nonmetallic underground conduit (PVC) Schedule 40

(heavy wall) or schedule 20 [called EB, for “encased bur-ial” (in concrete)]

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Flexible metallic tubing, Greenfield, spiral metal flexible conduit.

Liquidtite flexible metallic conduit, Sealtite, flexible metal conduit with an overall PVC waterproofing covering Surface metal and nonmetallic raceways, Wiremold Multioutlet assembly, Plugmold.

Cellular metal floor raceways, underfloor duct, Q-deck, Walker duct.

Cellular concrete floor raceways, Flexicore, Trenchduct Wireways.

Busways, bus duct.

Cablebus.

Boxes.

Auxiliary gutters.

Condulets and similar fittings.

The most common types of metallic raceways are elec-trometallic tubing (EMT) and rigid galvanized steel or rigid aluminum conduit Rigid galvanized steel is available in both a standard wall thickness and a lighter-weight type

known as intermediate metal conduit (IMC) Figure 11-1

shows the various common wiring methods in use today, along with the proper application for each to facilitate the correct selection of the wiring method type for a specific application

The outside dimensions (OD) of these raceways are all approximately the same for each trade size, so the cross-sec-tional area available within each type of raceway for the placement of conductors varies with the type of raceway Figure 11-2 on p 315 shows the available cross-sectional areas within each type of raceway for the installation of con-ductors

Pulling conductors into conduits requires considerations

of pull-in friction and cable pinching at conduit bends, and operating conductors within raceways requires considera-tions of heat retention and temperature increase within the

Figure

raceway These considerations are both lessened in effect by limiting the raceway fill to 40 percent for three or more con-ductors, to 31 percent for two concon-ductors, and to 53 percent for one single round conductor Accordingly, Fig 11-2 also provides the usable cross-sectional area within each type and size of raceway for 100 and 40 percent fill Figure 11-3

on p 316 provides a sample conduit fill calculation

Many different types of enclosures are available, and they are selected to match the environment into which they will

be located Figure 11-4 on p 317 is a listing of the National Electrical Manufacturers Association (NEMA) enclosures along with a description of the installation condition for which each is intended

Heavy-wall rigid steel conduit in an industrial plant.

Figure

Figure 11-3 Solve for minimum conduit size given wire insulation type, wire size, and wire quantity.

Figure