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

Raceways 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)]. Chapter 11 311 v Copyright 2001 by The McGraw-Hill Companies, Inc. Click here for Terms of Use. 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 312 Chapter Eleven Figure 11-1 Common wiring methods and locations where each is normally implemented. 313 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 conductors, 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. 314 Chapter Eleven Heavy-wall rigid steel conduit in an industrial plant. Figure 11-2 Cross-sectional areas of commonly used different types of raceways. 315 316 Chapter Eleven Figure 11-3 Solve for minimum conduit size given wire insulation type, wire size, and wire quantity. Figure 11-4 This is a listing of NEMA enclosures and the environments for which each is suitable. 317 . Chapter Eleven Heavy-wall rigid steel conduit in an industrial plant. Figure 1 1-2 Cross-sectional areas of commonly used different types of raceways. 315 316 Chapter Eleven Figure 1 1-3 Solve for minimum. so the cross-sec- tional area available within each type of raceway for the placement of conductors varies with the type of raceway. Figure 1 1-2 on p. 315 shows the available cross-sectional areas. Schedule 40 (heavy wall) or schedule 20 [called EB, for “encased bur- ial” (in concrete)]. Chapter 11 311 v Copyright 2001 by The McGraw-Hill Companies, Inc. Click here for Terms of Use. Flexible metallic