Instructor: Radian Belu, PhD EET 323 – Electrical System Design Lecture 8: Conduit and Raceway Systems Instructor: Radian Belu, PhD

Lecture Objectives

Rigid Metal Conduit (RMC) RMC – circular cross-section and are the heaviest wall of all metal conduit, made of steel or aluminum, with trade ranges from ½” to 6”. The standard length of RMC is 10 ft. Article 344 of NEC discuss the requirements for use and installation of RMC

Intermediate Metal Conduit (IMC) IMCs are rigid steel conduit, in circular cross-sections with trade ranges from ½” to 4”. Article 342 of NEC discuss the requirements for use and installation of IMC. IMC can be cut, reamed, and threaded, as required in the field (the same as RMC) In accordance with the section 342.210 of the NEC, the IMC conduit must be clearly identified at least 5 ft along its length.

Electric Metallic Tubing (EMT) EMTs are rigid steel or aluminum conduit, in circular cross-sections with trade ranges from ½” to 4”. Article 358 of NEC discuss the requirements for use and installation of EMT. The outer finishing of EMTs must make them clearly distinguishable from RMTs EMT is not permitted to be threaded in the field

Rigid Non-Metallic Conduit (RNC) RNCs are rigid PVC conduits, in circular cross-sections. Article 352 of NEC discuss the requirements for use and installation of RNC. The standard length of RNC is 10 ft, including a coupling at one end.

Electrical Non-Metallic Tubing (ENT) ENT is coruggated, pliable raceway of circular cross-section. Article 362 of NEC discuss the requirements for use and installation of RNC. ENT is easily bent to accommodate field installation. The standard length of ENT is 10 ft, and in trade sizes of ½” to 2”. Couplings and fittings must be listed for the use with this type of conduit.

Flexible Metal Conduit (FNC) FMC consists of a helical-wound metal strip formed to produce a circular cross-section (Figure 8-8). Article 362 of NEC discuss the requirements for use and installation of FNC. FNC is available in trade sizes of 3/8” to 4” and may be used in both exposed and concealed applications, and is commonly used for equipment connection. Section 348.12 of NEC lists uses not permitted for FMC. Note that FNC does not seal the conductors form the environment. Support requirements for FMC are given in section 348.30 of the NEC.

Conductor Sizing Proper sizing of the conduit is necessary to ensure easy installation and to prevent the damage of the conductors during the installation. Factors use in the selection of the conduit size are: fill and the probability of jamming.

Expansion Characteristics A conduit exposed to a wide temperature variations may expand or contract. The expansion or contraction of a non-metallic conduit can be significant if the conduit is of any appreciable length and if the temperature variable is large. Expansion of non-metallic conduit must be take into account in the design of raceway. If the conduit expansion exceeds ¼”, expansion fittings must be installed to allow the movement of the conduit, to avoid the bucking, the conduit damage or the damage of the termination enclosure at each end of the conduit run (Figure 8-12).

Wire-ways and Auxiliary Gutters Wire-ways and auxiliary gutters are short section of metallic or nonmetallic rectangular or square cross-section troughs. Wire-ways (Article 376 and 378 of the NEC) are generally used to connect electrical equipment such as meter sockets, panels, and disconnect enclosures located in the same general area. Auxiliary gutters (Article 366 of the NEC) are usually connected directly to panels, disconnect enclosures, and so on to allow for a larger wire space for banding and termination purposes. Splices and taps are permitted to be made in wire-ways and auxiliary gutters.

Wire-ways and Auxiliary Gutters Wire-ways are generally available in the following trade size: 21/2”x21/2”; 4”x4”, 6”x6”; 8”x8”; 10”x10”; and 12”x12” Standard length are 12”, 24”, 36”, 48”, 60”, and 120”. Figure 8-16 shows examples of wire-ways and auxiliary gutters. The support requirements for metallic and nonmetallic gutters are discussed in Sections 366.4(A) and 366.4(B) of the NEC, and support requirements for metallic and nonmetallic wire-ways are discussed Sections 376.30 and 378.30 of the NEC. Articles 366, 376, and 378 discussed the uses permitted and the uses not permitted for the various wire-ways and auxiliary gutters. Expansion and contraction of these must be taken into account in design.

Wiring Methods Article 300 of the NEC discussed wiring methods and deals with primarily with installation methods; important in the design of electrical systems. Generally, conductors of circuit rated 600 V and below must be kept physically separated from the conductors of circuits above 600 V (Section 300.3(C) (2) of the NEC). In addition, Section 300.3(C) (1) of the NEC requires conductors of different circuits occupying the same raceway to have an insulation rating equal to the maximum circuit voltage in the raceway. Nonmetallic sheathed cable installed in bored holes in wood framing members must be protected from penetration by nails or other objects (Section 300.4(B)(2) of the NEC). A minimum distance of 1 1/4” in bored holes from the edge of the wood framing is required, or by placing a steel plate or bushing at least 1/4 “ thick over the wiring to provide the required protection. Steel plates are not required to protect rigid metal conduit, intermediate metallic conduit, electric metallic tubing, or rigid nonmetallic conduit.

Cable Puling Calculations Cable puling calculations are necessary to ensure that maximum tension on the cable is not exceeded during the installation, to prevent the cable damage. The maximum allowable tension on a conductor is a function of maximum allowable conductor stress, expressed in terms of lb/cmil.

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