5424Ch05 - FM 5-424 CHAPTER 5 Conduit Wiring Conduit wiring...

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FM 5-424 Conduit Wiring 5-1 CHAPTER 5 Conduit Wiring Conduit wiring provides mechanical protection and electrical safety to persons and property and provides convenient and accessible ducts for the conductor. A well-designed electrical raceway system has adequate capacity for future expansion and is readily adaptable to changing conditions. Section I. Rigid Conduit USES AND ADVANTAGES Black-enamel or galvanized rigid metal con- duit is approved for use under most condi- tions and in most locations. Metal conduit and fittings that are protected from corro- sion by enamel may only be used indoors and in areas that are not subject to severe corrosion. Although rigid conduit is gener- ally the most expensive type of wiring installation, its inherent strength permits installation without running boards and provides additional damage protection. Its capacity facilitates carrying more conduc- tors in one run than in any other system, and its rigidity permits installation with fewer supports than the other types of wir- ing systems. Moreover, the size of conduit used in the system's installation usually provides for the addition of several more conductors in the conduit when additional circuits and outlets are required in the run. MATERIALS Though the materials used in rigid conduit wiring have been outlined in detail in Chap- ter 2, the following discussion will review the advantages of these standard materials as well as their limitations. Rigid conduit (Figure 5-1, page 5-2) has the same size designations as water pipe. Con- duit smaller than 1/2 inch can only be used in finished buildings where extensions are to be made under plaster. In these installations, 5/16-inch conduit or tubing is permitted. The size of conduit is determined by the inside diameter. For example, 1/2-inch conduit has an inside diameter of approximately 1/2 inch. Standard conduit sizes used in interior wir- ing are 1/2, 3/4, 1, 1 1/4, 1 1/2, 2, and 2 1/2 inches. Larger sizes (up to 6 inches) are avail- able for special use in certain commercial and factory installations. Though conduit is made in dimensions simi- lar to water pipe, it differs from water pipe in a number of ways. It is softer than water pipe and thus can be bent fairly easily. In addi- tion, the inner surface is smooth to prevent damage to wires being pulled through it, and the finish is rust-resistant. Black-enamel conduit is used for dry and indoor installa- tions, and galvanized conduit is used in out- side installations to provide moisture protection for the conductors. For wiring installations in corrosive atmospheres, alu- minum, copper-alloy, or plastic-jacketed con- duit is available.
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FM 5-424 5-2 Conduit Wiring CONDUCTORS Rubber-covered, insulated Types R and RH wire are used with conduit in most interior wiring installations; but thermoplastic, insulated Types T and TW are gaining favor because of their superior insulating charac- teristics. Underground or wet installations require the insertion of lead-covered cables in galvanized conduit for permanent protec- tion.
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