INTERIOR WIRING SYSTEMS Every interior wiring job you will be assigned will have its own particulars, depending upon what type of building or structure you are assigned to wire. Some of these particulars are whether to use rigid, flexible, or thin-wall conduits. The type of conductors you will use, whether single or a cable with multiconductors, such as Romex or flexible cord. These particulars, as mentioned above, depend on what type of job you are assigned, temporary or hard-wired, block or drywall construction. Use this information provided here to become familiar with installation requirements. Learn to use the proper cable, conductors, and conduit in the correct place. Make sure you are familiar with the various methods of bending, joining, and installing the various materials, and learn to select the proper fittings and accessories that you will need to install those materials. As you are referred to the NEC(c) throughout the chapter, look up the articles indicated This action will help you get acquainted with the NEC(c). A good CE takes pride in doing a neat, safe, and proper job.

CONDUCTORS AND CABLE SYSTEMS Electrical conductors generally consist of drawn copper or aluminum formed into a wire. They provide paths for the flow of electric current and usually have insulating material encasing the metal. The insulation material is provided to minimize short circuits and to protect personnel. Atmospheric conditions, voltage

Table 5-1.- Percentage of Current-Carrying Capacity of Conductors

PERCENT OF NORMAL NUMBER OF CURRENT-CARRYING CONDUCTORS CAPACITY 4 through 6 80 7 through 24 70 25 through 42 60 43 and above 50

06NP0182 requirements, and environmental and operating temperatures are factors considered in the selection of the type of insulating material for a particular job. 

Single Conductors A conductor may consist of a single, solid wire or a combination of a number of solid wires (stranded) that are not insulated from each other and share in carrying the total current.

A stranded conductor has the advantage of being more flexible than a solid conductor, thus making it more adaptable for pulling through bends in the conduit. Conductors vary in diameter. wire manufacturers have established a numerical system called the American Wire Gauge (AWG) standard. Table 8 of the NEC(c) shows how this numerical system eliminates the necessity for cumbersome, circular mil or fractional inch diameters in the description of wire sizes. Notice that the wire gauge numbers increase from 4/ 0 through 18 as the diameter of the wire decreases.

Size, Number, and Ampacity The wire size most frequently used for interior wiring is No. 12 AWG, used as a solid or stranded copper conductor. Table 310-17, column 2, of the NEC(c) shows the allowable ampacity of a single conductor in free air. No. 12 AWG (for types FEPW, RH, RHW, THW, THWN, XHHW, and ZW insulation) to be 35 amperes. However, the minute that same conductor is not alone in free air and is placed in a raceway, cable, or direct burial, you see, by referring to table 310-16, NEC(c), that its ampacity is reduced to 25 amperes, provided that not more than three conductors are in the raceway or cable. Table 5-1 of this training manual indicates the reduced ampacities for a variety of numbers of conductors in such a situation, according to the NEC(c). Suppose now that you have four to six No. 12 AWG wires in a conduit. The allowable current-carrying capacity would be only 80 percent of the normal, or 20 amperes. To ensure a current-carrying capacity of 25 amperes, you would have to use No. 10 wire that has a normal current-carrying capacity of 35 amperes, 80 percent of which is 28 amperes.

Cables A cable is an assembly of two or more conductors insulated from each other with an additional insulating or protective shield formed or wound around the group of conductors.

Nonmetallic Sheathed Cable Nonmetallic sheathed cable is more commonly called nonmetallic cable, NM cable, or Romex.

Nonmetallic cable consists of two or three insulated conductors in an outer sheath. It may have an added insulated or bare conductor to be used as an equipment ground. The outer sheath is made of a moisture-resistant, flame-retardant, nonmetallic material either of thermoplastic or treated braid.

Nonmetallic cable has copper, aluminum, or copper-clad aluminum conductors. Copper conductors used in cable range in size from No. 14 to No. 2 AWG. The size of aluminum conductors is from No. 12 to No. 2 AWG. Specific descriptive information must be marked on the exterior of nonmetallic cable, repeating at intervals of at least every 24 inches. The information required to be shown includes the manufacturer's name or trademark, maximum working voltage, wire size, and cable type. Most cable is also marked to show the number of conductors and whether it has a ground, as shown in figure 5-8. The ground wire is used to ensure

Figure 5-8.- Markings on nonmetallic cable.

the grounding of all metal boxes in the circuit, and it also furnishes the ground for the grounded type of convenience outlets.

Nonmetallic cable comes in two types: NM and NMC. Type NM cable has a flame-retardant and moisture-resistant cover. Type NMC cable is corrosion-resistant. Its covering is flame-retardant, moisture-resistant, fungus-resistant, and corrosion-resistant.

Refer to the NEC(c), Articles 336, for Types NM and NMC uses permitted and not permitted.

In naval installations, Romex is used primarily for temporary work, such as on Quonset huts. Civilian contractors, however, use it extensively for residential wiring. All connections in Romex must be at the junction or outlet boxes. Saddle, straight clamps, or cable connectors must be used for securing the cable to the boxes. In installations where Romex is permitted, the ground wire is fastened securely to create a good mechanical and electrical ground. When a bend is made in Romex, the radius of the bend should be not less than five times the diameter of the cable.