Size, Number, and Ampacity

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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©  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©  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©, 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©. 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. 5-5

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