Table 5-1. - Percentage of Current-Carrying Capacity of Conductors
|PERCENT OF NORMAL CURRENT-CARRYING CAPACITY||NUMBER OF CONDUCTORS|
|80||4 through 6|
|70||7 through 24|
|60||25 through 42|
|50||43 and above|
requirements, and environmental and operating temperatures are factors considered in the selection of the type of insulating material for a particular job.
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.
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.
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 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.Continue Reading