Table 5-1.Percentage of Current-Carrying Capacity of
PERCENT OF NORMAL
4 through 6
7 through 24
25 through 42
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.
Size, Number, and Ampacity
The wire size most frequently used for interior
wiring is No. 12 AWG, used as a solid or stranded
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
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
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 manufacturers 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.