the grounding of all metal boxes in the circuit, and it also
furnishes the ground for the grounded type of
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©, 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.
There is no firm requirement that a certain type of
box be installed for a specific purpose. The usual
practice is to install octagonal boxes for lighting outlets
and to install rectangular and square boxes for switches
and receptacle outlets. Round boxes are normally
installed overhead for lighting purposes especially
where the fixture canopy must cover the box. However,
when the need arises because of inside space
requirements, wall-surfacing materials used, number of
electrical devices to be mounted, or the availability of
boxes, almost any box can be used for any purpose.
The size and number of conductors to be installed in
a box have a definite impact on the selection of a box.
Each conductor in a box must have some free air space
to prevent a buildup of heat. As a result, the more
conductors, or the larger their size, the bigger the box
must be in which they are installed. The cubic inch
capacity of a box is determined by its length, width, and
depth. An increase in one or more of these dimensions
increases box capacity. When gangable boxes are
assembled together, box volume is the total of all
sections assembled together. Also, when a raised cover
or a box extension with volume markings is added to a
box, as shown in figure 5-9, its capacity is added to the
The box to be mounted in each location is selected
after the number and size of conductors it will contain
Figure 5-9.Extension ring.
are known In many cases, the usual box for the
intended purpose is not adequate. To ensure proper air
space for a conductor in a box, consult the table 370-
16(a) for metal boxes in the NEC©. This table lists
dimensions for common boxes, their cubic inch
capacity, and the maximum permitted number of
conductors in wire sizes No. 14 through No. 6. The
number of conductors listed does not make allowance
for fixture studs, cable clamps, grounding conductors,
switches, or receptacles, or straps for mounting these
devices. One conductor must be deducted from those
listed when the box contains one or more fixture studs or
cable clamps. Each strap containing one or more
devices requires deduction of one conductor.
Also, one conductor is deducted for one or more
grounding conductors that enter the box. A conductor
that runs through the box is counted as one conductor.
Each conductor that terminates in the box counts as one.
Fixture wires and conductors that do not leave the box,
such as an internal grounding wire, are not counted.
Let us use an example to see how the table works.
Two receptacle outlets are to be installed using No. 12
nonmetallic cable. One of the outlets is to be installed as
an extension to the other. Device boxes with cable
clamps are to be used if possible. The first step is to
determine the number of conductors that will be wired
through the box. Two conductors plus a grounding
conductor enter this box. Also, if a second outlet is to be
connected to this one, then two conductors plus a
grounding conductor must leave the box. If the
preceding rules are followed, we have four conductors
plus one for the grounding conductors, the equivalent of
one conductor for the cable clamps, and the equivalent
of one conductor for the receptacle outlet. This method
gives us an equivalent of seven conductors. In looking