Nonmetallic-sheathed cable (NMC) (fig. 9-14,
view A) is more commonly called by the trade
name ROMEX, ROMEX (NMC) comes in
sizes No. 14 through 2 for copper conductors and
No. 12 through 2 for aluminum or copper-clad
aluminum conductors. This type of cable comes
with a bare (uninsulated) ground wire. The ground
wire is laid in the interstices (intervals) between
the circuit conductors and under the outside braid.
The ground wire is used to ensure the grounding
of all metal boxes in the circuit, and also to
furnish the ground for the grounded type of
convenience outlets that are required in Navy
installations. Nonmetallic-sheathed cable is used
for temporary wiring in locations where the use
of conduit would be unfeasible. The use of Romex
as service entrance cable, in garages, in storage
battery rooms, imbedded in poured concrete, or
in any hazardous area is NOT authorized.
Metallic-armored cable (fig. 9-14, view B), also
called BX cable, is used in naval installations for
temporary wiring, but unlike Romex, its use in
commercial installation is restricted. Most city
building codes restrict the use of BX cables to oil
burner control circuits and the like. A difficulty
with BX is the fact that it tends to ground after
installation. Small metal burrs on, the armor can,
because of vibration, penetrate the insulation and
cause a ground.
BX cables come in sizes from No. 14 to 2
AWG, and each cable may contain one, two,
three, or four conductors. The armor on the cable
furnishes a continuous ground between boxes.
As mentioned earlier, electrical conductors are
available with various kinds of insulating
materials. Some of these are rubber, thermo-
plastic, and varnished cambric. Special types of
paper, glass, silk, and enamel are also used to
insulate conductors, but with less frequency than
those previously mentioned. The NEC® recom-
mends insulation of certain kinds for use in
dry, damp, and wet locations. Underground
installations, those in concrete slabs and masonry,
those in direct contact with the earth, and those
subject to saturation with water or other liquids
are considered wet-location installations.
Another factor to consider in the choice of
insulation is temperature. Different insulations
have different maximum temperature ratings.
Check the NEC® and applicable LOCAL
CODES to be sure you are using the appropriate
insulation for the location and temperature
considered in the plans. Some examples of the
composition of insulation, the location that
applies, and their maximum temperature rating
Type RH is a heat-resistant compound, that
will stand higher temperature than Type R. This
type is commonly used in dry locations. The
maximum temperature rating is 167°F.
Type RHW is a moisture-resistant rubber
compound for use where the wire may be subject
to wet conditions. This type is used in both wet
and dry locations. The maximum temperature
rating is 167°F.
Type RUH is a high grade rubber compound,
consisting of 90-percent latex. This type is often
used for direct burial in dry locations. The
maximum temperature rating is 140°F.
Thermoplastic insulation has the advantage of
long life, toughness, and a dielectric strength (that
is, a capacity for insulating) equal to that of
rubber. It requires no protective covering over the
insulation. Common types of thermoplastic
insulation are Types T, TW, and TA. Type T is
suitable only for dry locations with a maximum
temperature rating of 140°F. Type TW is
moisture-resistant, and again, with a temperature
rating of 140°F. Type TA is a thermoplastic-
asbestos compound that combines the character-
istics of Types T and TW. This type has a
maximum temperature rating of 194°F. Its use is
restricted to switchboard wiring.
Varnished cambric insulation has an insulating
quality midway between that of rubber and paper.
It is more flexible than paper; its dielectric strength
is greater than that of rubber. This type is not
adversely affected by ordinary oil and grease. It
is manufactured in either standard type (black
finish), or in the heat-resistant type with a yellow
finish. Varnished insulation is restricted to dry
locations in areas such as motor leads,
transformer leads, and high-voltage cables.
Conduits and Fittings
An electrical conduit is a pipe, tube, or other
means in which electrical wires are installed for
protection from accidental damage or from the
elements. If pipes or tubing is used, the fittings
depend upon the pipe or tubing material. The