Chapter 5 Wiring Methods 63
Copyright Goodheart-Willcox Co., Inc.
Figure 5-9. Nonmetallic-sheathed cable. A—Type NM is often
called Romex. B—Type UF cable is used both indoors and
outdoors.
A
B
Type NM is used strictly indoors while types NMC
and UF are suitable for either indoor or outdoor applica-
tions. Further, type UF can be directly buried in the
ground.
Nonmetallic cable must be supported every 4′ - 6″
and within 12″ of a terminal box, junction box, outlet, or
fitting. It may be installed alongside or through studs,
joists, rafters, and other building members with the same
protection requirements and restrictions as with armored
cable. This type of cable may also be installed in unfin-
ished basements, attics, and crawl spaces, and may be
terminated in metal or nonmetallic boxes.
Metal-clad cable (MC)
This type of cable is a heavy-duty commercial and
industrial assembly consisting of one or more conductors
individually insulated and enclosed in an interlocked metallic
armor that consists of corrugated tubing or interlocking tape.
This type of cable looks similar to armored cable (BX). MC
cable is available in sizes 14 AWG up to 1000 kcmil. Its use
is permitted in a wide range of applications:
• Indoors or outdoors.
• Exposed or concealed.
• For direct burial.
• As open run cable, or in a raceway or a conduit tray.
• As aerial cable on a messenger wire.
• In hazardous locations.
• In wet and dry locations.
• For feeders, branch circuits, and service conductors.
• For power, control, lighting, and other circuit
applications.
Type MC cable cannot be used in destructive corrosive
environments, such as buried in concrete or earth that
exposes the cable to harmful chemicals and contaminants.
Raceways
A wiring method using raceways is more secure and
safe than a method using only cables. This is a result of the
added protection provided by the raceway. Unfortunately,
a method using raceways is also more expensive.
There are many different types of raceway. The
selection of a particular type depends on the specific
application.
NEC NOTE 100
Raceway: An enclosed channel of metal or nonme-
tallic materials designed expressly for holding wires,
cables, or busbars, with additional functions as
permitted in this Code.
Rigid metal conduit (RMC)
This extremely strong and versatile wiring method
may be installed inside, outside, in damp, wet, dry loca-
tions and in almost any kind of building. It can be
installed exposed or concealed in nearly all locations.
When connected and joined properly, the metal-to-metal
continuity is excellent and provides good grounding for
equipment fed by the conduit system.
Rigid metal conduit is manufactured in 10′ lengths,
but other sizes can be ordered. It can be field cut and
threaded. Threading must have a taper equivalent to
3/4″ to every foot. Bushings should be used at ends
where wires enter or exit the conduit to prevent damage
to the insulation.
There are numerous connectors and fittings used
with rigid metal conduit. See Figure 5-10. Most of
these fittings are also used with intermediate metal
conduit.
Generally, the conduit should be supported every 10′
and within 3′ of a junction, outlet, or fitting. The total
number of bends in a run of rigid metal conduit must not
exceed 360°, Figure 5-11.
NEC NOTE 344.30(B)(3)
For vertical rigid metal conduit risers from
machinery with threaded coupling, the distance
between supports can be increased to 20′, provided
that both ends of the riser are securely fastened and
no means of intermediate support is readily available.