58 Modern Commercial Wiring
Copyright Goodheart-Willcox Co., Inc.
General Rules and Requirements
The Code, under Article 300—General Requirements for
Wiring Methods and Materials, addresses the acceptable
methods of installing conductors. Choosing the right method
for a wiring job depends on the environment where the wiring
is to be installed. Certain wiring methods are only permitted
in a limited range of conditions. Others are acceptable in a
broad variety of situations.
Regardless of the specific wiring method, there are some
general rules common to many of the methods. These provi-
sions should be understood before beginning any wiring
installation. A brief look at these general provisions follows:
• Whether in cable or as single individual units,
conductors should be used within the voltage and
temperature range for which they are designed.
WARNING
Conductors placed in service where the voltage or
temperature may exceed their specific rating will
represent a serious hazard to the electrical system,
equipment, structure, and personnel.
• Normally, single conductors may only be
installed as part of a wiring method recognized
by the Code. Thus, routing individual conductors
without the protection or support of an approved
wiring method is strictly prohibited.
• For a circuit, the current-carrying conductors,
neutral conductor (where used), and equipment
grounding conductor must all be run within the
same conduit, cable, duct, tray, or enclosure.
Failure to comply with this requirement could
lead to inductance problems.
NEC NOTE 300.3(B)
All conductors of the same circuit and, where
used, the grounded conductor and all equipment
grounding conductors shall be contained within the
same raceway, cable tray, trench, cable, or cord. See
exceptions in Code.
• Circuits of different voltage may be run in the
same raceways or occupy the same spaces (enclo-
sures, equipment) providing that the voltage is
less than 1000 volts and the conductors each
have insulation ratings greater than or equal to
that of the circuit with the highest voltage rating.
For example, conductors rated for 300 volts can
be run in the same raceway as conductors rated
for 600 volts as long as the maximum circuit
voltage is 300 volts.
• Measures must be taken to protect conductors
against physical damage. When running cable
through bored holes in framing members, the
edge of the hole should be 1 1/4″ away from the
nearest edge of the member. If a cable or raceway
is placed along notches made in wood framing
members, the notches must be protected from
nails and screws by using a metal plate (minimum
1/16″ thick) to cover the notches. If cables or
nonmetallic raceway is run perpendicular to
exposed studs, rafters, or joists and subject to
damage, strips of 1 × 1 or 1 × 2 wood stock should
be placed along either side to protect the wiring
from damage. Figure 5-2 illustrates methods of
protecting conductors from physical damage.
Rules for Buried Conductors
Conduit, cables, and other raceways that are buried
must meet specific criteria. The conductors must be
protected so that damage does not occur.
Table 300.5 of the Code lists the minimum burial
depth for conductors 1000 volts nominal or less. The
table in Figure 5-3 is a condensed version of Table 300.5.
Deeper burial is not uncommon due to soil conditions,
interferences with other utilities, and structural footings.
Cables under buildings must be installed in raceway
and the raceway must extend past the exterior walls of the
building. Further, where buried conductors and cables
emerge from the ground, protection must be provided by
raceways that extend from the minimum cover distance
to at least 8′ above grade or to the point of entry into a
building. See Figure 5-4. At the point where under-
ground conductors emerge from a raceway, the raceway
should be fitted with a bushing or sealed.
NEC NOTE 300.5(D)(4)
Where the enclosure or raceway is subject to
physical damage, the conductors shall be installed in
rigid metal conduit, intermediate metal conduit,
RTRC-XW, or Schedule 80 PVC, or equivalent.
Regardless of the method of wiring used, all under-
ground installations must be grounded and bonded as
required by the Code. Refer to Chapter 10 of this text and
Article 250—Grounding and Bonding of the Code.
Splices and taps are permitted in buried conductors
and cables. Be sure to use materials that are suitable for
underground use. All splicing materials must be suitable for
the conditions and environment in which they are installed.