70 Modern Commercial Wiring
Copyright Goodheart-Willcox Co., Inc.
Overcurrent protection of busways is required at the
supply end. If a smaller busway is tapped from a larger
busway, overcurrent protection at the tap point should
protect the smaller busway. An exception allows a short
tap (50′ or less) with an ampere rating of at least 1/3 that
of the overcurrent device protecting the larger busway to
be exempt from the required overcurrent protection at the
tap point.
Further information and requirements concerning
busways can be found within Article 368 of the Code.
CAUTION
Busways must not be used where subject to
mechanical or physical damage, where there are
corrosive surroundings (such as in battery rooms),
where it is damp or wet, where there are explosive
gases or vapors, where there is ignitable dust or
fibers, where embedded in concrete, underground,
in any outdoor or hazardous area—except where
expressly approved for the purpose.
Cable Trays
Cable trays are open cable-supporting assemblies used
in a variety of commercial and industrial buildings. Cable
trays are not enclosed, so they do not fit the description of
raceways, but they have the same function. Tray systems
are fully recognized as an approved method for wiring.
Cable trays resemble troughs, open at the top
(although covers are often used), with ventilated bottom
sections, Figure 5-20. There are two main types of cable
trays: trough and ladder.
There are many specific rules regarding the types of
wire and cable that can be used in a tray system, and how
those wires and cables must be arranged within the tray.
Article 392 of the Code permits trays to be used as a
support system for wiring methods that can be used without
a tray. Uses permitted and not permitted are as follows:
• Where single conductor building wire is used in a
tray, only size 1/0 AWG or larger is permitted and
must be marked as suitable for tray installation.
Further, this only applies to industrial installa-
tions; only multiconductor cable is permitted in a
tray within commercial premises.
• A metallic cable tray is acceptable as the equip-
ment grounding conductor for the circuits within
the tray.
• Nonmetallic cable trays are permitted in areas
where there are corrosive conditions.
• Multiconductor cables rated 600 volts or less
may be placed in the same cable tray. Tray cable
rated over 600 volts can be placed in the same
tray with cables rated under 600 volts if a
noncombustable barrier is installed to separate
the high and low voltage cables.
• Cables within a cable tray can be spliced.
• Single-conductor cable can be used only if multi-
conductor cable is not available.
Multioutlet Assemblies
The Code addresses multioutlet assemblies in Article
380. Essentially, these are two-piece assemblies. The top
piece (cover) is prepunched to accept receptacles at close (6″
and up) intervals. The receptacles can be factory installed.
This method of wiring is particularly useful and
commonly found in laboratories, workshops, stores,
schools, and offices. It is installed exposed on the surface
like other surface raceway. It may pass through walls
provided there is no receptacle within the partition and
the covers on either side can be easily removed.
Review Questions
Answer the following questions using the information
provided in this chapter.
1. What is the maximum ambient temperature for
PVC conduit?
2. What is the maximum interval for vertical support of
a 4 AWG copper conductor?
Figure 5-20. Cable trays are used to support cables in industrial
settings. (PW Industries, Inc.)