Cable trays feature flexibility unmatched by
conduit, as cables are easier to mark, remove
and find in cable trays. Cable trays are available
in a number of different configurations,
including ladder, ventilated trough, ventilated
channel, solid bottom, wire mesh, single rail,
and other similar structures. They are manufactured
in steel, aluminum, and fiber reinforced
plastic (FRP), although aluminum
accounts for about 70% of the cable trays used
in industry today.
Overloading cable trays
Cable trays come in a wide variety of sizes.
The appropriate size and number of cable trays
depends directly on the number and size of
conductors intended and the allowable fill area
as specified in the NEC. Also, since cable trays
offer flexibility for modification and expansion,
engineers and designers should plan cable tray
systems to be sized and designed to anticipate
both current and future needs.
Cable tray fill is addressed in the 2005 edition
of NEC Sections 392.8, 392.9, 392.10, and
392.12. The type of cable tray (e.g., solid, ventilated),
ampacity requirements, and the type
and voltage rating of cable used determines
the allowable fill for each cable tray — ventilated
cable trays provide for the greatest allowable
fill due to increased airflow. A generic
guideline provided by The Cable Tray Institute
indicates that cable trays should not be filled in
excess of 40-50% of the inside area of the tray
or of the maximum weight based on the cable
tray specifications. The NEC provides specific
and more detailed requirements for cable tray
fill. In any case, the best strategy is to review
and follow the rules set out in the NEC and
the manufacturer’s installation guides when
installing cables in cable trays.
Hazards associated with
overloaded cable trays
Overfilling and improperly securing wires in
cable trays can lead to a number of serious
hazards. Weight is one issue; all cable trays
and their associated supports are rated for a
specific maximum weight, based partly on the
allowable fill area and the spacing of the cable
tray supports. Overloading cable trays can
lead to a breakdown of the tray, its connecting
points, and/or supports, causing hazards to
persons underneath the cable tray and even
leading to possible electric shock and arcflash/
blast events from component failure
when the cables are suddenly no longer supported.
Additionally, cables in trays can be
damaged by improperly securing and installing
other cables and wires in the same cable tray.
The NEC requirements for cable tray fill also
consider the heat buildup in conductors while
current flows. When cable trays are overloaded,
excessive heat buildup in and around
live conductors can cause the insulation to
break down, leading to potential shock hazards
or fires. Fires can occur either in the
cable tray (which may provide a fire path) or
in combustible materials near the cable tray.
Furthermore, the improper use of flexible cord
could lead to the spread of toxic fumes if a fire
were to occur.
Grounding of cable tray systems is essential
for personal safety and protection against arc-
ing that can occur anywhere in the wiring system.
Proper grounding must be done before
cables are installed and tested before cables
are energized. In addition to these general
requirements, metallic cable tray systems supporting
electrical conductors must be electrically
continuous and effectively bonded as per the
requirements of the 2005 edition of NEC
Section 392.7.
Recognizing overloaded cable trays
Recognizing overloaded cable trays is not difficult.
The fill values for cable trays specified in
the 2005 NEC range from a single layer to
roughly a 50% fill of the cross-sectional area of
the cable tray. If visual observation reveals a
cable tray that is completely full and/or overflowing
with cables, chances are that the cable
tray is in violation of both the National Electrical
Code and OSHA requirements. One of the
major culprits associated with overloaded cable
trays are abandoned cables within the tray.
These abandoned cables should be removed;
and in fact, section 590.3(D) and various sections
in Chapter 8 of the 2005 NEC specifically
require removal of abandoned temporary
wiring and communication cable installed within
a cable tray.
Wiring methods permitted in cable trays
Any wiring methods used in cable trays must
be listed by a Nationally Recognized Testing
Laboratory as suitable for use in cable trays
and in the environment in which it is installed.
Table 392.3(A) of the NEC and OSHA’s 1910.305(a)
(3)(i) provide corresponding lists of conductors
and raceways permitted in cable tray systems.
Additionally, NEC Section 392.3(B) and OSHA’s
1910.305(a)(3)(i)(B) allow other specific conductors
in industrial establishments where maintenance
and supervision assure that only qualified
persons will service the cable tray systems.
Flexible cords are not currently listed
for use in cable trays (NEC Article 400, OSHA,
1910.305(g)) as they are prohibited as a
replacement for the fixed wiring of a structure.
The insulation on flexible cords can break
down and become brittle over the years, which
can lead to shorts and fires containing toxic
fumes.
Standards and regulations that apply to
cable trays
Cable trays were first covered in the 1965 edition
of the NEC, under Continuous Rigid Cable
Supports. Today, the use and installation of
cable trays is covered by Article 392 of the
NEC, and by OSHA regulations in 29 CFR
1910.305(a)(3) and 1910.399, or comparable
standards promulgated by States operating
OSHA-approved State plans. Specific permitted
uses of cable trays are covered by the 2005
edition of NEC Section 392.3 and OSHA’s
1910.305(a)(3)(i); uses not permitted are
addressed in NEC 392.4 and OSHA’s
1910.305(a)(3)(ii). Other sections and articles of
the NEC are referenced throughout Article 392
for specific installation and use issues. The
National Electrical Manufacturers Association
(NEMA) also publishes three standards that
apply to the proper manufacture and installation
of cable trays: ANSI/NEMA-VE 1-1998,
Metal Cable Tray Systems; NEMA-VE 2-1996,
Metal Cable Tray Installation Guidelines; and
NEMA-FG-1998, Nonmetallic Cable Tray
Systems.