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U.S. Department of Labor
Occupational Safety and Health Administration
Directorate of Technical Support and Emergency Management
Office of Technical Programs and Coordination Activities |
Safely Installing, Maintaining and Inspecting Cable Trays
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Safety and Health Information Bulletin |
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SHIB 01-16-2009 |
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This Safety and Health Information Bulletin (SHIB) is
not a standard or regulation, and it creates no new
legal obligations. The Bulletin is advisory in nature,
informational in content, and is intended to assist
employers in providing a safe and healthful
workplace. The Occupational Safety and Health Act
requires employers to comply with safety and health
standards promulgated by OSHA or by a state with
an OSHA-approved state plan. In addition, pursuant
to Section 5(a)(1), the General Duty Clause of the
Act, employers must provide their employees with a
workplace free from recognized hazards likely to
cause death or serious physical harm. Employers can
be cited for violating the General Duty Clause if there
is a recognized hazard and they do not take
reasonable steps to prevent or abate the hazard.
However, failure to implement any recommendations
in this SHIB is not, in itself, a violation of the General
Duty Clause. Citations can only be based on
standards, regulations, and the General Duty Clause. |
Purpose
The purpose of this Safety and Health Information
Bulletin is to:
- Review the proper methods for safely
installing, maintaining and inspecting electrical
cable trays;
- Provide information regarding the hazards of
overloaded cable trays;
- Identify specific Occupational Safety and
Health Administration (OSHA) regulatory
requirements and National Electrical Code®
(NEC) guidance that address the proper
installation and maintenance of cable trays;
- Recognize electrical cable tray misuse that
can lead to electric shock and arc-flash/blast
events and fires caused by overheating.
OSHA Regulations and Industry Consensus
Standards that Apply to Cable Trays
The use and installation of cable trays is covered by
legally enforceable OSHA regulations in 29 CFR
1910.305(a)(3), or comparable standards
promulgated by States operating OSHA-approved
State plans. In addition, this document contains
several references to provisions of the National
Electric Code (NEC), which is published by the
National Fire Protection Association (NFPA). The
2005 edition of NEC is listed as a reference in
Appendix A – “Reference Documents” of OSHA
Subpart S, Electrical (1910.301 through 1910.399).
While these references provide nonmandatory
information that can be helpful in understanding and
complying with Subpart S, compliance with the
referenced provisions of the NEC is not required and
is not a substitute for compliance with any applicable
OSHA standards.
Although the recently promulgated electrical
standards for general industry at 29 CFR 1910
Subpart S (72 FR 7136—7221, February 14, 2007)
are based on the 2002 edition of the NEC, OSHA
has not conducted rulemaking to adopt all of the
requirements of the NEC (or subsequent revisions)
and, therefore, cannot enforce those requirements.
However, industry consensus standards such as the
NEC and others referenced throughout this Bulletin
can be used by employers as guidance for conducting
hazard analyses and selecting effective control
measures.
The National Electrical Manufacturers Association
(NEMA) also publishes three consensus 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.
Cable Trays
According to OSHA 1910.399, a cable tray system is
“[a] unit or assembly of units or sections and
associated fittings forming a rigid structural system
used to securely fasten or support cables and
raceways. Cable tray systems include ladders,
troughs, channels, solid bottom trays, and other similar
structures.” Cable trays are not raceways, but they
are treated as a structural component of a facility’s
electrical system. Cable trays are a part of a planned
cable management system to support, route, protect
and provide a pathway for cable systems. Cable trays
support cables across open spans in the same way
that roadway bridges support traffic.
Cable trays can provide a safe component of a power,
low voltage control, data or telecommunications wiring
distribution system. Cables in trays can be easy to
mark, find, and remove. Their flexibility makes cable
trays a good choice for installation situations that
require upgrading, reconfiguring, or relocation.
Cable trays are available in a number of different
configurations, including ladder, ventilated trough,
ventilated channel, solid bottom, wire mesh, single rail
and other configurations. They come in a wide variety
of shapes and sizes, with a host of hanging options that
are able to meet almost any installation need. Cable
trays are manufactured of steel, stainless steel,
aluminum and fiber reinforced plastic (FRP). They
also are available with special finishes including
polyvinylchloride (PVC) coated and galvanized finish.
A significant portion of cable trays used in industry
today are aluminum. Aluminum, steel and coated-steel
cable trays, all being metallic, may be used as
equipment grounding conductors in accordance with
OSHA 1910.305(a)(3)(iii). This requirement is
mirrored by the guidance provided by NEC Section
392.3(C). Depending on the need, covers and
ventilated louvers or slats are available for all trays.
Covers physically protect the cables as well as
shielding the cable jackets from the sun’s ultraviolet
radiation when used outdoors. Suitable guards or
covers must be installed to a minimum height of 2.5m
(8 ft.) above grade such as where cable trays are
exposed to physical damage from vehicular traffic.
Ventilated louvers also protect the cables and
facilitate cooling by allowing natural convection
(heat dissipation) to occur.
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Figure 1. Ladder cable tray, ventilated cable tray,
solid-trough cable tray. |
Cable Tray Use
Cable trays can be used in a variety of settings.
Cable trays can be rated for outdoors, indoors,
corrosive and classified hazardous locations, and
areas with high electrical noise and vibration. As
with any electrical equipment, cable trays and the
wiring contained in the trays must be listed, labeled
or otherwise approved, pursuant to the requirements
of 29 CFR § 1910.303(a). The National Electrical
Manufacturers Association (NEMA) Standard VE
1-2002 provides guidance for metal cable trays and
associated fittings designed for use in accordance
with the rules of the NEC. NEMA Standard VE 2-
2006 addresses shipping, handling, storing, and
installing cable tray systems; it also provides
information on cable tray maintenance and system
modification. Compliance with these standards helps
to ensure safe loading and the electrical continuity of
cable tray systems.
Cable trays may be designed to cross through
partitions and walls, as well as go vertically through
platforms and floors.
However, where cable trays (and the conductors
and cables they contain) pass through fire-rated
partitions, walls and floors, appropriate fire-stops
should be provided in accordance with NEC Section
300.21 to prevent the spread of a fire or the byproducts
of combustion. Typically, specific building
codes should be consulted and the design and
oversight should be done by a qualified engineer.
Use of cable trays is popular in hazardous locations
where concentrations of flammable or combustible
gases, vapors and dusts exist. However, the improper
use of cable trays in these environments could result in
an explosion. 29 CFR § 1910.305(a)(3)(iv) requires
that cable trays in hazardous (classified) locations
contain only the cable types permitted in such
locations (see 1910.307 for details on hazardous
(classified) locations). In addition, the NEC also
contains specific requirements for wiring in hazardous
or classified environments. For example, NEC
Section 392.3(D) states that cable trays in hazardous
locations should contain only the wiring permitted in
specific sections of Chapter 500 (Sections 501.10,
502.10, 503.10, 504.20 and 505.15).
Proper Loading of Cable Trays
Since cable trays come in a wide variety of sizes, they
can be designed to accommodate a wide range of
loading configurations. Because of their flexibility,
cable trays are especially subject to overloading. Safe
and permissible loading of cable trays is governed by
three criteria: manufacturer-specified weight
restrictions; limitations of cable fill because of crosssectional
area limitations; and conductor spacing
requirements. The appropriate size and number of
cable trays for an installation depends on the number
and size of conductors included and the allowable fill
area specified in the guidance provided by the NEC.
Because cable trays offer flexibility for expansion and
changes, engineers and designers should design and
size cable tray systems to anticipate both current and
future needs.
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Figure 2. Outdoor metal clad cable in cable tray. |
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Figure 3. Electrical wires in multiple cable trays. |
Load and Support Requirements
29 CFR § 1910.303(b)(8) requires the appropriate
mounting and cooling of electrical equipment.
Additionally, guidance provided in NEC Section
392.6(C) states, in part, that cable trays shoul be
supported at intervals in accordance with the
installation instructions. This straightforward approach
is corroborated in 29 CFR § 1910.303(b)(2), which
states that “listed or labeled equipment shall be used
or installed in accordance with any instructions
included in the listing or labeling,” and guidance
provided by NEC Section 110.3(B). The type and
number of cable trays, and the support required to
handle loads must take into account several factors,
including, but not limited to, environmental or weather
factors; the weight of the cable tray; current and future
cable needs; electromagnetic forces; and any
accessories installed. Manufacturers of cable tray
systems provide a wide range of parts and typical
support methods, as well as detailed installation guides
and tables for appropriate supports and support
spacing for cable trays. However, engineering
calculations are often required to determine where to
place supports so that the designed cable tray system
provides the proper strength.
Concurrently, NEC 392.6(J) permits cable tray
systems to externally support raceways, cables and
boxes, and conduit bodies as covered by Section
314.1, provided that the cable tray is listed and
designed for the application. Support for the
equipment also must be in accord with the appropriate
NEC article. This permission does not extend to
other non-electrical equipment, such as water pipes,
sprinklers, and gas pipes. In addition, cable tray
supports should also take into account dynamic loads
(e.g., loads caused by the motion of the cable tray
system and its contents) and lateral stressors. Cable
tray systems need not be absolutely rigid; most
systems are designed with about a 1/200 spandeflection
ratio (i.e., 1.2 inches in a 20-foot span)
when fully loaded.
Wiring Fill and Spacing Requirements
In industrial establishments, where the conditions of
maintenance and supervision assure that only qualified
persons will service the installation, OSHA
1910.305(a)(3)(ii) defines the wiring methods that
may be installed in cable trays. Cable tray fill is
addressed in NEC Sections 392.8, 392.9, 392.10,
and 392.12. The type of cable tray (e.g., solid,
ventilated), ampacity (current-carrying limit)
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 developed 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
tray’s maximum weight based on the cable tray
specifications. However, the NEC provides more
detailed requirements for cable tray fill (e.g., single
conductors sized 1/0 through 4/0 used in ladder or
ventilated cable trays must be installed in a single layer
and where multi-conductor cable is used 4/0 and
larger conductors must be installed in a single layer
and the sum of the diameters of the cables must not
exceed the width of the cable tray). In making cable
tray fill determinations, the best strategy is to review
and follow the requirements of the NEC and the
manufacturer’s installation guides to determine the
appropriate fill when installing cable in cable trays.
The ampacity (current-carrying rating) for conductors
and cables in cable trays provided in NEC Sections
392.11 and 392.13 is based on compliance with the
NEC cable tray fill requirements.
Importance of Supports and Allowable Fill
Overfilling cable trays can lead to a number of serious
hazards. The weight of the cables inside the cable tray
may pose a hazard. 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 arc-flash/blast events from
component failure that occurs when the cables are
suddenly no longer supported. Paragraph (b)(1) of 29
CFR 1910.303 requires that equipment shall be free
from recognized hazards that are likely to cause death
or serious physical harm to employees. Some of the
tests for suitability and use are mechanical strength and
durability (1910.303(b)(1)(ii)), heating effects under
all conditions of use (1910.303(b)(1)(v)), and arcing
effects (1910.303(b)(1)(vi)).
Paragraphs (b)(8)(ii) and (b)(8)(iii) of 29 CFR
1910.303 contain requirements for the cooling of
electrical equipment. Avoiding heat buildup is another
important issue. The NEC requirements for cable tray
fill address heat build-up in conductors while current is
flowing. When cable trays are overloaded, excessive
heat builds up in and around live conductors, which
can cause the insulation to break down and create
potential shock hazards or fires. Fires can occur
either in cable trays (which may provide a fire path) or
in combustible materials near cable trays.
Furthermore, the improper use of flexible cord within
cable trays could lead to the spread of toxic vapors if
a fire were to occur.
Types of Conductors to Use
Any wiring method used in cable trays must be
“acceptable” as defined in OSHA 1910.399. In other
words, the wiring method used must be listed by a
nationally recognized testing laboratory (NRTL) as
suitable for use in cable trays and in the environment in
which it is installed. 29 CFR § 1910.305(a)(3)(i) and
NEC Table 392.3(A) provide corresponding lists of
wiring methods permitted in cable tray systems.
Additionally, 29 CFR § 1910.305(a)(3)(i)(b) and
NEC Section 392.3(B) allow other specific
conductors in industrial establishments where
maintenance and supervision assure that only qualified
persons will service the cable tray systems.
Mixing of cable types and voltages is permitted in
cable trays provided that some specific requirements
are met. For example, NEC Section 392.6(F)
permits cables rated to carry over 600V to be
installed with cables rated 600V or less, provided that
the cable rated over 600V is Type MC, or if a solid
fixed barrier of material compatible with the cable tray
is installed to separate the voltage levels. Installing
barriers between power and control cables is a
recommended practice, regardless of the ratings and
voltages of the cables in question.
It is important to note that although NEC Table
392.3(A) states that “other factory-assembled, multiconductor
control, signal, or power cables that are
specifically approved for installation in cable trays”
may be used in cable trays, flexible cords and cables
are not approved for use in cable trays (29 CFR §
1910.305(g) - Flexible cords and cables, and NEC
Article 400). There are several reasons for this. First,
both OSHA and the NEC prohibit the use of flexible
cord as a replacement for the fixed wiring of a
building. Since wiring in cable trays is considered a
fixed or permanent wiring method by both OSHA and
the NEC, flexible cords are prohibited from being
used in cable trays (see NEC Article 400.4 for a list of
prohibited flexible cords). Secondly, flexible cord
insulation can break down and become brittle over
time which can result in electrical shorts, shock
hazards, and fires releasing toxic smoke.
Additionally, flexible cords and cables might not be
rigid enough to span the openings in ladder and
ventilated-type cable trays. Furthermore, the
conductor insulation might not withstand the load of
stacked cables.
Securing Cables within Cable Trays
There are many reasons for securing cables within
cable trays. Securing cables will maintain proper
spacing between cables, keep cables in the trays, and
confine the cables to specific locations within trays.
Those designing and installing the system must
determine the distances between fastenings in cable
trays. While the weight of the cable itself keeps it in
the tray in horizontal runs, the recommended practice
is to tie all cables down so that the cables are not
knocked out or “whipped” during abnormal or fault
current conditions. When cables are not installed in
horizontal runs, guidance presented by NEC 392.8(B)
indicates that said cables should be securely fastened
to transverse (crosswise) members. Smaller diameter
cables might need to be lashed or tied to the cable
tray more frequently than the stiff large diameter
cables to prevent them from hanging away from the
cable tray. Support is also required where cables are
routed from one cable tray to another or where cables
enter raceways or other enclosures (see OSHA
1910.305(a)).
Certain cable installations, such as in higher ambient
temperatures, might require the spacing between
adjacent cables to be increased to not less than one
cable diameter between cables pursuant to NEC
Section 392.11. Generally, multiconductor cables do
not need to be spaced.
Cable ties should be appropriate for the conditions in
which they are used. Factors such as moisture
resistance, ultraviolet resistance, extremely high or low
temperatures, chemical resistance, flammability, lowsmoke
characteristics, tensile strength, and length are
important to consider. In all cases, persons installing
or inspecting cable tray installations should refer to the
manufacturer’s instructions and specific NEC articles
for the proper wiring support method to be used.
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Figure 4. Cable ties. |
Proper Grounding and Bonding
Grounding and bonding help to prevent electrocutions
and arcing by facilitating the operation of over current
devices and preventing exposed non-current-carrying
metal parts and enclosures from being energized.
Proper grounding and bonding is done before the
cable is installed and must be tested before the cables
are energized.
Grounding and bonding are often associated together,
although, in reality, they represent two different
concepts. Bonding is the permanent joining of
metallic parts to form an electrically conductive path.
Equipment grounding is the connection of noncurrent-
carrying metal parts of equipment, raceways,
and other enclosures to the system’s grounded
conductor, the grounding electrode conductor, or
both, at the service equipment or at the source of a
separately derived system. Both grounding and
bonding are done to ensure electrical continuity and
also to assure the capacity to safely conduct any
current likely to be imposed upon those non-currentcarrying
metal parts, like cable trays.
Metallic cable tray systems used to support electrical
conductors must be grounded and electrically
continuous, and effectively bonded as required for
conductor enclosures (specified by 29 CFR §§
1910.304(g)(5), 1910.304(g)(6) and mirrored by the
guidance provided by NEC Section 250.96). If the
employer is following NEC guidelines, cable trays
may be used as equipment grounding conductors
provided that continuous maintenance and supervision
ensure that qualified persons will service the installed
cable tray system and that the provisions of NEC
Section 392.7 are met (i.e., cable trays are marked
for use as equipment grounding conductors and
minimum cross-sectional areas meet, and are marked
as meeting, the requirements of Table 392.7(B)). If
the cable tray system does not meet the marking
requirements of NEC Section 392.7, it also would not
meet OSHA’s suitablilty requirement in
1910.303(b)(1). Therefore it cannot be used as the
equipment grounding conductor for branch or feeder
circuits unless a single equipment grounding conductor
is installed in the tray and listed bonding connectors or
jumpers are used to effectively bond the cable tray
sections together to ensure electrical continuity.
Grounding of cable trays is so important that it has
become the industry practice to use grounding
conductors in cable trays for added reliability,
regardless of how the tray is listed and marked. It is
also recommended that cable trays be bonded to
building steel or earth every 60 feet in order to reduce
noise in the system. Where a cable tray includes only
multiconductor cables, there is generally no need to
use the tray as an equipment grounding conductor
because each multiconductor cable should have
integral equipment grounding conductor. Cable trays,
however, should be bonded in accordance with NEC
Section 250.96(A). Bonding jumpers on cable trays
are important to maintain the electrical continuity and
the ability to safely carry any fault current likely to be
imposed (in accordance with NEC Section 250.96).
As cable trays are typically fastened using direct
bolted connections, which provide bonding, bonding
jumpers are only required at adjustable splice plates,
expansion plates and non-continuous sections of trays.
However, as with the use of equipment grounding
conductors, industry practice is to use bonding
jumpers at all splice points to ensure continuity.
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Figure 5. Grounding and bonding hardware. |
Required continuity applies to all enclosures and
utilization equipment and to cable dropouts from the
tray system to enclosures. If channel dropouts or
conduit-to-cable tray adapters listed for grounding are
not used, grounding and bonding must be provided by
grounding conductors in the cable or by bonding
jumpers.
Recognizing Overloaded Trays
Recognizing overloaded cable trays is not difficult. If
visual observation of the cable tray reveals that the
cable tray is nearly full or overflowing with cables,
then the installation does not meet the guidance
provided by the NEC. Cable tray fill is addressed in
the 2005 edition of NEC Sections 392.8, 392.9,
392.10, and 392.12. The number of single
conductors or multiconductor cables that are
permitted in a cable tray as indicated by the NEC
range from a single layer to a fill value that might
represent 50% of the cross-sectional area of the
interior space within the cable tray. This is an extreme
limit and the cable tray will appear to be over half full
due to voids between the cables or conductors.
Another consideration for cable tray fill is not to
overload the cable tray or its support system beyond
their ratings.
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.
Solutions for Overloaded Cable Trays
If cable trays are overloaded because of poor design
and/or installation, the solution is to add additional
able trays in accordance with guidance provided by
the NEC. Another effective strategy for preventing
overfill is to review and follow the manufacturer’s
installation guides when installing cables in cable trays.
However, one of the major causes of overloaded
cable trays is abandoned conductors and cables for
circuits no longer in use, which often are not removed
from the cable tray when replacement or additional
cables are added. The solution in this instance is to
remove abandoned cable when they are no longer
necessary.
In fact, Section 590.3(D) and various sections in Chapter 8 of the NEC
specifically indicate that abandoned communication cable and temporary wiring
installed within cable trays should be removed upon the completion of projects.
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Figure 6. Overloaded Cable Tray. |
Preventing Damage to Cables and Conductors
The cables and conductors approved for use in cable
trays are required to be insulated. However, while the
insulation of the conductors does provide some
protection, it is important to use measures to prevent
damage to the insulation when working around
energized conductors or cables so as not to damage
the insulation. If the work the employee is performing,
such as adding boxes or other approved electrical
equipment using screws or bolts, drilling into the cable
tray, and pulling or dragging cables or conductors
across each other, could damage the insulation, then
the wiring must be de-energized when attaching boxes
or other approved electrical equipment to cable trays.
In general, 29 CFR 1910.333(a)(1) requires that live
parts to which an employee may be exposed shall be
deenergized before an employee works on or near
them.
Conclusion
Cable trays can provide a safe component of a wiring
distribution system. However, if not designed and
installed properly, wiring inside cable trays may pose
hazards such as fire, electric shock and arc-flash blast
events. During the maintenance, installation and
inspection of cable trays, the following concerns
should be taken into consideration.
- Cable trays, and the conductors and cable
they contain, must be listed or labeled by a
NRTL as suitable in the environment in which
they are installed.
- Where cable trays pass through fire-rated
partitions, walls and floors, appropriate fire
stops should be provided in accordance with
guidance provided by NEC Section 300.21 to
prevent the spread of a fire or the by-products
of combustion.
- Cable trays in hazardous locations must only
contain the wiring permitted in such locations.
- Cable trays must be properly supported in
accordance with the installation instructions.
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 arc-flash/blast
events from component failure when the
cables are suddenly no longer supported.
- When cable trays are overfilled, excessive
heat build-up in and around live conductors
can cause the insulation to break down,
leading to potential shock hazards or fires.
- 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.
- When cable trays are overfilled beyond the fill
criteria established by the NEC, add another
cable tray system above, below, or next to the
overfilled tray. Allow enough working space
around the added cable tray.
- Grounding of cable tray systems is essential
for personal safety and protection against
arcing that can occur anywhere in the wiring
system. Proper grounding must be done
before cables are installed and tested before
cables are energized.
- Abandoned cables within cable trays should
be removed.
- Work on cable tray installations may expose
employees to live parts. According to 29
CFR 1910.333(a)(1) deenergization of live
parts to which an employee may be exposed
is required before employees begin work on
or near them.
For more information
National Electrical Code® (2005) Article 392 (See
also NEC® Handbook).
OSHA Electrical Standard on Cable Trays, 29 CFR
1910.305(a)(3).
OSHA Fact Sheet - Electrical Safety Hazards of Overloading Standard Cable Trays [22 KB
PDF*, 2 pages]
Cable Tray Manufacturers:
Cooper
GS Metals
Accessibility Assistance: Contact the OSHA Directorate of Technical Support and Emergency
Management at 202-693-2300 for assistance accessing OSHA PDF materials.
*These files are provided for downloading.
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