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 arcCable 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

FactSheet

Electrical Safety Hazards of 

Overloading Cable Trays

According to the 2005 National Electrical Code® (NEC), a cable tray system is 

“[a] unit or assembly of units or sections and associated fittings forming a structural

system used to securely fasten or support cables and raceways.” Cable trays support

cable across open spans in the same manner that roadway bridges support traffic.

Cable trays are not raceways, and 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.
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

This is one in a series of informational fact sheets highlighting OSHA programs, policies or 

standards. It does not impose any new compliance requirements. For a comprehensive list of 

compliance requirements of OSHA standards or regulations, refer to Title 29 of the Code of Federal

Regulations. This information will be made available to sensory impaired individuals upon request.

The voice phone is (202) 693-1999; teletypewriter (TTY) number: (877) 889-5627.

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. 

For more information

National Electrical Code®, (2005 Edition) Article

392 (See also NEC Handbook). 

OSHA 29 CFR 1910.305(a)(3) and 1910.399. 

Cable Tray Institute (http://www.cabletrays.com).

The Cable Tray Manufacturer’s Installation and

Use Instructions.

U.S. Department of Labor

www.osha.gov

(800) 321-OSHA

For more complete information:

DSTM 03/2006