Cable tray
Encyclopedia
In the electrical wiring
of buildings, a cable tray system is used to support insulated electric cable
s used for power distribution and communication. Cable trays are used as an alternative to open wiring or electrical conduit
systems, and are commonly used for cable management in commercial and industrial construction. They are especially useful in situations where changes to a wiring system are anticipated, since new cables can be installed by laying them in the tray, instead of pulling them through a pipe.
A ventilated tray has openings in the bottom of the tray, allowing some air circulation around the cables, water drainage, and allowing some dust to fall through the tray. Small cables may exit the tray through the ventilation openings, which may be either slots or holes punched in the bottom. A ladder tray has the cables supported by a traverse bar, similarly to the rungs of a ladder, at regular intervals on the order of 4 to 12 inches (100 to 300 mm).
Ladder and ventilated trays may have solid covers to protect cables from falling objects, dust, and water. Tray covers for use outdoors or in dusty locations may have a peaked shape to shed debris including dust, ice or snow. Lighter cable trays are more appropriate in situations where a great number of small cables are used, such as for telephone or computer network cables. These trays may be made of wire mesh, called "cable basket", or be designed in the form of a single central spine (rail) with ribs to support the cable on either side.
Large power cables laid in the tray may require support blocks to maintain spacing between conductors, to prevent overheating of the wires. Smaller cables may be laid unsecured in horizontal trays, or secured with cable ties to the bottom of vertically-mounted trays.
To maintain support of cables at changes of elevation or direction of a tray, a large number of specialized cable tray fittings are made compatible with each style and manufacturer. Horizontal elbows change direction of a tray in the same plane as the bottom of the tray and are made in 30, 45 and 90 degree forms; inside and outside elbows are for changes perpendicular to the tray bottom. These can be in various shapes including tees and crosses. Some manufacturers and types provide adjustable elbows, useful for field-fitting a tray around obstacles or around irregular shapes.
Various clamping, supporting and splicing accessories are used with the cable tray to provide a complete functional tray system. For example, different sizes of cable tray used within one run can be connected with reducers.
, stainless steel
, aluminum, or glass-fiber reinforced plastic. The material for a given application is chosen based on the corrosion resistance required for the location.
coatings applied to installed cables. Heavy coatings or long fire-stops may require adjustment of the cable current ratings, since such fireproofing measures may reduce the heat dissipation of installed cables.
Regular housecleaning is important for safety, as cable trays are often installed in hard to reach places. Combustible dust and clutter may accumulate if the trays are not routinely checked and kept clean.
Plastic and fibreglass reinforced plastic cable trays are combustible; the effect is mitigated through the use of fire retardants or fireproofing.
Ferrous cable trays expand with the increasing heat
from accidental fire
. This has been proven by the German Otto-Graf-Institut Test Report III.1-80999/Tei/tei "Supplementary Test On The Topic Of Mechanical Force Acting On Cable Penetration Firestop Systems During The Fire Test", dated 23 October 1984, to dislodge "soft" firestop
s, such as those made of fibrous insulations with rubber coatings. This also applies to any silicone
foam seals, but is easily remedied through the use of firestop mortars
of sufficient compression strength and thickness, as shown above. Also, some building code
s mandate that penetrant
s such as cable trays are installed in such ways so as to avoid their contribution to the collapse of a firewall
.
Electrical wiring
Electrical wiring in general refers to insulated conductors used to carry electricity, and associated devices. This article describes general aspects of electrical wiring as used to provide power in buildings and structures, commonly referred to as building wiring. This article is intended to...
of buildings, a cable tray system is used to support insulated electric cable
Cable
A cable is two or more wires running side by side and bonded, twisted or braided together to form a single assembly. In mechanics cables, otherwise known as wire ropes, are used for lifting, hauling and towing or conveying force through tension. In electrical engineering cables are used to carry...
s used for power distribution and communication. Cable trays are used as an alternative to open wiring or electrical conduit
Electrical conduit
An electrical conduit is an electrical piping system used for protection and routing of electrical wiring. Electrical conduit may be made of metal, plastic, fiber, or fired clay. Flexible conduit is available for special purposes....
systems, and are commonly used for cable management in commercial and industrial construction. They are especially useful in situations where changes to a wiring system are anticipated, since new cables can be installed by laying them in the tray, instead of pulling them through a pipe.
Types
Several types of tray are used in different applications. A solid-bottom tray provides the maximum protection to cables, but requires cutting the tray or using fittings to enter or exit cables. A deep, solid enclosure for cables is called a cable channel or cable trough.A ventilated tray has openings in the bottom of the tray, allowing some air circulation around the cables, water drainage, and allowing some dust to fall through the tray. Small cables may exit the tray through the ventilation openings, which may be either slots or holes punched in the bottom. A ladder tray has the cables supported by a traverse bar, similarly to the rungs of a ladder, at regular intervals on the order of 4 to 12 inches (100 to 300 mm).
Ladder and ventilated trays may have solid covers to protect cables from falling objects, dust, and water. Tray covers for use outdoors or in dusty locations may have a peaked shape to shed debris including dust, ice or snow. Lighter cable trays are more appropriate in situations where a great number of small cables are used, such as for telephone or computer network cables. These trays may be made of wire mesh, called "cable basket", or be designed in the form of a single central spine (rail) with ribs to support the cable on either side.
Large power cables laid in the tray may require support blocks to maintain spacing between conductors, to prevent overheating of the wires. Smaller cables may be laid unsecured in horizontal trays, or secured with cable ties to the bottom of vertically-mounted trays.
To maintain support of cables at changes of elevation or direction of a tray, a large number of specialized cable tray fittings are made compatible with each style and manufacturer. Horizontal elbows change direction of a tray in the same plane as the bottom of the tray and are made in 30, 45 and 90 degree forms; inside and outside elbows are for changes perpendicular to the tray bottom. These can be in various shapes including tees and crosses. Some manufacturers and types provide adjustable elbows, useful for field-fitting a tray around obstacles or around irregular shapes.
Various clamping, supporting and splicing accessories are used with the cable tray to provide a complete functional tray system. For example, different sizes of cable tray used within one run can be connected with reducers.
Materials used
Common cable trays are made of galvanized steelSteel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
, stainless steel
Stainless steel
In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....
, aluminum, or glass-fiber reinforced plastic. The material for a given application is chosen based on the corrosion resistance required for the location.
Fire safety concerns and solutions
Combustible cable jackets may catch on fire and cable fires can thus spread along a cable tray within a structure. This is easily prevented through the use of fire-retardant cable jackets, or fireproofingFireproofing
Fireproofing, a passive fire protection measure, refers to the act of making materials or structures more resistant to fire, or to those materials themselves, or the act of applying such materials. Applying a certification listed fireproofing system to certain structures allows these to have a...
coatings applied to installed cables. Heavy coatings or long fire-stops may require adjustment of the cable current ratings, since such fireproofing measures may reduce the heat dissipation of installed cables.
Regular housecleaning is important for safety, as cable trays are often installed in hard to reach places. Combustible dust and clutter may accumulate if the trays are not routinely checked and kept clean.
Plastic and fibreglass reinforced plastic cable trays are combustible; the effect is mitigated through the use of fire retardants or fireproofing.
Ferrous cable trays expand with the increasing heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
from accidental fire
Fire
Fire is the rapid oxidation of a material in the chemical process of combustion, releasing heat, light, and various reaction products. Slower oxidative processes like rusting or digestion are not included by this definition....
. This has been proven by the German Otto-Graf-Institut Test Report III.1-80999/Tei/tei "Supplementary Test On The Topic Of Mechanical Force Acting On Cable Penetration Firestop Systems During The Fire Test", dated 23 October 1984, to dislodge "soft" firestop
Firestop
A firestop is a passive fire protection system of various components used to seal openings and joints in fire-resistance rated wall and/or floor assemblies, based on fire testing and certification listings....
s, such as those made of fibrous insulations with rubber coatings. This also applies to any silicone
Silicone
Silicones are inert, synthetic compounds with a variety of forms and uses. Typically heat-resistant and rubber-like, they are used in sealants, adhesives, lubricants, medical applications , cookware, and insulation....
foam seals, but is easily remedied through the use of firestop mortars
Mortar (firestop)
Firestop mortars are mortars most typically used to firestop large openings in walls and floors required to have a fire-resistance rating. They are passive fire protection items...
of sufficient compression strength and thickness, as shown above. Also, some building code
Building code
A building code, or building control, is a set of rules that specify the minimum acceptable level of safety for constructed objects such as buildings and nonbuilding structures. The main purpose of building codes are to protect public health, safety and general welfare as they relate to the...
s mandate that penetrant
Penetrant (mechanical, electrical, or structural)
Penetrants, or penetrating items, are the mechanical, electrical or structural items that pass through an opening in a wall or floor, such as pipes, electrical conduits, ducting, cables and cable trays, or structural steel beams and columns...
s such as cable trays are installed in such ways so as to avoid their contribution to the collapse of a firewall
Firewall (construction)
A firewall is a fireproof barrier used to prevent the spread of fire between or through buildings, structures, electrical substation transformers, or within an aircraft or vehicle.- Applications :...
.
See also
- CableCableA cable is two or more wires running side by side and bonded, twisted or braided together to form a single assembly. In mechanics cables, otherwise known as wire ropes, are used for lifting, hauling and towing or conveying force through tension. In electrical engineering cables are used to carry...
- International Brotherhood of Electrical WorkersInternational Brotherhood of Electrical WorkersThe International Brotherhood of Electrical Workers is a labor union which represents workers in the electrical industry in the United States, Canada, Panama and several Caribbean island nations; particularly electricians, or Inside Wiremen, in the construction industry and linemen and other...
- Passive fire protectionPassive fire protectionPassive fire protection is an integral component of the three components of structural fire protection and fire safety in a building. PFP attempts to contain fires or slow the spread, through use of fire-resistant walls, floors, and doors...
- Circuit integrityCircuit integrityCircuit integrity refers to the operability of electrical circuits during a fire. It is a form of fire-resistance rating. Circuit integrity is achieved via passive fire protection means, which are subject to stringent listing and approval use and compliance.-Fireproofing:Providing fireproofing for...
- FirestopFirestopA firestop is a passive fire protection system of various components used to seal openings and joints in fire-resistance rated wall and/or floor assemblies, based on fire testing and certification listings....
- Fire testFire testA fire test is a means of determining whether or not fire protection products meet minimum performance criteria as set out in a building code or other applicable legislation. Successful tests in laboratories holding national accreditation for testing and certification result in the issuance of a...
- FireproofingFireproofingFireproofing, a passive fire protection measure, refers to the act of making materials or structures more resistant to fire, or to those materials themselves, or the act of applying such materials. Applying a certification listed fireproofing system to certain structures allows these to have a...
- IntumescentIntumescentAn intumescent is a substance which swells as a result of heat exposure, thus increasing in volume, and decreasing in density. Intumescents are typically used in passive fire protection and, in America, require listing and approval use and compliance in their installed configurations in order to...
- EndothermicEndothermicIn thermodynamics, the word endothermic describes a process or reaction in which the system absorbs energy from the surroundings in the form of heat. Its etymology stems from the prefix endo- and the Greek word thermasi,...
- Conduit (electrical)