A complete guide to cable trays and fittings

What is cable tray ?

As per national electrical code (NEC) , cable tray 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"

Types of cable trays

Types of cable trays :-

• 1) Perforated cable tray
• 2) Ladder cable tray
• 3) Ventilated trough cable tray
• 4) Solid bottom cable tray
• 5) Wire mesh cable tray
• 6) Single rail cable tray
• 7) Channel cable tray

 1) Perforated cable tray

• This type of cable trays consists of single metal sheets with side rails, each sheet has holes or openings at bottom which provide good ventilation for cables and allow cables to be fastened to tray by using cable ties .
• It is mainly used for medium heat generating power and control cables. 

Perforated cable tray

  2) Ladder cable trays

• It gets its name from its ladder like structure .
• It is mainly used for large power cables but it can be used for other cable types. 
• It is a prefabricated metal structure made up of two longitudinal side rails connected by individual transverse rungs that are placed at regular intervals , typically at a distance 9 inch from each other .
• The rungs are designed to support the cables by fastening the cables to the rungs using cable cleats. 
• It allows for maximum possible air circulation which makes it suitable for cables prone to overheating .
• Cables can enter or exit from the top or the bottom of the ladder between any two rungs .
• It provides easy access to cables installed on it for making inspection or future modifications .
• Its structure prevents moisure and dust accumulation.
• It is generally used in applications with intermediate to long support spans , 12 feet to 30 feet.

Ladder cable tray

 3) Ventilated trough cable tray

• It has openings at the bottom that allows for air circulation and water to drain and through these openings small diameter cables can be inserted or taken out of the tray .
• It is used to support small diameter control and multiconductor instrumentation cables. 

Ventilated trough cable trays

 4) Solid bottom cable tray

• As the name suggests , this type has a solid bottom so it provides maximum protection to cables from bad weather conditions like wing , rain and strong sunlight and also protects cables from any chemicals or other corrosive. 
• It can provide protection against radio frequency interference and electromagnetic effects .
• Unlike ladder and ventilated trough cable trays , it can collect and retain moisure and dirt .
• These trays can result in heat built up where cables are packed closed together and are prone to heating .
• Solid bottom steel cable trays with solid covers and wrap around cover clamps can be used to provide EMI/RFI shielding protection for sensitive circuits .
• It is preferred to use this type for supporting large number of small diameter control and multiconductor instrumentation cables. 
• It is used for minimal heat generating electrical or telecommunication applications .

Solid bottom steel cable trays

 5) Wire mesh cable trays (Basket trays)

• It is one of the most open structure cable trays so it allows for maximum air flow from all sides and this type is ideal for supporting cables that are prone to overheating  .
• It is mainly used for supporting low voltage , telecommunication and fibre optic cables

Wire mesh cable trays

 6) Single rail cable tray

• It is similar to ladder tray except that the longitudinal rail runs through the center of the horizental rungs .
• It is the fastest tray system to install .
• This type of cable trays is suitable for supporting cables that are prone to overheating and also suitable for use in situations where quick access to cables is needed .
• As this type has an open structure , so it makes cables vulnerable to environmental corrosion and it doesn't provide mechanical protection for cables. 

Single rail cable tray

 7) Channel cable trays

• Channel cable trays is a prefabricated steel structure consists of one piece channel section with width up to 4 inch.
• cable channel can be used very effectively to support cable drops from the cable tray run to the equipment or device being serviced and is ideal for cable tray runs involving a small number of cables and the cable channel or cables are subjected to some degree of vibration. 
• Cable channel can also be used to support push buttons, field mounted instrumentation devices, etc.
•  Small diameter cables can exit ventilated cable channel through the bottom ventilation holes, out the top or through the end.

Channel cable trays

Material and finish of cable trays

Types of material & finish of cable trays :-

• 1) Electrogalvanized steel cable trays
• 2) Pre-galvanized steel cable trays
• 3) Hot dip galvanized steel cable trays
• 4) Aluminium cable tray
• 5) stainless steel cable tray
• 6) fiberglass cable tray

 1) Electrogalvanized steel cable tray

This type of cable trays are fabricated by depositing a zinc coating on the surface of the fabricated steel through electrolysis from a bath of zinc salts .
The bath used contains acidic or alkaline zink and the anodes are zink while cathodes are the parts of steel to be coated 

After being coated the parts are chromated to increase their protection ability .
This type is only suitable for indoor use in dry areas .

 2) Pre-galvanized steel cable tray

Pre-Galvanized steel is produced by coating coils of sheet steel with zinc by continuously rolling the material

through molten zinc at the mills.

These coils are then slit to size and fabricated by roll forming, shearing, punching, or forming to produce pre-galvanized cable tray.

this manufacturing method is suitable for indoor uses

 3) Hot dip galvanized steel cable tray

In this case, cable trays are fabricated from steel then  completely immersed in a bath of molten zinc .

The thickness of resulting zink layer can be controlled by the amount of time each part of the cable tray is immersed in the molten zinc as well as the speed at which these parts are removed .

the resulting galvanizing layer is thicker than that of the previous case and it provides high protection against corrosion  so this manufacturing method for cable trays is suitable for outdoor applications and aggressive industrial environments. 

 4) Aluminium cable tray

- Lightweight, easy to install and requires  low maintenance .

- high resistance to corrosion. 

- high strength to weight ratio .

- As aluminium is non magnetic ,this decreases the electrical losses that may arise to minimum. 

 5) Stainless steel cable tray

This type has the following characteristics :

- superior corrosion resistance .

- withstands high temperatures .

- Excellent resistance to various chemicals

- Limited deflection 

- this type is usuallly used in an offshore oil installation, chemical plants and marine locations .

 Non metallic cable trays

 6) Fiberglass cable trays (FRP trays)

FRP : Fiberglass Reinforced Polyester 
Characteristics:
 - a fiberglass tray is lightweight , on average 15% to 20% that of steel cable tray .
 - It has high strength to weight ratio . 
 - it has high corrosion resistance 
 - it has low thermal conductivity and is non-conductive .
 - It is fire retardant and can be easily installed .
 - it is transparent to radio waves , microwaves and other electromagnetic frequencies
 - The coefficient of thermal expansion of fiberglass is marginally less than steel and aluminum.
 - FRP has excellent mechanical properties at low temperatures .
 - it is mainly used in power plants, paper plants ,chemical plants ,offshore platforms ,effluent treatment plants ,refineries.

Types of return flange of cable tray 

• Straight
• Return Flange Inside
• Return Flange Outside
• C-Type Inside
• C-Type Outside

 Cable tray size selection 

Dimensions of cable tray

Width

The width of cable tray shall be determined based on the sum of overall diameters of all cables intended to be installed inside the tray in addition to spaces between cables and the width standard values are :

50,100,150,200,250,300,400,600,700,900 mm

HEIGHT

The side height of cable tray is choosen based on the largest overall diameter of all cables intended to be placed on the tray and height's standard values are : 50,75,100,25 mm

THICKNESS

The cable trays thickness is choosen based on the sum of weights of all cables intended to be installed inside the tray and the weight of the tray itself .

The standard values of cable tray thickness are :  1,1.25,1.5,2 

The thickness of cable tray can also be expressed as follows :

Light duty ( 1 mm)- medium duty (1.25 mm) - heavy duty (1.5 mm) - very heavy duty (2mm)

Cable tray support types

  Types of cable trays support systems :-

• 1) Trapeze system (single or multi-tier)
• 2) Hanger Rod clamps support
• 3) Centre hung support
• 4) Wall and cantilever support brackets
• 5) Underfloor and toproof support
• 6) Vertical support
• 7) Overhead hangers

 1) Trapeze support (single or multi-tier)

It is similar to direct rod suspension but instead of clamp it uses steel channel (called C channel) or strut .

Single tier trapeze support

Multi-tier trapeze support

 2) Hanger Rod clamps support

A threaded rod and clamps are used for tray suspension

 3) Center hung support

It uses a rod that is installed at the center of the tray .

Center hung support

 4) Wall and cantilever support brackets 

Wall support brackets are an effective way of fixing any horizontal or vertical cable tray or ladder to a vertical support .
These cantilever brackets can be welded or bolted type .

Wall support of cable tray

 5) Underfloor and toproof support

Floor mounted cable trays

underfloor mounted cable trays

 6) Vertical support 

Cable trays or ladders can be vertically supported through three ways :

 1) vertical threaded rod support. 

 2) vertical Gusset cantilever support. 

 3) vertical strut support. 

  7) Overhead hangers

Overhead hangers enable cable tray to be supported from a single threaded rod giving an easy access for laying cables from only one side of the cable tray and there are two types of overhead hangers :

 1) Single channel cable tray support

 2) Double channel cable tray support

Cable tray fittings and accessories

1) Elbow / Bend

 there are three types of elbows :

Horizontal elbow (45)

It is a fitting that is used for changing the direction of cable tray by a degree of 45 .

Horizontal elbow (90)

It is a fitting that is used for changing the direction of cable tray by a degree of 90 .

vertical elbow

It is used for changing the direction of cable tray to a different plane . 

inside vertical elbow : changes direction upward from the horizontal plane .

Outside vertical elbow : changes direction downward from the horizontal plane .

  2) Tee Connection

It is a fitting that is used for joining three cable trays in three directions at 90 degree between each tray and other in the same plane .

 3) Reducer 

It is a fitting that is used for joining two cable trays of different widths in the same plane  and there are three types of reducers :
straight reducer
Left hand reducer : when viewed from the wide end , it has a straight side from the left .
right hand reducer : when viewed from the wide end , it has a straight side from the right 

 4) Cross connection

It is a fitting that is used for joining four cable trays in four directions at 90 degree between each tray and other in the same plane .

 5) Riser Bend

Types of riser bend :
Internal Riser : it is used for changing the direction of the cable tray from a lower level to the upper level .
External Riser : it is used for changing the direction of the cable tray from the upper level to the lower level .

internal and external riser

and all previous fittings are shown in the following figure :

Cable tray fitting and accessories

 6) Connectors / splice plates / couplers

These are fittings that are used to join two sections of cable trays to each other .

There are three types of connectors as follows 

• straight connectors
• angle connectors
• adjustable connectors

 Straight Connector :

- Angle Connector :

 - Adjustable connector : it allows changes in elevation where standard vertical fittings are not applicable and they include two types : 

• Horizontal adjustable connector : it adapts to changes in direction in a horizental plane .

• Vertical adjustable connector :  it adapts to changes in direction in a vertical plane 

Horizontal adjustable connector

Vertical adjustable connector

 7) Cable Tray Cover 

the cover protect the cables inside the cable tray from mechanical damage , heavy accumulation of dust and direct sunlight .
it also isolates cables from fires and radio frequency from interference. 

 types of tray covers 

- solid cover (plain or flanged) : 

provides maximum mechanical protection for cables with limited heat build up.

 - ventilated cover (plain or flanged) : provides excellent mechanical protection while allowing heat produced by cables to dissipate .

 - peaked cover (flanged) :

It is used in locations where there is a possibility of heavy rains 

Note : only cable trays containing instrument and communications cables may have unventilated covers .

Types of cover clamps :

Cable trays are used with a cover in outdoor applications and in mechanical rooms

  8) Fish plate 

it is a fitting that is used for extra strength when joining cable tray sections and also can protect cables from cut edges .

Fish plate joins tray sections

  9) End Plate

it is a fitting that is fixed to the end of the cable tray or ladder to lock it giving a neater appearance .

 10) Dividers / barrier strips

divider is a fitting that is installed inside the cable tray to physically separate different types or groups of cables within the same tray .

Barrier strips are also used to separate cable systems  above and below 600 volts installed in the same cable tray as per NEC 392.6(F).

 11) Flexible expansion coupler

it is a fitting that is installed between two cable trays or ladders across the expansion joints of the building structure for the following reasons :

• To compensate for the  variation of the length of the cable tray or ladder due to thermal expansion or contraction resulting from changes in temperature 
• To provide a semi-flexible joint when cable trays or ladders span two separate structures between them there is a possible relative movement. 

 12) Hold down clamps and clips 

They are used for fixing cable ladders to heavy duty  trapeze hangers , cantilever arms and channels .

Hold down clamps

Hold down clips

 13) Dtop-out plate 

It is a fitting that is installed in cable ladders to support the cables as they exit the ladder down between the rungs .

Drop-out plates can be located either on the side or at the bottom of cable ladder .

  14) Tray to box splice plate 

It is used to attach the end of cable tray to a distribution box or control panel .

 15) Roofing Bolts 

most of cable tray fittings are fastened to the tray using bolts and  these bolts are mushroom-head steel roofing type bolts and should be mounted with head inside the tray and washers and nuts located on the outside the tray to prevent injury to cable insulation . 

 16) Threaded rod   

It is a rod that is used for hanging cable trays or ladders directly to the ceiling from anchors and is fastened to the horizental support channel (strut channel) by using nuts and washers .

The rod's diameter is choosed depending on the weight of the cables installed on the tray in addition to the weight of the tray itself.

 17) Support Brackets

 support brackets are used for supporting cable trays on walls ,ceiling ,toproof and underfloor .

 18) protection strip

It is a strip that is used to protect the cables from cut edges as they enter or exit from the cable tray .

 19) Step down splice plate

It is used for joining cable tray/ladder sections having side rails of different heights .

  20) offset reducing connector

 It is used for joining cable tray/ladder sections of different widths , so :

•  To form an offset reduction, use a reducing connector.
• To form a straight reduction, use a pair (2) of reducing connectors.

 21) junction box plate 

It is used for the installation of junction box in cable trays. 

Grounding and Bonding of cable trays

Grounding and Bonding are often associated together but on reality they represent two different concepts .

Grounding is the connection of non current carrying metal parts of equipment to the system's grounded conductor , the grounding electrode conductor , or both at the service equipment .
Bonding is the permanent joining of Metallic parts to form an electrically conductive path .

The EGC conductor (Equipment Grounding Conductir) is the conductor responsible for grounding equipments to provide electrical safety .

According to the national electrical code (NEC) , cable trays can be grounded through three ways :
1) using a single EGC conductor in the cable tray .
2) using an EGC conductor for each multiconductor cable layed in the cable tray .
3) The cable tray itself can be used as the EGC and bonding jumpers are used at all splice points to ensure continuity .

Usually , the cable tray is used as an EGC and also a separate EGC cable is placed in the cable .

In case of installing an EGC conductor on the cable tray , it should be clamped at each cable tray section by using grounding clamps for two reasons :

• providing electrical continuity between all sections of cable tray .
• protecting the EGC conductor from being thrown out of the cable tray by the magnetic forces that arises during fault current conditions .

in a moist environment where aluminium cable trays are used , a bare copper EGC conductor should not be placed in the aluminium tray to avoid galvanic corrosion and in this case , an insulated EGC conductor can be used  but its insulation should be removed at bonding connections to the cable tray .

Metal Area Requirements for cable trays used as Equipment Grounding Conductors 

 

The values shown in the first column in table 392.7(B) NEC are the actual trip settings of the circuit breaker not the maximum trip settings which are the same as the frame size of the breaker in many cases .

If the ampere rating of the cable tray is not sufficient for the protective device to be used ,the cable tray can not be used as an EGC and in this case  a separate EGC conductor should be placed in the cable tray .

Minimum size of equipment grounding conductors for grounding cable trays (according to NEC)

Bonding jumpers for cable tray bonding

Bonding jumpers should be used at certain locations of the cable tray such as (adjustable splice plates  expansion joints and discontinuous tray sections) to ensure electrical continuity of the cable tray whether it is going to be used as an EGC or not .

Types of bonding jumpers 

there are three types of bonding jumpers :

• Insulated bonding jumpers
• Bare bonding jumpers
• Braided bonding jumpers

It is not necessary to use bonding jumpers at standard splice plates because its bolted connections provide adequate bonding .

Bonding jumpers are not used with fiberglass cable trays as fiberglass is non conductive .

It is important to install two bonding jumpers from both side rails of the cable tray section to the ground bus at switchgear or MCC .

   Installation of cable trays between electrical panels inside buildings

- If main distribution board (MDB) and  sub distribution board (SMDB) are located in the same floor in a building  , a perforated cable tray is installed between the two panels and the feeding cable from the MDB to the SMDB is laid over the tray .

But if the two panels are located on two different floors , the feeding cable from the main distribution board should pass over a perforated  cable tray till it reaches the floor's ceiling  ,in this case it will pass over a ladder tray that is installed vertically through shafts in all floors' ceilings till the floor where the sub distribution board is located then it again passes over a perforated cable tray till it reaches the sub-board . 

So perforated cable tray is used for carrying cables between panels and ladder tray is used for carrying cables through different floors passing through shafts .

Cable tray clearance requirements 

 - the cable trays shall be installed over all piping works in the building (such as plumbing pipes).

 - The minimum distance between the upper side of cable trays and the ceiling shall be 12 inches (300 mm)

 - there must be a minimum distance of 2 inches (50 mm) between the cable tray and the vertical wall ,also the other side of the cable tray should have a minimum clearance of 12 inches (300 mm) to provide easy access to the cables installed on the tray for inspection or future modifications. 

 - in multiple tiers trays : the vertical distance  between the two parallel trays shall not be less than  12 inches (300 mm) .

 - cable trays installed under raised floor : the minimum clearance between the top of the tray and the bottom of the floor tiles or floor system stringers (whichever are lower in elevation) is 3/4-inch (19 mm) .

 - The clearance between cable tray and tray cross overs should be not less than 3 inches (75 mm) .

  - The distance between any two cable trays in the same plane shall not be less than 4 inches (100 mm)