Picture 1 – Cable tray at high level inside a chiller plant room
Cable trays for the installation of electric power cables are usually specified as perforated hot-dipped galvanized sheet steel.
The term perforated usually is defined as holes provided to the sheet of the trays to allow the movement of airflow that can provide a more natural flow of air circulation around the electric cables. This air movement can effectively help cool the cables on the tray.
All electric cables carrying current dissipate some power because of the resistance of the current carrying conductors.
If the energy dissipated in the form of heat is not carried away from the cables, the cables will heat themselves up and effectively operate at the higher temperature than the actual surrounding air. This will lower the actual maximum current that the cables can actually handle.
This IEE Regulation provides a table of current carrying capacities of cables that are run on perforated cable trays. However, these cable ratings are only applicable if the holes occupy at least 30 percent of the surface area of the trays.
You can also see from the picture that the cable trays are installed at the highest level, which is above all the pipe works in the plant room. This does not happen by chance.
Prior to commencement of the installation work, the construction people should prepare a coordination plan that incorporate all services and structures in a given area.
With all construction layouts of all building services superimposed into a coordinated layout, then the horizontal and vertical alignment of the pipe works, the cabling works and building elements (i.e. floor beams and columns) can be properly adjusted and tuned so that each element of the works will actually be able to be installed where it is supposed to.
For example, the electrical cable trays in the picture should not be installed below pipe works. The reason being there is always a chance throughout the life of the buildings that a pipe or joint will leak. If the pipe work is above the cable tray, then the water or other liquid in the pipe can drop onto the cables, eventually travel along the cable length, and enter the electrical switchboards. The will surely result in damages to the switchboards and the electrical components inside.
In addition to the damages, this situation will lead to hazards of electrical accidents. If there is a trunking or conduit works that are jointed to the trays or the switchboards, then the travel path may finally lead to some switched socket outlets or other fixed electrical equipment.
Serious electrical accidents may happen before the problem is actually noticed by anyone in the building.
The second main reason for having the coordinated services drawings are that different mechanical and electrical services are usually done by different trade sub-contractors. Each has their own schedule of work and priorities.
Without a prior coordinated construction plan available to each one of the trade subcontractors, the first teams that start the installation works at the area would surely choose the piping or cabling route that is most convenient or the most economical for them.
Then the teams that come in at later time will find that the route they need to run their services have been taken, or the path has been blocked. Relocation of the already installed works can cost a lot of money and abortive works, something all contractors would avoid as best as they can.
So many times the owner of an installation is handed over with installations that are badly coordinated. These sorts of substandard works can present not only hazards, but also costly to run and sometimes impossible to maintain and carry out service or repair works.
One of the major and most difficult tasks of a supervision engineer is to make sure that this aspect of work is properly taken care of.
The cable trays and the cable ladders, among other elements of an electrical installation, are the major components that need to be coordinated. This is because the large cables carried by these trays and ladders are usually difficult to zigzag around obstacles.
Picture 2 – Cable tray hanger system and angle bend piece
The picture shows a closer view of the angle piece of the cable tray. This is another important matter need attention of a supervision engineer during construction. Many clients and engineers do not accept the angle bends that are fabricated at site.
Cable tray systems are usually proprietary systems. The components are supplied in standard pieces, but most types of factory-made pieces are available nowadays. The contractors or installers do not need to do self-fabricated piece by modifying the factory-supplied standard piece.
Angle bends, the piece that is the 90-degree bend in the photograph above, cost money. They are generally more expensive than the straight run pieces. In large installations, many of the angle bend pieces are required. So there is always a tendency by the installers and contractors to modify and improvise using the lower cost straight run pieces instead of using factory-manufactured angle bends.
The problem with bends fabricated at site is control of workmanship. A badly fabricated angle bends (and other types of bends and accessories fabricated at the construction site) can cause serious damage to the cables during the cable installation works.
Many of these damages are difficult to spot and even the task of thorough inspection on the cables are difficult to be carried after the cables are installed in place on the cable trays and the cable ladders.
This is the main reason all non-standard pieces and accessories of a cable tray system and a cable ladder system should be factory-manufactured.
Picture 3 – A straight run cable tray under soffit of slab along a corridor
The above photo shows a stretch of the cable tray along a building corridor that uses only the standard pieces. The contractors have not much room to cut corners here. Not like the angles and bends that are necessary in congested spaces like the plant room in Picture 1 and Picture 2 above.
Picture 4 – Perforated cable tray with the circuit protective conductor (3 mm x 25 mm copper tape) installed
The above picture shows a stretch of vertically mounted cable tray. One or two readers may notice that this stretch is not painted like the others in Picture 1, 2 and 3. This one and those in the other pictures above are actually at the same building.
However, the one in Picture 4 is not painted because their materials and installation are covered by different specifications, which is the mechanical specification. It is going to carry electric cables for the chiller plant. That is why it is under mechanical specifications, and those specifications did not require them be painted.
Those in Picture 1, 2 and 3 are actually for electrical distribution cables and are under electrical works contract. The contract specification requires that all electrical trays, trunking and conduit be painted with orange color.
Picture 5 - Another air-cond cable tray
Cable Ladder
This section is about the installation of cable trays and cable ladders. However, I have spoken only about the cable trays.
Actually, the installation of cable trays and the cable ladders are about the same.
You can compare the installation from the pictures. (UPDATE: You can see the cable ladder pictures at this post, Electrical cable ladder pictures.)
For building constructions, the cable ladders are usually used for large plant room areas and where larges cables are used. The constructions of the cable ladders are better suited to handle the weight and stresses imposed by large cables.
One important point to note is that the cables mounted on cable ladders are just like cable installed in free air. If the cable trays have 30 holes occupying the surface of the tray, the cables on cable ladder are like installed in free air. The cooling qualities of this method of installation is better and therefore the cables can carry higher continuous current for a given size and type of cables.
Picture 6 – Diagram of the cable tray hanger
The mounting methods for cable trays and cable ladders are generally the same. This diagram shows some details on the type of materials and the method of fixing.
Sample specifications on installation of cable trays and cable ladders
A. Cable tray materials
a) Wherever cable trays are required for use in the contract works, the contractor shall supply and install perforated type, hot-dipped galvanized cable trays complete with all the necessary bends, tee pieces and adaptors where changes in the cable tray widths are required. The cable trays shall be of heavy-duty construction, made from sheet steel of 1.6 mm minimum thickness (for tray dimension of up to 300 mm width), and a minimum width of 2 mm for sizes above 300 mm width.
b) All fittings and accessories for the cable tray system including the tee pieces, bends, intersections and other accessories shall be factory-manufactured and purpose-made by the same manufactures. Custom-made fittings fabricated at the construction site will not be acceptable.
c) Depending on the installation situation, the cable trays may either be suspended from the underside of the floor slabs or roof structural works, supported on columns or walls, or installed on the floor. All materials for the suspension units, angle supports, structures etc shall be hot-dipped galvanized.
d) Cable tray supports, hangers and structures shall be spaced adequately apart to cater for the weights of the cables and the trays supported by them. Under no circumstances will the cable trays and the cables be permitted to sag. Any sag found on the installation works shall be repaired or replaced by the contractor at his own cost. The spacing of the support shall not exceed 1.2 m, and supports shall be provided not more than 150 mm away from any bend, tee, intersection or riser. The contractor shall be required to submit technical calculations to justify the structural integrity of the cable tray supports.
e) In cases where a single layers of cable trays is insufficient to accommodate the number of cables to be laid thereon, the contractor shall install two or more layers of cable trays on a common set of cable tray hangers. The supports or the structure shall be sufficiently robust and with sufficient capacity to cater the additional weights of the trays and the electrical cables.
f) Fixing clips or cleats for cables on the trays shall be fixed by means of non-corrosive metal screws (or bolts, washers and nuts).
g) All cable trays shall be installed with the greater dimension in the horizontal plane unless otherwise agreed by the Employer’s Representative.
h) The whole cable tray system shall be completely earthed with equipotential bonding conductors using 25 mm x 3 mm copper tapes. All connections between the copper conductors shall be accomplished using square copper clamps.
i) Provisions of the cable tray for the telecommunication systems shall be as per the tender drawings. The color of the paint applied s shall complied with the requirements of the telecommunication authority
j) For all other services, the painting shall be in the form of color bands of 50 mm width at 1.5 m interval, or a minimum of one bend per section.
B. Execution
a) All cable trays and cable ladders shall run vertically, horizontal or parallel with features of the building and in accordance with BS 7671. The contractor shall be responsible for coordinating them with other services during the installation works.
b) Galvanized coating damaged by excessively rough treatment during transit and erection shall be repaired using at least two coats of good quality zinc-rich paint complying with BS 4652.
c) The maximum size of damaged area for which such repairs are acceptable shall be in accordance with BS EN ISO 1461: 1999.
d) Cable trays and cable ladders shall not be cut at site. Instead, they shall be supplied in appropriate lengths from factory for assembly at site.
e) Splice connector plates shall be located according to manufacturer’s recommendation. Where necessary the trays and ladders shall be cut at site to suit splice locations.
f) The splice connector plates shall be located outside of tray side rails. The adjacent tray or ladder sections shall be bolted using nuts and washers, on the outer side of the tray. The torque nuts used shall be to manufacturer’s specified values.
g) Expansion splices shall be positioned properly with the connector fasteners securely locked to permit the tray or ladder to expand or contract freely.
h) Trays and ladders shall be securely anchored to supports. They shall be secured such that the tray or ladder system will not move during cable installation.
i) Holes shall be punched or drilled in the side rails or troughs only as needed for splicing of sections cut at site.
j) Where cables are to be installed across dividers, divider strip protectors shall be installed. Where cables are to pass over the tray edge, a sheath edge with compassable materials similar to the cable sheath shall be installed.
k) Holes for attachment of conduit to blind end-plates shall be punched at site.
l) All cable trays or cable ladders damaged during installation or cable pulling shall be restored to new condition or replaced.
m) Cable trays and ladders installed above piping and other obstructions shall meet the required headroom. The minimum clearance from the top of the tray side rails shall be 300 mm.
n) The side of the cable tray facing the wall shall have a clearance as per the manufacturer’s recommendation. The other side of the cable tray shall have a minimum clearance of 100 mm.
o) Bends, elbows, hinged splices, etc shall be of proprietary manufacture and shall not be fabricated at site.
C. Installation of supports for the cable trays and ladders
a) Horizontal and vertical supports shall provide at least 30 mm bearing length from each rail and shall have provisions for hold-down clamps and fasteners.
b) The side rail shall bear on the support. Where necessary, shims shall be used to elevate the trays or ladders and provide bottom clearance to the supports. The trays or ladders shall not bear on the support.
c) Vertical straight lengths shall be supported by wall-mounted brackets at intervals as dictated by building structure but this shall not exceed 1 meter.
d) Horizontal cable trays and ladders shall be supported by either wall mounted support bracket or a hanger rod system. The intervals between support shall be as recommended by the manufacturer but this shall not exceed 1 meter for wall mounted support brackets, and 1.2 meter for the hanger rod system.
e) The hanger rods shall be positioned and installed according to the manufacturer’s recommendation. The selection of the rod sizes shall be determined based on the total loading of the support system. The contractor shall be required to submit technical calculations to justify the structural integrity of the structural system of the cable trays and ladders.
f) Sloping trays and ladders shall be supported at intervals not exceeding those used for the horizontal trays and ladders of the same design.
g) Cable tray support shall be installed at each cable drop-out.
h) Cables shall be supported by proprietary make hanger system and cable clamps on the cable tray and ladder. Cable straps are not acceptable in this contract.
D. Installation of splice connectors
a) Splice connectors shall be located as recommended by the manufacturers.
b) Splice connectors shall be attached by round head bolts with the nuts and washers located on the outside of the tray or ladder.
c) Thermal expansion splices shall be installed wherever expansion joints occur.
d) Where space constraint demands, reducer plates shall be required. The structural straightness of the tray shall be maintained using supplementary permanent framing.
E. Installation of cable exits from cable trays and ladders
a) Conduits shall be attached to the side rail with conduit clamps and brackets. Holes shall not be made in side rail for conduit attachment.
b) The minimum bending radius of cable exit from trays or ladders shall be maintained using drop-out plates, vertical riser elbow fittings, or other accessories designed for control bending.
c) The edges and flanges of the cable trays and ladders at locations of cable exit from the trays shall be suitably sheathed to prevent injury to cable insulation.
F. Installation of divider strips
a) These divider strips shall be installed where different system wiring is installed in a common tray or where shown in the design drawings.
b) Divider strips shall be anchored to every ladder rung using fasteners, which attach to the tray without the need for drilling or punching at site.
c) The faster shall attach to the tray without the need for any drilling or punching of hole at site. The fasteners shall also present minimum exposure to the cables by having rounded exposed parts.
G. Installation of fire barriers and fireproof enclosure
a) Fire stop seals shall be provided where cable trays and ladders pass through floors or walls.
b) Fire stop installation shall as specified in the FIRESTOPS section.
c) For cable ladders and cable trays passing through the floor slabs and walls, the installation of fire stops shall only be carried out after the utility or services provider has completed their cabling works.
H. Earthing
a) All cable ladders, cable trays and interconnecting trunking and conduit shall be earthed as described in earlier paragraphs.
b) Earth links shall be used to electrically interconnect joined sections of the trunking, cable trays and the cable ladder system. The resistance measured between adjacent sections shall not exceed 0.01 ohms.
I. Handling, transport and storage
a) Conduits, trunking, cable trays, cable ladders, fittings and accessories shall be separately packed. They shall be delivered in appropriately labeled packing in accordance with the manufacturer’s recommendations. All articles shall be securely packed to prevent any movement and damage during transport. To avoid wet storage staining, transporting of galvanized articles shall avoid damp and/ or badly ventilated conditions.
b) The manufacturer shall provide information for unpacking and safe handling of all articles.
c) On arrival at site, the consignment shall be checked against the delivery notes.
d) All parts and components shall be stored indoor, in a clean, dry and well-ventilated place. Galvanized articles shall not be installed on clinches or ashes.
J. Tests
1) Type tests – Cable trays and ladders shall be type-tested to NEMA VEI standard as follows:
a) VE 1-3.01 Destruction load test
b) VE 1-3.02 Deflection test
c) VE 1-3.03 Electrical continuity of connections
d) Provide complete type test reports
2) Acceptance tests at Manufacturer’s Work
The cable trays, ladders, fittings and accessories shall be subjected to the following:
a) Visual inspection:
i) Correct type of cable trays and cable ladders
ii) Correct markings
iii) Dimension checks
iv) Damage on trays and ladders
v) Damage on galvanizing
vi) Fittings and accessories are of proprietary type.
b) Inspection and testing of galvanizing for compliance with BS EN ISO 1461: 1999
i) Determination of coating weight
ii) Uniformity of coating
iii) Deflection tests
iv) Electrical continuity of connection
c) Provide complete acceptance test reports
d) The manufacturer test shall be verified and witnessed by Employer’s Representative, or his representatives.
3) Inspection and testing at site:
a) Upon delivery to site, check the condition as follows:
i) Damage on the cable trays and ladders
ii) Damage on galvanizing
iii) Fittings and accessories are of proprietary make
iv) Store indoor
b) Repair or replace damaged parts or damaged galvanized coating areas.
c) Upon completion of the installation, visual inspection and verification of:
i) Correctness of location and mounting
ii) Labeling and marking
iii) Damage on the cable trays and ladders or their accessories
iv) Damage on galvanizing
v) Earthing of the cable trays and ladder system
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