Cable trays and ladder installation

I have uploaded a few pictures and a diagram on the installation of electrical cable trays and cable ladders for those who have some need for them. At the end of these pictures, you will also find a sample specification for the cable tray and cable ladder installations in multi-storey office buildings. If you wish to see more photos of electrical installations, this post, Free electric installation pictures.
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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Light fitting specification

The following provides an example specification that can be used in a contract for the installation of the light fittings in a multi-storey office building.

General
All light fittings shall be designed to prevent ingress of vermin, accidental contact with any live part and to minimize the ingress of dust and dirt. Materials which may be liable to be attacked by termites or other insects shall be avoided.

All outdoor fittings shall be vandal, corrosion, rust-protected and fully weatherproof by means of carefully chosen materials, protective painting and sealing, gasketing and the application of protective compounds.

Each fitting shall be constructed and erected as to be easily accessible for inspection and maintenance.

Each fitting shall be complete with bulbs, tubes, control gears, suspension rods or chain, fixing brackets etc.

Terminals and contacts, as far as possible, shall be shielded as to afford protection from accidental contact. All terminals shall be suitable for connection with metal conduit, to meet with fire regulations.

Sample of each type of fittings must be submitted to the S.O. for approval prior to commencement of manufacture and/or installation.

Uniformity
All fittings, accessories and components in any items supplied shall be uniform of their type, similar parts being interchangeable.

Incandescent lamps
Tungsten filament lamps shall be of bayonet cap general service pattern, internally frosted and shall be suitable for operating on a 240/250 volts A.C. system. The ratings shall be as stated in the drawings.

Fluorescent lamps
All fluorescent tubes shall be of the standard 26mm diameter energy saving type having a color temperature of approximately 6200 degree K (daylight color) if not otherwise stated with, bi-pin lamp cap.

Unless otherwise specified, all fluorescent tubes shall be of switch-start type.

Compact fluorescent lamps
Where SL lamps are specified, they shall be the type of fluorescent lamp integrated with low pressure gas discharge ballast and starter and with Edison screw lamp cap (B22). The wattage of the lamps shall be 9, 13 or 18 as specified in the drawings.

Where PL lamps are specified, they shall be of the type with unique single-ended fluorescent lamps with two pin connection with instant start adapters. The lamp base shall be incorporated with capacitor. The lamp wattage shall be 7, 9, 11, or 13 as specified in the drawings.

Discharge lamps
Where gas discharge lamps are specified, they shall be the type with good color rendering and complete with necessary control gear such as ballasts, capacitors and igniters. The ratings of the lamps shall be as specified in the drawings.

Fluorescent lamp lighting fixtures
All fixtures must be fabricated by manufacturers registered with Director General of Electrical Department, and be approved with The National Testing Department.

The housing, base frame, cover plates and reflector of the fixtures shall be constructed of sheet steel of a thickness not less than 20 SWG.

The sheet steel body shall be designed such that it can be fixed to ceiling or the surface by suspension or on a separate detachable tray.

An adequate mains input terminal block marked “L” and “N” together with a substantial earthing terminal shall be riveted, welded or brazed to the base frame.

Retractable type spring load lamp holders fitted with lamps cap earthing contact housed in a white plastic moulding shall be suitable for direct attachment to the base frame.

All materials liable to corrosion shall be suitably protected and sheet steel surfaces shall be prepared, painted and finished white in stove or vitreous enamel.

Ballasts shall be of the switch start, vacuum/pressure impregnated in polyester resin type comply to B.S. 2818 Part1 or MS. 141: 1973. They shall be compact in design and silent in operation and of the “low loss” type with losses not exceeding 6.5 watts. One ballast shall be used to control one fluorescent tube only.

All capacitors for power factor correction shall be suitable for operating on 250 volts, single phase, 50 Hz at a temperature between -10 degree to 100 degree Centigrade and shall ensure a power factor of not less than 0.85 lagging.

Starter switch shall be of the two pin small metal canister glow type.

All internal wiring shall be heat resistant type and suitably identified by color coding and shall be neatly arranged and adequately supported.

All diffusers, unless otherwise stated, shall be made from tough translucent prismatic plastic of UV-stabilized polystyrene of not less than 2.5mm thick and free from blemishes.

All louvered diffusers, unless otherwise stated, shall be made from UV-stabilized polystyrene material with silver-metallized and parabolic 13mm x 13mm x 11mm cells.

All diffusers and louvered diffusers shall be mounted onto the steel frame which hinged onto the frame work of the fixtures and form a detachable dustproof cover.

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Electrical wiring specification

The following provides a sample specification that can be used in an electrical wiring installation contract for a multi-storey office building.

Scope

The scope of this section is to set out the requirements, methods, materials, workmanship, standards and regulations in connection with the electrical equipment and their installation works for this project. 

General

These specifications shall be read together with the relevant drawings, and schedule of quantities if there is any, that form part of this contract. All the works of wiring and cabling are generally as indicated and specified on these drawings.

Ducts and trenches necessary to accommodate cables and switchgears that are generally shown on the drawings will be provided by Contractor unless otherwise stated. However, it shall be the contractor’s responsibility to ensure during the progress of the work that the various ducts and trenches are constructed in the correct manner and that they are adequate for the electrical works whether such details are specifically mentioned or not.

Wiring methods

All wiring shall be of the “loop-in” system and no joint shall be accepted except at the terminations of the electrical accessories and/or equipment.

No reduction in the number of strands of cables shall be allowed at the terminals. All strands shall be effectively terminated and secured by screws, nuts and washers or other approved means of fixing.

Concealed wiring

All wiring to be concealed shall be neatly run in the concrete or plaster. They shall be secured firmly to the surfaces on which they run by means of lead saddles at interval of not more than 600 mm. 

There shall be a plaster cover of at least 12 mm over all such concealed wires.

It is imperative that all the wiring is completed before surfaces are plastered.

Cables shall be concealed behind cement plaster in the walls and/or ceilings or concealed in roof spaces behind false ceilings. Where cables run over surfaces other than wood behind ceiling, they must be protected by conduit. Chases, not deeper or wider than is necessary to accommodate the cable runs, shall be cut in the concrete/brick walls and ceilings. The cables when fitted shall not protrude beyond the surface of the walls and ceilings. Final rendering and plastering shall be carried out by the Contractor. Cable runs shall always be parallel or perpendicular to walls and shall be adequately secured by non-rusty wiring clips and brass nails at appropriate intervals. Any cutting and chipping of concrete shall be approved by S.O.

Electrical accessories where possible shall be flush fitting housed in open metal or aluminum boxes of appropriate thickness sunk into the walls and fastened by rawl plugs and brass screw. The open side of the metal box shall flush with the plaster wall surface and a hole shall be drilled at the back to permit cable entry. 

Accessories which cannot be fitted flush with wall surface shall be mounted on hard wood blocks of suitable size and thickness which shall be fixed to the walls by means of rawl plugs and brass screws so that the surface of each block shall finish flush with the plastered wall surface.

Surface wiring

Cables shall run on the surface of walls and ceilings or in the roof of spaces and secured by lead alloy saddles of approved design. Saddles shall be fixed by brass nails or screws spaced and not more than 150mm apart and not more than 10 cables shall be clipped together using the same saddles. 

Where cables run over surfaces other than wood, they must be secured on treated hardwood battens firmly fixed in position with rawl plugs and brass screws. Cable run shall always be parallel or perpendicular to walls and earth wires shall be fixed on the outside of the cable runs. All wiring which is installed at less than 1200 mm above floor level or subject to mechanical damage easily shall be protected with a wooden or plastic casing.

Electrical accessories shall be of the surface pattern type fixed on hardwood blocks with brass screws. Where more than one piece of accessory are grouped together, a single wooden block shall be used to accommodate all the accessories. However, it shall not be larger than necessary and it shall be cut at the side of the wooden block to permit cable entry.

PVC insulated cable

PVC insulated cables shall mean Polyvinyl chloride insulated cables. The conductors shall be of high conductivity stranded copper conductors. They shall manufactured in accordance with the specifications of BS 6004 or MS 136 and be of the 600/1000 volts grade.

The colors of the insulation shall be in accordance with Table 51A of the 16th Edition of IEE Wiring Regulations.

Cables to be used for surface and concealed wiring shall be PVC insulated and PVC sheath.

All cables shall be supplied at maximum required lengths and no joints are permitted.

Conduit installation

All conduits, fittings and associated accessories shall be galvanized and shall comply with B.S. 31. Conduits shall be screwed and welded Class “B” and fittings shall be manufactured from steel or malleable cast iron.

Where PVC conduits are specified, they shall be of high quality rigid type with all approved type joints, tee off and jointing materials.

Concealed conduit shall be fixed securely to prevent movement before casting of floor slabs, floating of plaster and casting of columns and beams.

Conduits and associated accessories shall be painted with one coat of red lead whenever the exposed galvanized surface has been cut or otherwise damaged including exposed threads and connections after erection.

Conduits shall be properly and tightly screwed into the full depth of box spouts and butted in sockets between lengths to ensure maximum mechanical strength and electrical continuity so that the wiring is continuously and effectively protected throughout its whole length, is not in anyway under mechanical stress.

The whole of the conduit system shall be continuous throughout. A separate earth continuity conductor shall be provided in all metal conduits. All conduits shall be earthed at terminations.

Flexible metal conduits shall not be accepted as a means of providing earth continuity. A separate earth continuity-conductor shall be provided with every part of the system formed by such conduit.

Conduit sizes shall be selected carefully for the number and size of cables they are to contain. The conduits shall be arranged with an adequate number of boxes to allow easy draw in and draw out of any one or all of the cables at any time. 

The conduit sizes shall not in any circumstances be less than 20mm and the number of cables drawn in shall not be greater than the appropriate number permitted in the 16th Edition of IEE Wiring Regulations.

Cables for lighting and power circuits shall not be drawn into the same conduit as those for extra low voltage systems. 


Lighting and power final circuits shall not be run in the same conduits, except where an adaptable box is employed as a final distribution point. A number of final circuits may be grouped together in a larger circuit between the distribution board and the adaptable box provided that all sub-circuits are of the same phase. 

In case of three phase circuits, all three phases and neutral if any should be drawn into the same conduit. Where condensation is likely to occur in surface conduits they shall be laid in falls to drain off condensed moisture so it does not gain entry into terminations.

Conduit work and accessories where not concealed shall be fixed effectively by means of heavy patterned spacing saddle and some approved metal or other non-disintegrating plugs of proprietary manufacture.

On straight runs the conduit shall be supported by saddles at intervals not exceeding 900 mm in addition to supports provided by any structure, box or fittings included in the run. For 40 mm conduit saddles maybe spaced at intervals not more than 1220 mm.

Hanging or suspending conduits using wires are not permitted 

Bends must in all cases be supported on each side by two saddles as near thereto as possible and a draw in box shall be provided after two bends and after not more than each 9 m of straight run.

Where conduits cross expansion joints they shall be installed in such a way so as not to resist relative movement of the sections. A suitable crossing shall comprise conduits telescoped one inside the other with the free ends or ends projecting immediately to one side of the crossing. Earth bonding of the telescoped end, which shall be suitable bushed, shall be affected inside the box to maintain earth continuity of the system.

Immediately on the completion of erection of any conduit during building construction all exposed switch, socket and conduit risers shall be plugged effectively against the ingress of water and dirt particularly where concrete shall be poured. Such seals shall be maintained in good order for such times as is necessary to complete wiring and connection of fittings and switches.

All conduits shall be swanned out and free from moisture to the S.O. satisfaction before wiring is commenced. Draw in tapes with absorbent cloth, such as flannel or army pull through cloth shall be used for this purpose.

On completion of the installation all exposed conduits shall be painted with two coats of good quality approved paint and to the satisfaction of the S.O.

Cable trunking installation

Cable trunking may be employed in lieu of conduit where multiple runs would otherwise occur.

All cable trunking shall be manufactured from good quality hot dipped galvanized mild sheet steel of not less than 18 SWG for sizes up to 100mm x 100mm and not less than 16 SWG for sizes up to 150mm x 150mm and not less than 14 SWG for larger sizes.

The trunking shall be installed complete with all necessary accessories such as bolted flanged outlets, blank ends, reducers, outlet bushes, bends, tees, sleeve couplings, intersection four way boxes and fitting adapters. 

Bridge pieces to act as cable retainers shall be readily removable, but positive fixing by machine screws for cover shall be provided. The inner radius of any bend shall not be less than 2.5 times the minor dimension of rectangular section trunking.

A 25mm x 3 mm copper tape shall run throughout the whole length of trunking from main switchboards to sub-switchboards, from main switchboards to distribution boards and from sub-switchboards to distribution boards to provide earth continuity.

All trunking shall be supported adequately by suitable brackets fabricated from galvanized mild steel sheet flat.

Whenever permitted by the S.O., cables for power and lighting circuits and extra low voltage systems shall not be run in the same trunking unless they are segregated effectively by means of rigidly fixed metal barrier or screen. 

The erection work of a trunking must be completed before any cable is drawn in. 

The number of cables run in a trunking shall be such that a minimum space factor of 45 percent is provided.

Cable tray installation

Perforated hot dipped galvanized mild steel cable trays of not less than 16 SWG may be employed in lieu of conduit.

Trays shall be of appropriate width with an up-turned flange both sides 20 mm deep and shall be with all necessary long radius bends and tees and fixing brackets fabricated from mild steel flat.

They shall generally be supported by directly beaming into top side of the concrete rib construction at 1820 mm centers forming the ceiling and in this event only, a simple and efficient approved clamping arrangement to the ribs shall be affected to prevent lateral displacement of the tray. 

Trays may be employed in other situations at the discretion of the contractor in order to carry out multiple runs of the M.I.C.C. and multi core cables as an alternative to fixing by saddles to the structure.

If trunking or cable trays are used in lieu of conduits, care shall be taken to ensure that all trunking, cable trays and cable runs in areas known to contain corrosive vapors are painted with an approved type of anti-corrosive paint and it shall be deemed that the cost of such painting has been included in the contract sum.

A 25 mm x 3 mm copper tape shall run throughout the whole length of the cable tray to provide earth continuity.

Ducts and trenches

Unless otherwise stated, ducts and trenches necessary to accommodate cables and equipment will be provided by the building Contractor in accordance with the drawings. However, it shall be the Contractor’s responsibility to ensure during the progress of work that the various ducts and trenches are constructed in positions as are required by the electrical distribution works and as such, are adequate for these requirements whether specifically mentioned herein or not.

If these ducts are not provided the contractor must advise the S.O.’s as soon as possible.

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Lightning protection specifications

The following provides some example specifications that can be used in a contract for the installation of the lightning protection system in a multi-storey office building.

General
The installation of the Lightning Protection must comply with the specifications below, and the information given in the accompanying drawings, but in any case must always be in accordance with the British Standard Code of Practice BS 6651: 1985 and to the satisfaction of the S.O.’s representative. All materials should conform to the latest edition of B.S. Specifications.

Air Termination
Materials for the air termination must be medium or hard drawn copper or phosphor bronze rod of at least 15mm in diameter.

The terminations shall project at least 460mm above the highest point of the building.

They shall be firmly and permanently fixed with saddles or brackets and protected from mechanical damage.

Roof conductors
Materials for the roof conductors must be high conductivity annealed copper tape with the dimensions of 25mm x 3mm.

The roof conductors shall interconnect all air terminations to all down conductors and form a conducting air termination network covering the area to be protected with closed metal loops.

All exposed metallic structures shall be bonded in a solid and permanent manner to the roof conductor network.

Care should be exercised to avoid corrosion by the bonding of dissimilar metals.

Ferrous metals should not be used.

All joints and conductors shall be protected against mechanical damage.

Roof conductor runs shall be as direct as possible with spacer bars or tape clips at two meter intervals.

Bends shall be made as large as possible and in any event the bending radius shall not be less than 150mm.


Down conductors
Materials for the down conductors shall be of high conductivity annealed copper tape with the dimensions of 25mm x 3mm.

Down conductors shall interconnect the roof conductor network to the earth termination.

The route shall not interfere with the architecture and shall as direct as possible.

The down conductors shall be firmly and permanently fixed to the outside of the building or structure with spacer bars or tape clips at one meter intervals.

All independent metallic structures and other building structures within 600mm of any down conductor route shall be bonded to the nearest down conductor.

Lift shafts and duct shafts shall not be used as down conductor routes.

There shall not be any “up-turn” for the down conductor routes.

Joints and bonds
All joints shall be tinned and soldered and double riveted.

Clamped, bolted and screwed joints shall not be used, except at testing points and at rod connections.

Joints or bonds shall be as few as possible. All joints shall be coated with bituminous paint and protected against moisture and corrosion.

The maximum resistance of a joint shall not exceed 0.5 miliohms.

Tape to tape bonding clamps shall be “Furse” type having tinned contact surfaces complete with 8mm phosphor bronze fixing screw.

Test and junction clamps
Materials for the clamps should be phosphor bronze.

Test clamps shall be provided on each down conductor in an easily accessible position for testing purposes at a height of 2000mm from finished ground level.

Test clamps shall be protected from unauthorized interference.

Test clamps shall be provided on the roof conductor network in such a way that all parts of the network can be tested independently.

The test clamps shall be of approved type and shall not constitute an electrical resistance within the system.

After installation and testing they shall be painted with bituminous paint to prevent corrosion.

Earth termination
Materials for the earth terminations shall be of hard drawn copper or phosphor bronze rod. The minimum diameter of the rod shall not be less than 16mm.

The diameter of the rod shall be chosen according to the site condition but it shall always be such that it can be driven into the soil without bending or deforming the rod. Proper driving head and coupling similar to “Furse” type shall be employed.

The length of the rods shall be made up from the standard 1200mm lengths with internal screws and socket joints.

Minimum lengths of earth driven electrodes shall be 2400mm.

The length or the number of rods shall be increased if the maximum permissible earth resistance of five ohms cannot be achieved with the standard electrodes as specified.

The distance between two driven electrodes shall be equal to or greater than their driven length.

When more than one down conductors are used and terminated at earth terminations, all earth rods shall be bonded together to form one network.

The lightning protection earthing shall not be used in whole or in part as part of electrical earthing without the approval of the S.O.

Chemical treatment of soil at termination points to decrease the earth resistance shall not be carried out without the approval of the S.O.

Connections to the earth electrodes shall be by means of approved connector clamps adequately tightened. Earth connections shall be protected from damage by means of suitable electrode housing. The actual connection of the rods must be accessible and clearly visible when the housing cover is removed.

Testing
Testing of earth resistance and conductor continuity shall be carried as specified in the British Standard Code of Practice BS 6651: 1985.

The test shall be carried out in the presence and to the satisfaction of the S.O.

The resultant earth resistance of the electrodes shall not exceed five ohms and this resistance value shall be determined by an Earth Resistance Megger.


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