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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