Electrical - Installation Of Wiring System

Wiring in Steel Conduit System

1 Type of Cable

Wiring system installed in conduits shall be non-sheathed copper cables. Cables for 3 phase circuit shall be 450/750V grade or above.


2 Concealed Steel Conduit System

Unless otherwise specified in the Particular Specification or on the Drawings, conduits shall be concealed within walls, floor slabs, false ceilings, or other suitable space.
Chase for conduit to be concealed in wall, column or beam shall be vertical or horizontal.


3 Surface Conduit System

Where surface conduits are specified, they shall be fixed by distance (spacing) saddles, and shall run in a vertical or horizontal direction.

Conduit boxes, adaptable boxes and metal boxes for accessories shall be securely fixed to walls, ceilings or other substantial parts of a structure by means of suitable brass screws correctly spaced. The fixing of these boxes shall be independent of the fixing of the associated conduits.

Particular precaution shall be taken in situations where the high temperature cables may be touched, or where they may touch other materials.


4 Minimum Size of Conduit

The minimum outside diameter of any conduit shall be 20mm.


5 Flexible Conduit

Flexible conduit shall be used as short as possible and in any case shall not exceed a conduit run of 2m. (Note : PVC pliable conduit may be used in place of steel flexible conduit wherever appropriate. In such case, requirements in Sub-section B1.3 shall apply.)


6 Conduit Continuity

The steel conduit installation shall be mechanically and electrically continuous throughout and effectively earthed. Where the circuit protective conductor is formed by the conduits, the terminal of a socket outlet shall be connected by a separate circuit protective conductor having the same cross-sectional area and type as the live conductor to an earth terminal incorporated in the associated
metal box or enclosure.

Flexible conduit shall not be used as a circuit protective conductor. A separate circuit protective conductor having the same cross-sectional area and type as the largest live conductor shall be drawn into the flexible conduit for earth continuity. This circuit protective conductor shall be fixed to an earth terminal incorporated, at each end, in the metal boxes or enclosures onto which the ends of the flexible conduit are locked.


7 Joint in Conduits

Joint in steel conduits shall be made by means of a solid coupler into which the adjacent ends of the two conduits shall be inserted to approximately half into it and screwed up tightly in order to make the conduit run mechanically and electrically continuous. No threads on either conduit shall be exposed.

Running couplings shall not be used. In case where such a coupling cannot be avoided approval from the Architect must be obtained before it is adopted. In this case, the coupler shall be screwed up tightly onto the short threaded portion of one conduit, the threaded portion of which shall project approximately half way into the length of the coupler. The threaded portion of the mating conduit shall also project approximately half way into the coupler. Electrical continuity shall be ensured by
tightening up a hexagon locknut against the coupler. Any exposed threads on either conduit shall be painted with at least two coats of antirust paint.


8 Provision of Adaptable Box

An adequate number of suitably sized adaptable boxes shall be provided in all conduit runs to enable cables to be drawn in and out easily without damage. Adaptable boxes shall be provided immediately after every two bends, or after a bend plus a maximum straight run of 10m, or after
a maximum straight run of 15m.


9 Spacing between Conduits

Adjacent or parallel conduits cast in concrete shall be separated by a spacing of not less than 25mm so as to allow concrete aggregate to pass and set between them.


10 Termination of Conduits at Metal Casing of Equipment

Where a steel conduit terminates at a metal casing, a coupler and a brass male bush shall be used. The brass male bush shall be screwed into the coupler or adaptor from the inside of the metal casing through a clearance hole drilled in the metal casing to suit the bush. Both the conduit and the bush shall be screwed tightly into the coupler or adaptor so as to grip the metal casing securely for mechanical and electrical continuity.

Threads on the steel conduit shall be at least half the coupler length. Connection between flexible conduit and the adaptor shall be securely fixed. No threads on the metal casing shall be allowed. In the case of flexible steel conduit, a brass adaptor together with a brass male bush shall be used. The brass adaptor shall comprise two parts, an inner core and an outer ferrule. The inner core shall screw into the bore of the conduit together with an outer ferrule which caps off the
end of the conduit, so that the adaptor can provide an extremely strong joint. The core shall lock against the outer ferrule and isolate any sharp cut edges in the conduit.

Where a metal casing is painted or enamelled, the electrical continuity between the conduit and the casing shall be achieved by means of a separate circuit protective conductor connecting the conduit and an earth terminal of the casing. Alternatively, a copper continuity connector may be placed between the bush and the metal casing. The cross-sectional area of the separate circuit protective conductor or the copper continuity connector shall have the same current carrying capacity as the largest live conductor drawn into the conduit in accordance with the IEC 60364. Neither the paint nor the enamel shall be damaged or removed in order to achieve the electrical continuity.


11 Conduit Bend

Conduit shall not be bent with an acute angle. The internal radius of the bend shall not be less than 2.5 times the outside diameter of the conduit.


12 Conduit Crossing Expansion Joint

Where a steel conduit crosses an expansion joint, special arrangement shall be made to allow relative movement to occur on either side of the expansion joint. A separate circuit protective conductor shall be installed to maintain an effective electrical continuity across the expansion joint. The circuit protective conductor shall have a crosssectional area rated to suit the largest live conductor drawn into the conduit in accordance with IEC 60364.


13 Use of Extension Ring

An extension ring shall be fitted to a conduit box where the plaster wall finish (including plaster) is more than 35mm from the conduit box. Only extension rings of the correct depth shall be used. Under no circumstances shall multiple extension rings be permitted.


14 Prevention of Ingress of Foreign Matters

During the building construction, all open ends of the conduit termination, which are liable to be filled with water, moisture or other foreign matters, shall be plugged with approved conduit stopping plugs;
paper, rag or similar materials shall NOT be used for this purpose. Steel conduit boxes in similar circumstances shall also be similarly plugged to prevent concrete aggregate or plaster from entering into the boxes during building construction.


15 Prevention of Accumulation of Water or Moisture

The conduits shall be so laid to prevent accumulation of condensed moisture and the ingress of water in any part of the installation. Approved type of sealant for the prevention of condensed moisture shall be applied to ceiling conduit outlets installed in a cooled space subject to the influx of warm air.


16 Conduit Laid Direct in Ground

Steel conduits laid direct in ground or buried in soil shall be painted with two coats of bituminous paint and wrapped with selfamalgamating tapes or other wrapping materials for protection against
corrosion and approved by the Architect.


17 Fixing of Distance (Spacing) Saddle

Saddles, for the support of surface conduits, shall be provided throughout the entire route at regular intervals. The spacing between adjacent saddles for steel conduits shall not be more than those. Each bend of a surface conduit shall be supported by a saddle on either side of the bend, and the saddles shall be fixed as near to the bend as practicable.
Saddles shall be fixed with brass screws in expanding plugs or other purpose-designed fixing devices approved by the Architect.


18 Conduit Installed Outdoors or in Damp Situation

Where steel conduits are installed outdoors or in damp situations, all conduit fittings and conduit accessories shall not be placed in contact with other metals with which they are liable to set up electrolytic action. In addition, where the conduit installation is subject to weather, the conduit fittings and conduit accessories shall be provided with weatherproof sealant or other materials to ensure that the installation is completely weatherproof.


19 Swabbing Out of Conduit

Conduits shall be swabbed out and free from moisture before wiring work is to commence. The swabbing operation shall be witnessed by the authorised representative of the Architect. Only approved draw-in tape or steel wire of appropriate size and absorbent cloth shall be used.


20 Fire Barrier

Where a conduit passes through fire resistant structural elements, such as walls and floors designated as fire barriers, the opening made shall be sealed according to the appropriate degree of fire resistance of the wall and/or floor. In addition, where a conduit is installed in a channel, duct,
ducting, trunking or shaft which pass through such elements, suitable fire-resistant barriers shall also be provided to prevent the spread of fire.


21 Identification

Conduits, where required to be distinguished from pipelines or other services, shall use orange (BS colour reference No. 06 E 51 to BS 4800) as the basic identification colour in compliance with BS 1710.


22 Cable Capacity of Conduit

The number of cables drawn into a conduit shall be such that no damage will be caused to the cables or to the conduits during their installation.
In determining the size of the conduit, the “unit system” method shall be adopted.


23 Method of Drawing Cables into Conduit

Conduit system shall be completely erected before cable is drawn in and there shall be adequate means of access for drawing cable in or out. Cables shall be drawn into a conduit by using approved draw-in tape or steel wire of the appropriate size. Under no circumstances shall the use of lubricants, e.g. grease, graphite, talc power, etc., be permitted to assist in the drawing the cables.


24 Segregation of Cables of Different Circuit Categories

Separate conduits shall be provided for cables of different circuit
categories or using different voltage levels.


25 “Looping-in” Wiring System

Each cable in a run shall be of one continuous length and wired on the
“loop-in” system. No joint in cable run shall be allowed.


26 Grouping of Cables in Conduit

Conduits running from a distribution board to the final distribution point may each contain all live conductors of a number of final circuits provided that the effective current-carrying capacity of all circuits, upon taking the correction factors into consideration, shall exceed the rating of their respective over-current protection devices and all conditions as stipulated in Sub-section 22 above are satisfied.
For conduits serving final circuits direct from a distribution board or from an adaptable box used as a final distribution point, each conduit shall contain all live cables of one final circuit only except in the case of lighting final circuit where two circuits with 1.0mm2 or 1.5mm2 cables will be permitted in a 20mm diameter conduit.
The neutral cable of a lighting final circuit using single core cables may be routed in the conduit direct to the lighting point without passing through the switch box.


27 Termination of Bonding Conductors at Conduit Installation

For surface conduit installations, the supplementary bonding conductors shall be terminated at the nearest conduit or conduit box forming an integral part of the conduit installation.
For concealed conduit installations, the supplementary bonding conductors shall be terminated at a copper earthing terminal fitted inside a metal box forming an integral part of the conduit installation. The metal conduit box shall be located as near as possible to the bonding position and the exposed part of the supplementary bonding conductor shall be made as short as possible.