Conduit wiring

What is a conduit wiring?

An electrical conduit is a tube used to protect and route electrical wiring in a building or non building structure. Electrical conduit may be made of metal, plastic, fiber, or fired clay. Most conduit is rigid, but flexible conduit is used for some purposes.
What are the advantages of conduit wiring and what are its disadvantages?

The main advantage of running wires in a conduit is that running large amounts of wire can be more efficient, not to mention after a certain size it’s the only option. Don’t forget there’s greater protection and also greater ability to expand your electrical system in the future when using conduit.

The disadvantages would be expertise in running the conduit, the time factor in running a conduit over a multi-conductor cable, and there are some limitations on the ampacity of the wire that runs through the conduit. Because of the heat factor and how wires lay in a conduit the NEC disallows any more than 3 current carrying conductors (hot & neutral — not ground) to be calculated at 100% of their ampacity. That is, the more current carrying wires beyond 3 you have in a conduit the more you must derate the ampacity of the wire.

According to Table 310.15(B)(3)(a), just going from 3 current carrying conductors to 4–6, the maximum allowable ampacity is 80%. It keeps listing conductors up until > 40, which only allows 35% of ampacity of the wire. As you can see this can cause a lot of headaches in wiring.

There can be other factors involved depending on the situation. I think, though, for the most part, we are looking at efficiency (including costs) vs. protection from damage.

Advantages and disadvantages of conduit wiring

Advantages:

PVC conduits offers a highly resistant against corrosion.It is a durable and very popular system.It is a waterproof wiring system.No risk of electric shock.Alternations are possible.Maintenance is easy.It has a long life.

Disadvantages:

It is difficult to install. More time is needed.Fault finding process is very difficult.In case of steel conduit, it is a more expensive system compared to other wiring systems.

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Specification For Conduit Wiring In Electrical Construction Contract Works

Scope of Work of this this sample specification covers the supply, installation, testing and commissioning of all wiring materials for power and lighting in concealed and/or surface conduits.

1 Application 

The conduits shall conform to IS 9537 I & II  or equivalent standards. Switches, Plug Sockets shall conform to IS 3854 and IS 4615, however, contractor shall comply with the latest or superseded versions of IS/BIS to be used for the complete work.

2. Wiring System Details

1. All wiring to lights, fans, convenience outlets, call bells, etc. will be considered on 'point rate' basis and shall include the required length of wires from sub-distribution board outlet to the ceiling roses, switches, convenience outlet sockets, bell pushes, call bells, connectors, etc. The wiring shall be carried out separately for 'lighting' and power as indicated in the drawings.
2. All conduits shall be run as far as possible along the walls and ceiling and above false ceilings so as to be easily accessible for inspection if need be. While the Architects drawings indicate the distribution for light and power points, it shall be the contractors' responsibility as well, to balance the load on all the three phases for an even distribution. Before commencement of work, the contractor shall seek the approval of the Consultants on the distribution of balancing of loads and circuits. The wiring shall be done by the process of looping the live conductors and the neutral wires. 
3. Bare or twist joints shall be carried out with due care and preferably through proper junction boxes.
4. Before commencement of work, the contractor shall seek the approval of the Architects/Consultants on the layout of the conduit runs. Separate conduits shall be run for lighting and power circuits and also for telephone cables. To avoid any cross talk and extraneous interference in the telephone circuits, all telephone wiring conduits shall be run with a sufficient clearance from the power and lighting conduits.
5. All separate circuits from DB's shall have separate neutral to the points. Common neutral between separate circuits are not permissible.
6. As far as possible wiring shall be run in conduits. Any open wiring to be done shall first be approved by the Architects/Owners.  All conduit wiring shall be complete with continuous earth as per I.E.R.
7. The wiring shall be carried out as specified.  'Power' and 'heating' wiring shall be kept separate and distinct from 'Lighting' wiring. The wiring shall be done on the distribution system with main and branch distribution boards at convenient physical and electrical centers and consideration shall be given for neatness and good appearance.
8. The wiring shall be done in the 'Looping System. 'Phase' or 'live' conductors shall be looped at the switch box and neutral conductor can be looped from the light fan or socket outlets. 1.5 sq. mm PVC wires in green colour are to be run continuously in conduits for continuous earthing. The earth wire should be connected to GI Switch boxes and DB boxes by tapped screws.
9. No bare or twist joints shall be made at intermediate points in the through run of cables.
10. If any joint becomes unavoidable such joints shall be made through proper cutouts or through proper junction boxes open to easy inspections.

3. Cables, Wires & Flexible Cords

1. All conductors shall be of electrolytic copper. The wires and cords shall have adequate cross-section and current carrying capacities for the intended loads. The minimum size of conductor used shall have a nominal cross section area 1.5 sq. mm copper for lighting and 2.5 sq. mm copper for power. In case of aluminium wires, 2.5 sq. mm wires for lighting and 4 sq. mm wires for power and 2.5 sq. mm wires for earthing shall be used.
2. The cables, wires and cords shall be PVC insulated type. Cables which are to be run in areas where there  is likelihood  of damage to the PVC insulation, all such cables shall be of the armored type. Cables and wires run through walls shall be run in conduits only.
3. The rates for laying of low tension cables shall include all hardwares like trays, racks, cleats, etc.  Separate rates for such items shall not be considered.

4. Conduits & Accessories - MS/FLO Coated Conduits

1. Steel conduiting shall be conforming to relevant IS specifications (IS 9537). All steel conduits shall be of heavy gauge, welded and threaded type.  Conduit accessories such as boxes, bends, inspection bends, boxes, elbows, reducers, etc. shall conform to relevant standards.  As far as possible, boxes shall have internally tapped spouts to receive the conduits.
2. All conduits and accessories shall be cleaned and painted with two coats of bituminous paint before  installation.  Where conduits are installed in straight run, draw boxes shall be provided at intervals not exceeding 30 feet(9 meters). Between two consecutive draw boxes, the right angle bends shall not exceed two in number. Conduits shall be properly threaded and screwed in to the accessories.
3. The minimum size of conduits used shall be 20 mm. The minimum thickness of the conduits shall be 16 SWG.
4. Wherever steel conduits terminate into points control boxes, distribution boards, etc. conduits shall be rigidly connected to the boxes, boards, etc. with check nuts on either side of the entry to ensure electrical continuity and with PVC or bakelite bushes. Turning joints in conduits wherever necessary shall be rigidly held in aligned position by check nut tightened on the running side. After conduits, junction boxes, outlets, etc. are fixed in position, their outlets shall be properly plugged with PVC stop cover or with any other suitable material so that water, mortar, vermin or any other foreign material do not enter into the conduit system.
5. All steel conduit runs shall have 3/22 PVC green copper earth continuity wire throughout the lengths inside the conduits.

5. Conduits & Accessories - PVC Conduits

1. PVC conduits shall be ISI grade and it shall be rigid type.  Where conduits are laid in straight run, draw boxes shall be provided at intervals not exceeding 10 meters.  Between two consecutive draw boxes the right angle bends shall not exceed two in number.
2. PVC conduits shall have continuous run of PVC insulated wires inside the conduit for earthing. Terminations of PVC conduits into switch boxes, DBs, etc. shall be with adapters.  PVC conduits shall be fixed to accessories such as coupler, circular boxes, etc. with vinyl cement.

6. Laying of Conduits

1. Conduits which are to be taken in the ceiling slab shall be laid on the prepared shuttering work of the ceiling slab before concrete is poured. The conduits, boxes, accessories, joints, etc. shall be laid along with the conduits.
2. Conduits recessed in walls be fixed rigidly with staples at maximum 1 M intervals. All chases shall be properly filled and concreted and finished. The conduit boxes, accessories, joints, etc. shall be laid along with the conduits. The chases shall be sufficiently deep and properly filled with cement mortar.
3. All chases, grooves shall be neatly made to proper dimensions to accommodate the required number and size of conduits and staples. The outlet boxes, point control boxes, inspection and draw boxes shall be fixed as an when conduit is being laid. The recessing of conduits in walls shall be so arranged as to allow atleast  12 mm plaster over the same. 
4. All grooves, chases, etc. shall be refilled with cement mortar and finished upto the wall surface before plastering of walls is taken up by the General Contractor.  Where conduits pass through expansion joints in the building, adequate expansion fittings or other approved services shall be used to take care of any relative movement. As far as possible, chasing of wall to embed the conduits to be avoided. Conduits with switch boxes to be fixed and supported before brick work has commenced.
5. Wherever surface conduit system is specified, only steel conduits shall be used and conduits shall be fixed by means of GI saddles. Conduits shall be run in square and symmetrical line with proper drawing.
6. Concealed conduits in slabs shall be brought out as vertical drops in beams, wherever such drops are required.  All vertical conduits in beams shall be left protruding from the bottom of the beam, atleast 150 mm in length. All such ends of conduits protruding from bottom of beams shall be provided with couplings  to receive extension. The open ends should be protected during concreting.
7. Vertical conduits in wall shall be laid and terminated in switch boxes, before the brick walls are erected.  Chiseling of brick walls for laying such conduits will not be allowed. The terminations of conduits shall be in GI PVC boxes and the size of the boxes shall be suitable to accommodate all control switches required for that particular location.
8. The switch boxes shall be of GI/PVC and shall be fixed to the conduits with adapters. The switch box shall be flush with the plaster and it shall be contractors' responsibility to cut open the brick work and plaster wherever necessary to make good the surface for achieving flush finish. The height of the switch boxes shall be as indicated in the drawings. The switch boxes should be sufficient depth to give minimum 20 mm  plaster cover to conduits embedded.

7. Branch Switches, Convenience Outlets and Ceiling Rose

1. All branch switches shall be installed in the phase circuit in the distribution. All switches shall be of rocker pattern type incorporating solid silver contacts suitably enclosed.  Where conduits are  concealed, switches will be of the flush type with single or multiple gang plates. Where conduits are on surface, the switches shall be mounted on surface type GI boxes after obtaining approval of the Architects/Owner.  All switches shall be of adequate current carrying capacities for specified load application.
2. The convenience outlets shall be of 5A/15A/20A rating as specified. All sockets shall be of 3 pin type and 5 pin as required and shall conform to the Indian Standards. The socket outlets shall be mounted  on heights specified on drawings and in consultation with the Architects/Owners. All wiring shall be on looping system of wiring.
3. All ceiling rose shall be suitable for use on 250V. Ceiling rose shall not be used for voltage level higher than 250 volts. Only one flexible cord shall be attached to ceiling rose.
4. All light points on ceiling shall be terminated in bakelite lamp holders of fluorescent as per drawings. All points shall be left connected to PVC terminals. Terminations to light fixtures shall be through 3 way terminals. All light points on wall shall be terminated on angular lamp holders fixed directly on PVC round box.
5. The lamp holders shall be white in colour.  Samples shall be submitted for Architect's approval PVC Boxes in the ceiling are to be covered with round cover flush with the ceiling and screwed to the box properly. The flexible wire extension is to be connected by means of suitable connectors inside the box.  Wooden round blocks are not to be used any where in the ceiling or wall. All point opening shall be properly finished in plaster.
6. The single pole circuits switches, decorative type 5 Amps and 15 Amps with or without plug sockets shall be approved by Architects before installation.
7. The following Indian Standard Specifications are applicable for decorative switches.
   1. IS 3854 1966: Switches for domestic and similar purpose.
   2. IS 1293 1967: Three pin plug and socket outlets.
   3. IS 4614 1968: Switch Socket outlet (non-interlocking type)
   4. IS 6538 1971: Three pin plugs made of resistant materials