1. 5.3 : BUILDING ELECTRICAL SUPPLY
CHAPTER 5
5.3 : BUILDING ELECTRICAL SUPPLY

2. Electrical Supply for Building
The purpose of electrical installation in buildings is to supply and distribute power.
Buildings that are used for different purposes have different requirements.
Building structure must be known before the power supply and distribution system can be planned and designed.

3. Electrical Supply in Small Building
For residential buildings, the public utility provides the supply voltage for the distribution systems via separate transformer stations.
Each residential building is connected to the power supply system via a low voltage cable.

4. Supply Cable to Building
Service cable to meter cupboard fixing on the inside wall.
Service cable to meter cupboard fixing on the outside wall.

5. Supply Cable to Building
External cabinet for easy meter reading

6. Supply Control
The service cable usually terminates inside the building in a main cut-out, fitted as near as possible to the service cable entry.
A meter is fitted after the main cut-out and everything up to and including the meter are the property and responsibility of the electrical supply company.
A switch or circuit breaker is fitted after the meter and a distribution board follows the switch. Everything from the switch to furthest outlet point is the property and responsibility of the building owner.
The service intake and the control unit is 240 V single-phase for a domestic or similar small building and for larger buildings is 415 V three-phase supply may be required, depending upon the load.
The control unit must be sited so as to fulfil the following conditions:
It must allow the supply cable to be brought in without undue difficulty.
It must be reasonably accessible for meter reading and general maintenance.
It must be separated from any gas meter by a fire-resisting partition.
It should be placed in a position where heavy condensation is unlikely.
There should be an easy and accessible route for the outgoing cables.

7. Protection Against Excess Current
All circuits must be protected against excess and three devices are available, -miniature circuit breaker
rewireable fuses
cartridge fuses
Miniature circuit breakers are virtually tamper-proof, their use is to be recommend for circuit protection.
miniature circuit breakers are designed to protect circuit conductors by opening automatically before conductor damage.
Cartridge fuses is used to protect the equipment form overloaded current or damage.
miniature circuit breaker
Rewireable fuse
Cartridge fuses

8. Earthing
The basic principle of earthing is that of limiting the difference in voltage between live conductors and earth.
If a person touches a live conductor that is correctly earthed, the flow of electricity through the earth conductor should form a path of lower resistance than that of a person’s body and the person should not receive an electric shock.
An earth leakage circuit breaker

9. Conductor and Cable Rating
The amount of current which a conductor or cable can carry is limited by the heating effect caused by the resistance to the flow of electricity.
The maximum permissible current under normal conditions, must not be so high that dangerous temperatures are attained, which could lead to fires.
Even with cables inside metal conduits or ducts or where mineral insulated copper, even though the cables are completely fireproof, the transmission of heat to other materials in proximity may still lead to fires.

10. Conductor and Cable Rating
When choosing a cable for a particular job, it is necessary to take into account not only the maximum current the conductor will have to carry, but also the drop in voltage that will occur when the current is carried.
IEE Regulations stipulate that the maximum permissible drop in voltage in a conductor shall not exceed 2.5 % of the nominal voltage when the conductor is carrying its full-load current.
The temperature reached by a cable is also affected by the following operating conditions:
Whether the cable is surrounded by the room air or is enclosed in a conduit or duct.
The closeness to other cables which may cause heat to build up, due to induced currents.
The temperature of the ambient of surrounding air.

11. Enclosed in conduit or trunking
The British Standards provide tables of cable size for various operating conditions.
Conductor
Enclosed in conduit or trunking
Nominal cross-sectional area (mm2)
Number and diameter of wires (mm)
Two core cable single phase a.c or d.c
Current rating (A)
Voltage drop (mV) per ampere, per metre
1.0
1/1.13 13 44
1.5
1/1.38
16.5 29
2.5
1/1.78 23 18 4
7/0.85 30 11 6
7/1.04 38
7.3 10
7/1.35 52
4.4
Table 5.0: Electrical cable capacities. (Part tables of B.S and 6346)

12. Conductor and Cable Rating
Example 1:
One PVC insulated two core cables with copper conductors, non armoured and enclosed in conduit is 14 m in total length and is required to supply a 5 kW electric heater. If the nominal voltage of the supply is 240 V, find the size of cable required, coarse excess current protection being provided.
Solution:
(i) Current I=P/V= 20.8 A
(ii) From Table 4.1; a 1/1.78 cable will allow a current of 23 A to flow with drop of 18 mV (0.018 V) per ampere, per metre run.
Voltage drop in cable = 20.8 x x 14 = V approx
Check percentage of nominal voltage: = % (less than 2.5%)
(v) Maximum voltage drop allowable for the cable = 240V x 2.5% = 6V
Therefore, the cable is suitable.

13. 5.4 DOMESTIC DISTRIBUTION SYSTEM
Chapter 5
5.4 DOMESTIC DISTRIBUTION SYSTEM

15. Lighting Circuit
Every sub-circuit which originates from the lighting distribution fuse board is generally limited to a total load of 1000 W and requires 5 A fuses and switches.
In large buildings, 15 A fuses and wiring are sometimes used, due to the higher total load on the circuit.
Wiring to lighting points should be carried out on what is known as the loop in method shown in figure.
Loop-in method of wiring.

16. Switch Control Cable connection One-way Switch Control
A one way light switch is connected with wiring to control a lamp.
If required, several lamps may be controlled from one switch.
One-way Switch Control

17. Switch Control
Theoretical of one-way switch and two-way switch

18. Switch Control
The two-way switch is, in principle, a single-pole changeover switch. When connected in pairs, the switches provide control of a lamp from two positions and may therefore be installed in bedrooms, landings and corridors.

19. Switch Control
Used in conjunction with two, two-way switches, provides control of a lamp from three or more positions.
Suitable for long corridors with several doors, long halls and multi-flight staircases, require intermediate switch control for reasons of both safety and convenience.

20. Switch Control

21. Socket Outlets
to avoid long runs of flexible cables and multi-point adaptors, the number of socket outlets must be adequate for the consumer’s needs.
The location of the items of furniture and electrical equipment should be considered when positioning socket outlets, lighting points and switches.
1 gang 13A
2 gang 13 A
multiple socket
3 phase socket

22. Socket Outlets
Travel plug adapter

23. Number of socket outlets for small house
Space / Location
Desirable number of socket outlets
Dining room 2
Living room 5
Double bedroom 3
Single bedroom
Kitchen (working area) 4
Hall 1
Landing
Garage

24. Lighting and socket outlets for a small house.

25. Bath and Shower Rooms
Special precautions are required in bathrooms and similar rooms to avoid danger from the electrical installations.
Lighting fittings should enclose the lamp completely and there should be no exposed metalwork on the fittings.
Switches inside bathrooms should be of the pull-cord type, mounted on the ceiling or high on the wall.
Ordinary switches or other means of control should be situated so as to be normally inaccessible to a person using a fixed bath or shower.
In any room containing a fixed bath or shower, there must be no stationary appliance with elements that can be touched by a person in a fixed bath or shower, nor there must be any socket outlet for connecting a portable appliance.

26. LARGE BUILDING / INDUSTRY
hospitals, factories and office blocks will require a three phase supply due to the higher electrical load.
The loading may be too high therefore a private sub-station is needed which fed electricity from the high voltage cables from the nearest switching station.
The electrical installation in a large building is similar to a small building, but is divided into sections.
There may be one main intake panel incorporating large fused switches or circuit-breakers, each of which controls a feeder cable to subsidiary distribution panels in different parts of building, or each separate building in group.
The subsidiary distribution panels are smaller versions of the main intake panel and they control distribution boards for each sub-section.

27. CURRENT RATING
The current rating of the cable and components must never be less than the protective device which controls it.
A fused switch of 100 A rating can only serve a cable having a current rating of 100 A or more and the switchgear at the opposite end of the circuit must be of 100 A rating up to the next smaller fuse or miniature circuit-breaker.
It is important that fuses or miniature circuit-breaker should provide discrimination, e.g. each subsidiary fuse or circuit-breaker should isolate a fault in its own section before the fuse or circuit-breaker one stage further back in the installation can operate.
To ensure this, the ratings of two successive fuses or miniature circuit-breakers must differ by at least 30 per cent and preferably 50 per cent. To achieve this, it may sometimes be necessary to increase the cable and switchgear ratings so that the main fuse or miniature circuit-breaker can not operate before the subsidiary one.

28. VOLTAGE DROP
In a small electrical installation the voltage drop in the circuit wiring is not usually significant, but in larger installations the voltage drop in the cables between the main intake and the subsidiary distribution panels may be very high.
The IEE Regulations require that the voltage drop between the main intake point and each supply point shall not exceed 2.5 per cent of the nominal supply voltage.

29. METHODS OF DISTRIBUTION
Radial distribution
The main intake normally consists of a main switch connected to fused switches through a bus-bar chamber.
Several separate feeder cables are run from the main intake panel to the subsidiary distribution panels which may be situated in separate buildings or at strategic points inside one building.

30. METHODS OF DISTRIBUTION
Ring main distribution
In the case of a large development scheme having several buildings around the perimeter of the site, a ring-main circuit would be taken around the site with supplies taken into each building.
advantages:
Each building and individual sections of the ring may be isolated without switching off the entire installation.
The current may flow in either direction which reduces the voltage drop.
The ring may be sized to take account of the diversity factor for all the buildings, since a heavy load may be required for any one of the buildings, but it is unlikely that such a load will be required for all the buildings simultaneously

31. METHODS OF DISTRIBUTION
Rising main distribution
For buildings above five storeys in heights, it is normally preferable to pass conductors vertically through the buildings. The supply to each floor is connected to the rising main by means of tap-off subsidiary units.
Types in use:
PVC or vulcanized rubber insulated cables mounted on porcelain cleats inside brick or concrete ducts with hardwood or metal access doors on each floor.
Paper, mineral, PVC or vulcanized rubber insulated cables run in sheet steel vertical ducts.
Uninsulated copper or aluminium bars run in steel sheet vertical ducts.

32. FLOOR DUCTS
The main purpose of a floor duct system is to enable desks to be moved to any position in the office or within building. The duct can be used to carry both low-voltage electrical supplies for machines and lighting and extra low-voltage supplies for private and public telephones.
Steel and pitch fibre ducts are materials that generally use for ducts.
There are three types of floor duct layouts; grid, branching and perimeter

33. FLOOR DUCTS-Grid
This method provides adequate flexibility for telephone and electrical supplies and is used in open-plan offices. A suitable spacing of the ducts is 1.5 to 2 m, but other spacings may be used depending upon the degree of flexibility required

34. FLOOR DUCTS-Branching
This method uses a central feeder duct with branches to each window bay. The branches may either terminate just short of the wall, or extend to wall outlets. Figure 4.26 shows a branching duct layout with wall outlets. The layout provides reasonable flexibility for open-plan offices, but is also used for partitioned offices with the central feeder duct in the corridor.

35. FLOOR DUCTS-Perimeter
This is the cheapest method but does not provide the flexibility obtained by the grid and branching layouts. A main feeder duct is located about 450 mm form the outside wall with short branches taken from junction boxes to wall outlets for telephones or both telephones and electrical supply.

36. End of Chapter 5
Thank you