WIRING INSTALLATIONComplied and Presented by Doren Nedrick

Wiring InstallationThe service entrance includes all the parts of the assembly that bring electricity into a building. Its conductors extend from the point at which the service-drop conductors are attached at the house to the load center. The complete service entrance is generally thought to include the conductors, the watt-hour meter, and the load center. Usually the power company takes care of the wiring from the power line up to and including the meter.

Service-entrance Cables The service-entrance cable has three conductors. One of these, the neutral conductor, is bare. When the cable is to be connected to a terminal point, the strands of the conductor are twisted together to form a single-stranded conductor.

Pictorial diagram of a Domestic Installation

The Watt Hour MeterWatt-hour meters measure and record the amount of electric energy (in kilowatt-hour units) supplied to a building. The meter is usually, but not always, installed by and belongs to the power company.

Load CenterLoad centers, also called fuse or circuit-breaker panels, are the boxes from which electricity can be distributed to different locations in a house. It contains the main breaker or fuses that can disconnect all the electrical service from a house. The load center also contains the fuses or circuit breakers that protect the branch circuits of the wiring system. The size of the load center is designated by the number of fuses or circuit breakers that can be placed into it, and by its load, or ampere, rating.

Service Grounds To reduce the danger of shock and to protect against lightning, the bare conductor of the service entrance cable is grounded at the meter socket by being connected an earth rod. This is the safety ground.This neutral wire is further connected to the neutral bus bar and earth lug in the load center.

BRANCH CIRCUITS Branch circuits of a residential wiring system distribute electricity from the load center to the rooms of the house. The three common kinds of branch circuits are discussed below.

General-Purpose Circuits General-purpose branch circuits are used for lighting and outlets. The outlets are intended to serve radios, television sets, clocks, and other small appliances used for food preparation (such as toasters, broilers, etc.). Lighting circuits are usually wired with number 1.5mm2 cable and are protected with 15A fuses or circuit breakers, respectively

Connecting Convenience Outletsone wire of each general-purpose circuit is connected to the neutral bus bar of the load center. The grounded neutral wire is color coded black. The wire connected directly to the fuse or circuit breaker is generally red. This circuit wire is often called the hot wire. The grounded black wire of a two-wire, general-purpose circuit is never used as a safety grounding wire. When the cabinet or case of a product operated from such a circuit is to be grounded, a third grounding wire is used.

Small Appliance CircuitsSmall appliance circuits are used for such appliances as refrigerators, toasters, broilers, coffee makers, and irons. They are wired with number 2.5mm2 wires and are protected with 20A fuses or circuit breakers.In practice a maximum of two (2) outlets should be placed in the kitchen circuits

Individual CircuitsAn individual circuit is used for only one piece of equipment. Examples of individual circuits are those used with electric ranges, dryers, water heaters, heating systems, and air conditioners.A range circuit is usually a four-wire, 220 V circuit. It has two hot wires, a neutral wire, and a ground. There is a fuse or circuit breaker in each of the hot wires and are ganged. Called double pole circuit breaker

Circuit breaker Rating for individual circuitThere current rating is based on its wattage.Example: An 8kW, 220V range will draw I = P/V = 8000/220 = 36.36ATherefore a 40A Double pole CB should be used to protect the circuit

Types of Wiring Methods Non-metallic Conduit: When wiring needs to be protected from the environment, either because of the weather safety considerations or to protect it from mechanical damage, a conduit system is used.Non-metallic conduit systems have the following advantages: absence of condensed moisture in the tube, non-corrosive (rust-free), non-inflammable.

Preparing the Conduit: Conduit comes in lengths (usually about 3.5 m) and is threaded at both ends.Cutting: Lengths should be cut to size with a hack saw having a fine tooth blade. The blade must be held at right angles to the conduit during cutting. Conduit should always be cut in a pipe vice.The thread is cut by using stocks and diesBending Conduit: Conduit can be bent with either a bending spring, bending block or a bending machine.

Bending Rigid Conduit

Termination of Conduit. Metallic conduit must always be terminated at an incombustible outlet, preferably a metal box.

Non Metallic Sheathed CableNonmetallic sheathed cable usually has two or three insulated wires and a grounding wire. NM cable is used for all kinds of indoor wiring work and is never to be buried in cement or plaster hence its use in board house. NM cable has a moisture-resistant overall covering, which allows it to be used for both exposed and concealed work in dry, moist, and damp locations, and on outside or inside walls of masonry, block, or tile.

PVC Trunking

Regulations Regarding Electrical Installation1. All cables must be rated at, or above, the current which they will normally be expected to carry without undue heating or voltage drop (normally 2.5 per cent of nominal voltage).2. All conductors, cables and equipment used in the installation must be of the correct Voltage rating, for example, 250V grade switchgear should not be used on 415V 3-phase installations: 500V switchgear is necessary.3. The fuse in a circuit must be capable of protecting the smallest conductor in that circuit.

Regulations Continued4. All conductors and equipment must be properly labelled; this saves time and also minimizes the danger of opening the wrong circuit when isolating or fault-finding.5. All equipment must be protected against: high temperatures, moisture, corrosion, and mechanical damage.6. The main distribution point should be situated, if possible, at the centre of the installation in order to keep cable runs short.7. Sufficient capacity should be installed at the outset (in main switches, bus-bar chambers, and cables) to allow for future expansion.

Used for stairs and corridors

Used for long corridors in hospitals and schools

Testing Verification of Polarity Test The purpose of the verification of polarity test is to check that the phase conductor is taken through the fuse or circuit breaker and the switch to the appliance.The neural must never be broken by a CB or switch. If the CB opened under these conditions, then the circuit would still be live.

A case was reported where a man was serious electric shock while cleaning the reflector of an electric fire with a damp cloth. The fire had been switched off but the plug was not removed.

Preparation for verification of polarity testSupply offLamps and appliances outAll switches off, Neutral links in; fuses out

Test InstrumentOhm meter or bell set

Reading Zero on ohmmeter and continuity on the bell set. (The insulation resistance tester megger should not be used as the smallest reading is about 10 000)

Polarity test