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Wiring systems
WIRING SYSTEMS
The choice of wiring system is affected by many factors such as the type of building, temperature, safety, cost,
etc. The electrician needs to be able to decide upon the most suitable type of cable or wiring for a given
situation.
A wiring system consists of the conductor, its insulation, its mechanical protection, and the various accessories,
such as joint boxes, etc. The systems are named mainly in terms of the mechanical protection used.
Factors affecting the choice of wiring system
1. Type of building construction – to decide the routing, fixing and terminations
2. Flexibility of the system – change of location of equipment such as in machine shops and temporary
buildings.
3. Installation conditions – protection against mechanical damage requirements and working heights.
4. Ambient temperatures – to be withstood, that is, in boiler houses, and heat treatment installations.
5. Appearance of the finished installation – can it be run on the surface or concealed?
6. Durability – life of the installation.
7. Economics – cost of the installation and money available.
8. Safely aspect
CABLES
A cable is a length of single conductors having one or several wires stranded together, or two or more such
conductors, each provided with its own insulation and lay up together. These insulated conductors may or may
not be further covered with an overall protective sheath to prevent mechanical damage and to minimize risk of
fire and shock. The insulated conductors are referred as the cores of the cable. They comprise very wide range of
sizes and types. The necessary requirements of a cable are that it should conduct electricity efficiently, cheaply
and safely.
Cables consist of essential parts:
 The conductor to carry the current
 The insulation (dielectric) to provide the means to prevent the leakage of current, and
 External overall protection against mechanical damage, chemical attack, fire or other external damaging
factors to the cable.
When cables are to be drawn in or enclosed in metal conduits or trunking, they are to be chosen with the above
temperature. The above temperatures also apply to flexible type cables supplying heating appliances or high
temperature light fittings.
All fixed wiring systems must be suitably protected against mechanical damage by heat, fire or explosion and
against damp and corrosive elements, which may be present on the installation.
Detailed information on the types of wiring systems and the regulations which apply to them is given in the
I.E.E. Regulations.
A number of points must be considered for the wiring system:1. Neatness of the finished job;
2. The durability of the installation;
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3.
4.
5.
6.
7.
Future extensions and alterations;
The time required to do the work;
Damage to the fabric of the building by cutting away;
Special conditions to be withstood, such as fumes, dampness; and
the total cost of the job.
The basic requirement for the installation of conductors and wiring systems are listed below.
1. All conductors and cables should be sufficient in size and power rating for the purpose for which they
are to be used.
2. All live conductors should be effectively insulated or so positioned and protected as to avoid danger.
3. Every circuit should be protected against excess current and earth leakage.
4. Any metalwork associated with current carrying conductors should be connected to earth or c.p.c.
(Circuit Protective Conductor)
5. Special precautions must be taken where conductors and wiring systems are exposed to inflammable
surroundings, or in wet or damp situations.
Conductor materials
Copper and aluminium are the materials used as conductors in power and lighting cables. Copper has lower
resistivity and higher conductivity than aluminium. This means that copper conductors have smaller crosssectional area and take up less space than aluminium for the same current capacity. Aluminium has about onethird the weight of copper and will have an advantage in some circumstances.
Copper conductors may be annealed or hard-drawn. Annealed copper conductors are comparatively soft and
flexible and are most suitable for indoor and outdoor wires and cables laid or fixed in position. Hard-drawn
copper conductors, which have a very high tensile strength, are used as overhead wires mainly in the form. The
great majority of cables in use are of copper. With some insulation materials, but not all, copper conductors need
to be tinned.
Aluminium conductors are made in all standard sizes but at present only in the larger sizes. I.E.E. Regulation B1
lays down that all cable conductors of cross-sectional area 10mm² or smaller shall be of copper.
The respective resistivities of copper and aluminium are given in Table 2 as 17.24μΩ mm and 28.2μΩ mm.
Stranding.
To ensure flexibility and ease of handling, conductors are stranded: a number of small wires twisted together
spirally forming a core equivalent to a single of the required size. The numbers of strands used are 1, 7, 19, 37,
61 and 127. The sizes of conductors range from 1.0 mm² (1/1.13 mm) to 630mm² (127/2.52mm).
The latter conductor, for example, consists of 127 strands of circular conductor each strand of 2.52mm diameter,
with a total cross-sectional area of 630mm².
Insulation
The function of insulation is to confine the electricity to the conductor. The insulation itself must have a very
high resistance. For normal work the insulation is arranged to surround the conductor throughout its length. For
overhead wires it is normally sufficient to provide insulation (e.g., a porcelain insulator) at the point of
suspension of the wire. The remainder of the cable is insulated by the air surrounding it.
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Voltage ratings for cable are given as Eo/E, where Eo is the system voltage to earth, E is the voltage between
lines, e.g. 600/1000 V.
Large cables may be single-core, twin-core, triple-core or multi-core, in which form the cores are separately
insulated and laid side by side with a slight continuous ‘lay’, packed with wormings and further insulated overall
to form a circular shape. They may also be made as twin concentric or triple concentric. Smaller cables for
lighting and power are laid up in flat form to give neatness of installation.
The types of insulating material commonly used are:
Polyvinyl Chloride (P.V.C.)
Cheap & resistance to corrosion. Use in temperature not exceeding 65°
Rubber:
- Vulcanized rubber (v.i.r.)
- Butyl rubber (b.r.)
- Ethylene-propylene (e.p.)
- Silicone rubber (s.r.)
Very cheap, perishes. Use in temperature not exceeding 65°.
Impenetrable to water, flexible, high resistivity.
Use in materials not exceeding 80°.
Same as butyl rubber used in flexible cords in lighting fittings.
Use in temperature not exceeding 145°.
All of the above are made up according to requirements as textile-braided and compounded, glass-braided and
compounded or with various qualities of heat-resistance, oil-resistance and flame-resistance sheathing.
Impregnated paper
Mineral Insulated
Enamel
Nominal Cross
Section Area (mm²)
1.0
1.5
2.5
4
6
10
mainly for underground laying 600V/1000V to 19KV/33KV. All cable
ends are sealed by special oil-filled or compound-filled sealing boxes.
Use in temperature not exceeding 75°.
copper-aluminium conductor with compressed powdered mineral
insulation enclosed in a copper of aluminium sheath (MICS). Ambient
temperature up to 150°. For light duty 600V, heavy duty 1000V.
Only a very thin coat required
Wire Diameter
(mm)
1.13
1.38
1.78
7 / 0.85
7 / 1.04
7 / 1.35
Single-strand Wire
Use single strand wire where
rigid wire is required.
Multi-Strand Wire
Use multi-strand wire where a flexible wire required.
Note:
Both single and multi-strand conductors are classified and the nominal cross sectional area in mm.
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Mechanical Protection
The larger cables, used for underground work and for large interior power installation, may be mechanically
protected in various ways.
Unarmoured cables may be run without further protection than the lead sheathing. A further protection is one or
two layers of compounded jute or hessian tape yarn lay over the lead sheath. Aluminium is used as an alternative
to lead for sheathing cables.
Armoured cables include single-wire armouring (a single layer of galvanized iron wire laid spirally upon a
bedding of jute or Hessian), double-wire armouring (two layers of armouring), and double steel tape armouring
(two layers of steel tape laid spirally over the bedding with an overall finish of jute and hessian).
Aluminium strip armouring is sometimes used as an alternative to wire armouring.
For underground cables, wire armouring is used where the ground is liable to subsidence, to prevent the cable
from breaking, whereas steel tape armouring is needed where physical damage from stones or workmen’s tools
may be expected.
Cable types
The range of types of cables used in electrical work is very wide: from heavy lead-sheathed and armoured paperinsulated cables to the domestic flexible cable. Lead, tough-rubber, PVC and other types of sheathed cables used
for domestic and industrial wiring are generally placed under the heading of power cables.
Flexible cords and cables
Flexible cords is defined as a flexible cable in which the cross sectional area of each conductor does not exceed
4 mm² (56/0.30) which varying from 0.5mm² (16/0.20) to 4 mm² (56/0.30). The most common types of flexible
cords are used in domestic and light installation work. The diameter of each strand or wires varies from 0.21 to
0.31 mm. In general they must not be used for fixed wiring.
Flexible cords come in many sizes and types; for convenience they are grouped as follows:Vulcanized rubber-insulated
Braided, twisted twin
Braided, circular twin and 3-core, ordinary and unsinkable.
Rubber or PCP-sheathed, single-core, twin, 3-core and 4-core.
PVC-insulated (other than heat-resisting)
Single-core twisted twin and parallel twin.
PVC-sheathed, single-core, twin, 3-core and 4-core.
PVC-insulated (heat-resisting)
Single-core, twin, 3-core and 4-core.
Butyl-rubber or ethylene-propylene rubber-insulated
Single-core, twin, 3-core and 4-core, sheathed with either PVC compound (heat resisting) or H.O.F.R.
compound.
Silicone rubber-insulated
Single-core, twisted twin, circular twin and 3-core, all glass fibre braided.
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Varnished glass fibre braided
Single-core, twisted twin, circular twin and 3-core.
These flexible cords are manufacture in 300/300 V and 300/500 V ranges according to types.
Flexible cables
Flexible cable is a cable specially designed to afford flexibility. The cable conductor therefore consists of a
number of fines wires. These cables are made with stranded conductors, the diameters varying from 6 mm²
(84/0.30) to 630 mm² (2257/0.60). These are made in the 600/1000V range and include single-core, two-core,
three-core, circular twin and multi-core cables. The conductors are of tinned annealed copper and the insulation
consists of vulcanized rubber, butyl rubber or EP rubber.
Length of cables
The smaller cables, as used for house lighting and power are supplied in lengths of 100 m on drums or in coils,
whilst flexible cords are supplied in lengths of 50m.
Current carrying capacity of cables
When a cable carries electric current there are inevitable power losses in the cable, which may be calculated
from the formula, P = I²R, where R is the resistance of the cable in Ohms and I is the current in amperes. This
wasted power is used in heating the cable, causing temperature of the cable to rise. If the cable is overloaded the
temperature may rise sufficiently to damage or even destroy the insulation. Heat is lost from the cable by
conduction, radiation and convection of air currents; the rate at which it is lost depends upon various factors, the
surface area of the copper, the material and thickness of the insulation, and upon the difference in temperature
between the copper and the surrounding atmosphere.
The current ratings are to be modified by factors in respect of:
Ambient temperature
Class of excess-current protection
Grouping
Disposition
Type of sheath
Voltage Drop
The cable conductor has resistance and whin it is carrying current, a drop in voltage will occur along its length of
run.
Voltage drop = conductor current (A)  conductor resistance (Ω)
Because of this voltage drop, the voltage of the load is less than the incoming supply. The I.E.E. Regulations
states that conductor sizes must be such that the voltage at any point on an installation does not drop by more
than 4% of the incoming voltage at the consumer’s terminals.
For an installation where the incoming voltage is 240V, the maximum allowable voltage drop is;
4 x 240 = 9.6V.
100
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Calculation of voltage drop in a cable, using Ohm’s Law
By considering the formula, V = R x I it will be seen that the voltage drop in the conductors of a cables is the
product of the current in the conductors and the resistance of the conductors. This applies to direct current
circuits, and to those alternating circuits where the effect of the inductance and the capacitance of the cable can
be neglected.
The resistance of a conductor is calculated from the formula, R 
pl
,
a
where R is the resistance of the conductor in ohms,
p is the reisitivity of the conductor material in mircrohm millimeters (μΩ),
l is the length of the conductor in millimeters (m) and
a is the cross-sectional area of the conductor in square millimeters (mm²)
When calculating the voltage drop in a 2 core cable, the lengths of the both lead and return conductors are taken
into account, and the value for l is twice the cable length.
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CABLE COLOUR CODE
BS7671:2008 IEE Wiring Regulations 17th Edition
FUNCTION
SYMBOL
Protective conductor Functional
earthing conductor
AC Power Circuit
Single Phase Circuit
- Phase
- Neutral
OLD COLOUR CODE
NEW COLOUR CODE
GREEN-YELLOW
CREAM
GREEN-YELLOW
CREAM
RED
BLACK
BROWN
BLUE
Three Phase Circuit
- Phase 1
- Phase 2
- Phase 3
- Neutral
L1
L2
L3
N
RED
YELLOW
BLUE
BLACK
BROWN
BLACK
GREY
BLUE
DC Two-Wire Unearthed Circuit
- Positive
- Negative
L+
L-
RED
BLACK
BROWN
GREY
DCTwo-Wire Earthed Circuit
- Positive (of negative earth)
- Negative (of negative earth)
L+
M
RED
BLACK
BROWN
GREY
Positive (of Positive earth)
Negative (of Positive earth)
M
L-
BLACK
BLUE
BLUE
GREY
DC Three-Wire Circuit
- Positive
- Mid-wire (may be earthed)
- Negative
L+
M
L-
RED
BLACK
BLUE
BROWN
BLUE
GREY
-
From 1st April 2006 onwards the electrical installations on site are to comply with Section 514 and , as
appropriate, cores are to be identified with the harmonized colours.
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