Eat 105 Chapter 3

Electrical Installation Designs
CHAPTER 3
CABLE SELECTION AND WIRING SYSTEM

CABLE SELECTION AND WIRING SYSTEM
Cables are means by which electrical energy is
distributed from its source to its point of use.
Defined: length of insulated single conductor or of two
or more such conductors each provided with its own
insulation which are laid up together.
The insulated conductor or conductors may or not be
provided with overall covering for mechanical protection.
A single core cable refers to a cable that has only one
insulated conductor with its own cable sheath, and a
multi core cable refers to a cable that has multiple cores
of insulated conductors within one common sheath.

A conductor may be defined as the conducting portion of
a cable, which consists of a single wire or group of wires
in contact with each others.
The ability of a material to be a good or bad conductor
of electricity depends on the composition of that
material, i.e. its resistivity.
Conductors may be divided into two groups:
a) solid conductor
b) stranded conductor
CABLE COMPONENTS: CONDUCTOR

Cables are usually a classified according to the type of
insulation used. The type of insulation to be used must
have the following properties:
It should have a high specific resistance
It should have high dielectric strength
It should be tough and flexible
It should not be hygroscopic, i.e, it should not absorb
moisture from air or surroundings.
It should be capable of standing high temperatures
without much deterioration.
It should be non inflammable.
It should not be attacked by acids or alkalies.
It should be capable of withstanding high rupturing
voltages.
CABLE COMPONENTS: INSULATION

CABLE COMPONENTS: SHEATHING
Sheath - a metallic component over the insulation of a
cable. Various metals may be used as the sheath of a
cable such as lead, copper, aluminium, bronze, steel,
etc.
The choice of sheathing material depends on its
environmental performance: ambient temperature;
flexibility; resistance to abrasion, water, oil and other
chemicals; performance under fire conditions; and
compatibility with other materials with which a cable is in
contact during its operational life.
The sheathing material must also be chemically
compatible with the other materials used in the cable
both during and after processing. While insulations are
chosen primarily for their electrical characteristics,
sheaths are selected on their physical properties.

Thus, not all insulations are suitable sheaths. However,
in general, insulation and sheath materials are similar:
e.g. a thermoplastic sheath protecting thermoplastic
insulation.
CABLE COMPONENTS: SHEATHING

CABLE COMPONENTS: ARMOURED
When cables are not installed in conduit or trunking,
they may require armour, most commonly provided by
galvanized steel wire (GSW) helically applied in a single
layer and known as `SWA' (single-wire armour).
Most types of power cable require mechanical protection
and/or an earth conductor to carry fault currents. For
most distribution cables this is provided by SWA. Cables
with aluminium sheaths seldom require armour. Cables
with lead sheaths may be armoured with steel tape,
which is cheaper, but SWA is preferred in the UK for the
heavier, higher voltage cables for 10 kV and upwards
because it increases corrosion resistance and the
longitudinal strength of the cable for installation
purposes. Steel tape is normally protected against
corrosion by bitumen, but if better corrosion resistance is
required, the tape may be galvanised

Stranded Cable
Non Armoured Cable
Armoured Cable

TYPES OF INSTALLATION
The final choice of a wiring system must rest
with those designing the installation and those
ordering the work, but whatever system is
employed, good workmanship and the use of
proper materials is essential for compliance
with the IEE Regulation 130-02-01. The
necessary skills can be acquired by an
electrical trainee who has the correct attitude
and dedication to his craft.

PVC insulated and sheathed wiring systems are
used extensively for lighting and socket
installations in domestic dwellings. Mechanical
damage to the cable caused by impact,
abrasion, penetration, compression or tension
must be minimised during installation
(Regulation 522-06-01).

a) Conduit Installation – Steel conduit/GI or uPVC
b) Trunking Installation
c) Tray Installation
d) Others – IEE Regulation – Table 4A1 page 210-215

CABLE SIZE CALCULATIONS
The size of a cable to be used for an
installation depends upon:
a) the current rating of the cable under
defined installation conditions and
b) the maximum permitted drop in voltage as
defined by Regulation 525-01(pg 100).

Some symbols and definitions will be used in
cable size calculations as:
Iz is the current carrying capacity of the cable
in the situation where it is installed.
It is the tabulated current for a single circuit
at an ambient temperature of 30°C.
Ib is the design current, the actual current to
be carried by the cable.
In is the rating of the protecting fuse or circuit
breaker.

I2 is the operating current for the fuse or
circuit breaker (the current at which the
fuse blows or the circuit breaker opens).
Ca is the correction factor for ambient
temperature.
Cg is the correction factor for grouping.
Ci is the correction factor for thermal
insulation.
Cr is the 0.725 correction factor to be applied
when the semi enclosed fuses protect the
circuit.

Current Cable Rating
The factors which influence the current rating
are:
a) the design current, Ib , the actual current
to be carried by the cable. The cable
current carrying capacity, Iz must be equal
to or greater than the circuit design current,
Ib;
b) the type of cable either PVC, PVC/PVC,
MICC, copper conductor or aluminium
conductor (refer Table 5.2)
bz II ≥

c)
the installed conditions either clipped to
surface or installed with other cables in a
trunking. The correction factor may be
need applying for installation condition is
the grouping correction factor, Cg given in
Tables 4B1, 4B2 and 4B3 (pg 217&218), of
the IEE regulations and the correction
factor to be used when cables are enclosed
in thermal insulation, Ci. The Cg is not
applied applied if the cable is to be clipped
direct to a surface or not in contact with
other cables.

Cont…c)
Regulation 523-04 gives us three possible
correction values:
where one side of the cable is in contact
with thermal insulation, we must read
the current rating from the column in
the table which relates to Reference
Method 4.
where the cable is totally surrounded over
a length greater than 0.5m, we must
apply a factor of 0.5.
where the cable is totally surrounded over
a short length, the appropriate factor is
given in Table 52A of the IEE
Regulations should be applied.

d) the ambient temperature, the cable
resistance increases as temperature increases
and insulation may melt if the temperature is
too high. The correction factor may be need
applying for this condition is the ambient or
surrounding temperature correction factor,
Ca, which is given Tables 4C1 and 4C2 of
Appendix 4 of the IEE Regulations.

e) the type of protection – for how long will the
cable have to carry a fault current? Choose
the type and rating of protective device (fuse
or circuit breaker) to be used, In.
To ensure correct protection from overload, it
is important that the protective device
operating current or the operating current for
the fuse or circuit breaker (the current at
which the fuse blows or the circuit breaker
opens), I2 is not bigger than 1.45 times the
current carrying capacity of the cable, Iz.

It is important to appreciate that the operating
current of a protective device is always larger
than its rated value. In the case of a back-up
fuse, which is not intended to provide
overload protection, neither of these
requirements applies.
bnz III ≥≥
zII 45.12 ≥
zn II ≤

When the semi enclosed fuses is used to
protect the circuit, the 0.725 correction factor
may be need applying for this condition.

Here then are the eight basic steps in a
simplified form:
• Determine the design current Ib.
• Select the rating of the protection, In.
• Select the relevant rating factors (CFs).
• Divide In by the relevant (CFs) to give
tabulated cable current carrying capacity,
It.
• Choose a cable size to suit It.
• Check the voltage drop.
• Check for shock constraints.
• Check for thermal constraints

Cable Capacities of Conduit and Trunking
Single PVC insulated conductors are usually drawn
into the installed conduit to complete the
installation.
Having decided upon the type, size and number of
cables required for a final circuit, it is then
necessary to select the appropriate size of conduit
to accommodate those cables.

The following three cases are dealt with:
Single core thermoplastic (pvc) insulated cables
1. In straight runs of conduit not exceeding 3m in
length Table 5A & 5B.
2. In straight runs of conduit exceeding 3m in
length, or in runs of any length incorporating
bends or sets. Table 5C & 5D.
3. In trunking. Table 5E & 5F.
Conduit Capacities

The tables in Appendix 5 of the On Site Guide
describe a factor system for determining the size
of conduit required to enclose a number of
conductors. The method is as follows:
1. Identify the cable factor for the particular size
of conductor. (This is given in Table 5A for
straight conduit runs and Table 5C for cables
run in conduits which incorporate bends.)
2. Multiply the cable factor by the number of
conductors, to give the sum of the cable
factors.

3. Identify the appropriate part of the conduit
factor table given by the length of run and
number of bends. (For straight runs of
conduit less than 3m in length, the conduit
factors are given in Table 5B. For conduit
runs in excess of 3m or incorporating bends,
the conduit factors are given in Table 5D.)
4. The correct size of conduit to accommodate
the tables is that conduit which has a factor
equal to or greater than the sum of the cable
factors.

PPKSE

Trunking Capacities
The ratio of the space occupied by all the cables
in a conduit or trunking to the whole space
enclosed by the conduit or trunking is known as
the space factor. Where sizes and types of cable
and trunking are not covered by the tables in
Appendix 5 of the On Site Guide a space factor of
45% must not exceeded. This means that the
cables must not fill more than 45% of the space
enclosed by the trunking. The tables of Appendix
5 take this factor into account.

To calculate the size of trunking required to
enclose a number of cables:
1. Identify the cable factor for the particular size
of conductor (Table 5E).
2. Multiply the cable factor by the number of
conductors to give the sum of the cable
factors.
3. Consider the factors for trunking (Table 5F).
The correct size of trunking to accommodate
the cables is that trunking which has a factor
equal to or greater than the sum of the cable
factors.

Eat 105 Chapter 3

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