Page 135 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

References

I.S. 732-1963
I.S. 4648-1968
N.E. Code

Selection of the type and size of cable for a given wiring

installation and voltage drop concept

Objectives: At the end of this lesson you shall be able to:
• state the factors to be considered for selecting the cable for a circuit
• apply the factors and select the cable.

In order to determine the type and size of the cable for a given circuit, the following points should be taken
into account.
• Suitability of the type of cable for the location of the circuit and the type of wiring.
• Size of the cable depending upon the current carrying capacity of the cable.
• Size of the cable depending upon the length of the wiring and permissible voltage drop in the cable.
• Minimum size of the cable based on the economy.
Location of the circuit and the type of wiring decide the type of cable.
It is necessary to consider whether the installation is for industry or domestic use and whether the atmosphere
is damp or corrosive. Accordingly the type of cable has to be chosen.
Further the type of wiring determines the type of cable suitable for the installations.
The current carrying capacity of the cable decides the size of the cable.
In this, the first step is to find out the current expected to flow in the circuit when the total connected load is
fully switched on. This current is the maximum current that would flow through the circuit in case all the loads
are working at the same time. But this is not the case in actual situations.

Diversity factor
In the case of lighting installation all the lamps in a domestic installation may not be switched ‘on’ at the
same time. Hence, it is assumed only two thirds of the lights (say 66%) only will be ‘on’ at a given time. This
introduces a factor called ‘diversity factor’.
When the connected load is multiplied by the diversity factor you get a load value which can be said as
normal working load. Use of this diversity factor enables the technician to use a lesser size cable than the
one calculated, based on the connected load.
Based on the working load the current in each circuit is to be calculated and the size of the cable suitable to
carry the current has to be chosen.
Voltage drop in the cable
In any current carrying conductor, voltage drop takes place due to its internal resistance. This voltage
drop in a premises as per BIS 732 should not be more than 3 percent of the standard supply voltage when
measured between the consumer supply point and any point of the installation when the conductors
are carrying the maximum current under the normal conditions of service.
Tables 3 and 4 for aluminium cable and 5 for copper cable give the relation between voltage drop and
length of the cable run for various cables. In case the voltage drop found in the cable exceeds the stipulated
limit of 3% voltage drop, the technician has to choose the next bigger sized cable to maintain the voltage
drop within limits.

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CITS : Power - Electrician & Wireman - Lesson 20-25