Page 136 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

If the cable size is increased to avoid voltage drop in the circuit, the rating of the cable shall be the current
which the circuit is designed to carry. In each circuit or sub-circuit the fuse shall be selected to match the load
or the cable rating whichever is minimum, to ensure the desired protection (BIS 732).

Declared voltage of supply to consumer
On the other hand according to IE Rule No.54, the voltage at the point of commencement of supply at the
consumer should not vary from the declared voltage by more than 5 percent in the case of low or medium
voltage or by more than 12 percent in the case of high or extra high voltage (Fig 1).

Fig 1

At this stage it is better to remember that when current flows through a conductor, the resistance offered
by the conductor produces heat. The increase in heat is proportional to the cable resistance which
in turn depends upon the cross-sectional area of the cable. Since overheating damages the insulation,
the conductor size must be adequate to prevent this from occurring.

While choosing the cable size, voltage drop is a more severe limitation than any other criterion. Hence,
it is advisable to select the cable size only after ascertaining the permissible voltage drop. Excessive voltage
drop impairs the performance of heating appliances, lights and the electric motors.
Calculation of voltage drop
In DC and single phase AC two-wire circuits
Voltage drop = Current x Total resistance of cables
= 2 IR

where I is the current and
R is the resistance of one conductor only
Wherever voltage drop is given as 1 volt drop per metre run of cable, we have to assume that both (lead and
return) cables are taken into account and the cable carries its rated current. In such cases the voltage
drop for X metre length of cable for a current of Y amps is calculated as given.

3-phase circuits

123

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