Domestic Wiring Chapter 3 

SPECI FICATIONS OF WIRES

Usually cables or wires have a no. of strands twisted together. 

Standard stranding consists of 6 wires around 1, then 12 around 6, followed by 18, 24 and so on. 

A stranded conductor is expressed

as 7/0. 1 where the first number stands for the no. of strands used and the second number corresponds to the diameter of each strand in mm. 

Some times the second number given corresponds to the gauge ofthe strand used e.g., 3/20 which means a cable with three strands each of 20SWG.

All electric cables consist ofthree essential points :

(a) the conductor for transmitting electrical power,

(b) the insulation, an electrical insulating medium, needed to insulate the conductor from direct contact with earth or other objects, and

(c) external protection against mechanical damage, chemical or electro-chemical attack,fire or any other dangerous effects external to the cable.

Copper conductor 

has extensively been used for cables but oflate aluminium is being used

to a considerable extent. 

To obtain flexibility a number of wires are made up into a strand which makes it easier to handle, less liable to kink and break and to a large extent eliminates risk to the conductor breaking through the dielectric. 

The wires in a stranded conductor are twisted together to form lays. 

The successive layers usually are stranded in opposite direction i.e., if the wires of

one layer have a right-handed lay, the next layer has a left-handed lay.

In regard to specification of cables, it is important to mention all possible details regarding the particular type of cable in terms of size (in mm or mm2) types of conductor whether copper or

aluminium, type of insulation PVC or VIR and voltage grading.

Size of Conductor

Following points must be kept in mind while determining the size of the conductor for internal wiring for a given circuit.

✓ Minimum size of the conductor for    mechanical reason

✓ Current carrying capacity

✓ Voltage drop

Minimum size of Conductor

✓ Wire. 

The minimum size of copper conductor in use is 1/1.2 mm based on permissible wattage of 800 watts in the subcircuit. 

The aluminium conductor of size 1/1.4 mm is used for a subcircuit in domestic wiring and minimum size of conductor for power wiring is 4 mm2 and 1/2.24 mm.

✓ Underground cables

The area of cross section of conductor for two core cables should not be less than 6 mm2 and for three and four core cables it should not be less than 25 mm2. 

The minimum area of cross section of a three and half core cable should be 50 sq. cm.

Current-carrying capacity

When current is passed through a wire a certain amount of power is wasted in the wire which appears in the form of heat.

Which results in increase temperature of the conductor which may again decrease the life of insulation. 

Therefore, the rating should be such that when that current is allowed to flow continuously it does not affect the

insulation of the cable. 

However, it should be able to carry a relatively larger current. (Varying

between 1.5 to 2 times the continuous rated current. ) for a short duration without affecting the insulation.

Voltage drop

Before deciding a proper size ofcable to be used in a circuit, due consideration

must be given to voltage drop. 

This voltage drop in the cable from distribution board to the point of device (lamp, fan, motor etc.) is due to the flow of current through the cable. 

The permissible voltage drop from supply terminal to any point on the wiring system should not exceed 2% + 1 volt for light loads and 7.5% for declared supply voltage of power loads.