The Importance Of Cable Sizing And Selection

Cable Sizing and Selection Introduction:
One of the important aspects of designing any electrical system is to select an appropriate type and size of a cable. Too much small size of cable generates a lot of heat in a cable which will caused permanent damage to an insulation of a cable and using a too much large cable size means we are wasting our money on copper because copper is the costly material for cables. So we must have to select a cable that is neither of too small nor too large size. Further there are many types of cable like stranded cable, ACSR, PVC insulation etc. Cable sizing helps us to select an accurate size (i.e. cross sectional area) and type of cable required for a particular system. Why

Although this current is removed within a second but this short circuit current will cause a temperature rise in the cable which will damaged the cable’s components such as insulation of cable, sheath materials etc and also degrade the condition of the cable.
When the short circuit occurs for a brief interval of time (maximum 5 seconds) it is assumed that all of the heat produced is within the cable without the transference of the heat through convection and radiation causing the temperature within the cable to rise, since all the heat produced is within the cable that is why it is also known as adiabatic temperature rise. In actual heat produced in the cable is not purely adiabatic but we considered it to adiabatic for the sake of ease of calculation.
When the cross-sectional area of cable is increased it will able to dissipate higher fault current with a given temperature rise. Therefore it is very necessary to consider short circuit temperature rise while selecting a cable so that a cable can withstand the worse short circuit current flowing through the cable in case of short

Given below is a table having initial and final temperature mention for different types of material, initial temperature is the maximum operating temperature of the cable and the final temperature is the limiting temperature of the cable.

Cable Selection By Considering Earth Fault Loop Impedance: The earth fault loop impedance is the most important factor that needs to be taken into account when sizing a cable. It is basically defined as the path followed by fault current when low impedance fault occurs between the phase conductor and earth or it can also be defined as the impedance of the path followed by the current during fault conditions.
The higher the impedance the lower will be the fault current and the longer it will take for the circuit protection to operate. For fast protection this fault loop impedance must be low. The reason for the low earth loop impedance is that when short circuit occurs between a live part and the earth, the short circuit current will follow the earth loop and this fault current is large enough that it will trip the circuit breaker and blow the fuses. Earth fault loop impedance directly affects the size of circuit breaker or any other protective devices and