Answer Posted / abdul haseeb khattal

Well, First try to calculate the total load of the system.
For example total load is 1000KW. The current can be
calculated as P=1.732VICOS(PHI). Hence total current will
be 1732A by considering V=415volt, power factor=0.8.

Step2: To calculate the cable size

Vd = mV/L/A

where Vd= Allowable voltage drop which is normally 2.5
percentage to 4 percent.
L = Lenght of the cable from the source to feeder
A = Ampere rating

Let us consider L = 1000Meters
A = 1732A
Vd = 2.5 percentage of 415V as in U.A.E.
= 2.5*415/100 = 10.375

Use these value in the above formula

mV= Vd/L*A
= 10.375/1000*1732
= 0.005 volt

Refer to the cable catalogue in your region and find out
the current carrying capacity of the cable which will be
totally depend upon the atmospheric condition, in cold
place a smaller diameter of cable can carry larger current
and in hot areas the same size will carry lesser current.

In U.A.E. we are normally using 11x1c 630sqmm cable with
2500A incomer breaker. But cross check the voltage drop as
calculated above is within the limit of voltage drop of
these cables else you have to go to higher size of cable
untill the voltage drop met the design milli volt drop of
the cable.

State angle criterion.

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