Sizing cable is important because you want the cable to be able to:
Operate continuously under full load conditions without being damaged
You also want the cable to endure potential short circuit currents flowing through the cable
The load on the cable must be provided proper voltage
Things you will need to know:
1) Information about cable, installation conditions and the load
2) Figure out min cable size due to current carrying capacity
3) Figure out the minimum cable size due to voltage drop considerations
4) Figure out the minimum cable size due to short circuit temperature rise
5) Figure out the minimum cable size due to earth fault loop impedance
6) Pick cable based on the highest of the sizes calculated in step 2, 3, 4 and 5
To find out cable size Example
motor kW = 9500 kW
Voltage = 11 kV
Power Factor = 0.91
Efficiency= 0.962
Motor full load current = Motor kW
1.732 x Voltage x Power Factor x Efficiency
= 9500
1.732 x 11 x 0.91 x 0.962
Motor full load current = 569.596 Amp
Derating Factors
The De-rating factors (Using Manufacturers Data based on your Site Conditions)
1) For Ground Temp correction (30°C) DF1=1.00
2) Derating factor for grouping DF2 =0.8
3) Derating for depth of laying DF3 =0.95
4) Derating factor for thermal resitivity of soil DF4 =0.92
Now group derating factor = DF1 X DF2 X DF3 X DF4 = 0.70
Now,Corrected Cable Ampacity is = (Motor full load current/Group derating factor)
Corrected Cable Ampacity =813.708 Amp
569.569 / .7 = 813.708 Amp
Now, as per the vendor catalogue cable ampacity for
3CX185 sqmm cable is = 433 Amp
So, no of runs requried is = 813.708 / 433
= 1.87
So the no of runs for 3CX150 Sqmm cable is 2
