Answer / kanchan dogra

Selection of HT cable is based on
1. short circuit calculation
2. Full load current capacity
1. Short Ckt. calculation
if total load is 3000 KVA, V 11 KV
short ckt MVA on HV side = 350 MVA (As per standard practice)
short ckt. current Ik= MVA/root3 x KV
350/root3 x 1= 18.4 KA
duration of short Ckt= 1 Second
cross section area of the conductor = Ik x root T/.094
18.4x root T/.094 = 95 SQ.mm

2. Full Load current
Total Load= 3000 KVA
System Voltage= 11 KV
Full load current = KVA/ root3 x KV
3000/root 3 x 11= 157 Amp.
derating factor = .7
current carrying capacity after derating = 157/.7 = 224.95 AMP.
Hence select cable for this current.

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