how can we know cable size for a given current?
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Answer Posted / vasu
Two things are very much common on the power system engineering field while doing a cable sizing, these are:
1)LT
2)HT
Cables are separately sized based on voltage levels as said above.
Formulas used for LT side:
Full load current = KW/(KV x sqrt3 x PF x η)
Voltage Drop =
sqrt 3*IFL*(R COS Ø + X SIN Ø)*L/(KV*10000*N.R.)
where
IFL = Full load current of the Load connected in Amps
KW = Rating of the Load connected
KV = System Voltage
R = Resistance of the cable in Ohm/Ph/KM
X = Reactance of the cable in Ohm/Ph/KM
L = Length of the cable in meters
N.R = No of Runs of the cable
cosØ = Power factor of the system
PF = Power factor
η =efficiency
Voltage Drop =
sqrt 3*Ist*(R COS Ø + X SIN Ø)*L/(KV*10000*N.R.)
where Ist=starting current which can be 6 times the FLA of current.In technical term it is called as LRC(locked rotor current)
HT side:
Full load current = KVA / KV x sqrt 3
Short circuit current :
A = Ish x sqrt(t)/ k
Where
t = short circuit time in sec
k = constant
A = Conductor size in Sqmm
Ish = Short circuit current in Amps
Derating Factor :
Ambient temperature correction factor for cables laid in 0.9 Trays at 50 degree C = 0.9
Group derating factor:
Multi Core cables
Cables are laid with a spacing of one Diameter and routed in cable trays and trays in tiers by 300mm =0.9
Ageing factor= 0.95
Therefore the Total derating factor = 0.9 x 0.9 x 0.95 = 0.7695
Total Derating Factor= 0.77
The rest calculation for voltage drop are same as that of LT side
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