CABLE SIZING CALCULATION FOR 2500kVA COMMON TRANSFORMER INCOMING CABLE

CONSIDERATIONS:

1. Line to Line Voltage = 4320 Volts2. Frequency = 60 Hz3. Max. Allowable Voltage Drop % of Line to Line Voltage

Starting = 15%Running = 5%

4. Cable Overall Derating Factor D 0.75. Length of the cable, L = 100 m (Assumed)

Transformer Parameters:

Transformer Rating = 1600 kVATransformer Efficiency at Full Load = 0.97Hence, Transformer FLC Im = kVA/(1.732*kV*Efficiency) = 228.93 A

Cable Sizing Based on Ampacity:

Cable equivalent current at 45 degree C, Ic = I/D 327.05 ACable Size Selected = 185 mm2 CU, XLPE INSULATED, ARMOURED CABLECable Rated Current in Air, Ca = 453 A (As per KEI catalogue)Cable Resistance at 90 deg. C (ohm/km) , R = 0.127 (As per KEI Catalogue)Cable Reactance (ohm/km) at 60 Hz , X = 0.10056 (As per KEI Catalogue)No.of parallel cables required Ic/Ca 1

Cable Sizing Based on Shortcircuit withstanding capacity:

A sample calculation has been done below for minimum cable size required for a motor feeder for the specified short circuit rating. As per IEC 60364-5-54, Clause No.543.1.1,

S = (Isc x √t) / KWhere, S: Cross sectional area, in Square milimeters

Isc: Value (a.c. rms) of fault current for a fault of negligible impedance, which can flow through the protective device in amperes.t: operating time of the disconnecting device, in seconds. K: factor depend on the material of the protective conductor.

Transformer Rating = 1600 kVAFull Load Current = 228.93 A4320V Symmentrical SC current Isc = 40000 AOperating time t = 0.16 SRequired Cross sectional Area S 111.89 Sqmm

K - Factor Copper AluminiumXLPE Cables 143 94PVC Cables 115 76

Cable Voltage Drop:

Transformer Running Voltage Drop (%) = L *Sqrt(3) * IFL( R cosφr+X sin φr) (VL-L/100)* 1000 *No.of runs

= 0.12 %

Where, L:Length of the cable in metresIFL: Full load current in Amps.IST: Starting current in Amps.

The % of Voltage drop is within the limit.

Hence, the Selected Cable Size is 3.6/6 (7.2)kV, 1RX3Cx185 sq.mm, (2XFY) XLPE, Flat Armoured,Copper cable

(As per tender specification: CEM/ISGEC/700558-60- SECTION -II -Design basis-serial no.19)