Mv Cable Sizing Calculation

CABLE SIZING CALCULATION FOR 2500kVA COMMON TRANSFORMER INCOMING CABLE CONSIDERATIONS: 1. Line to Line Voltage = 2. Frequency = 3. Max. Allowable Voltage Drop % of Line to Line Voltage Starting = Running = 4. Cable Overall Derating Factor D 5. Length of the cable, L =

4320 60

Volts Hz

15% 5% 0.7 100

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

Transformer Parameters: Transformer Rating Transformer Efficiency at Full Load Hence, Transformer FLC Im

= = =

1600 kVA 0.97 kVA/(1.732*kV*Efficiency)

=

I/D

=

228.93 A

Cable Sizing Based on Ampacity: Cable equivalent current at 45 degree C, Ic Cable Size Selected Cable Rated Current in Air, Ca Cable Resistance at 90 deg. C (ohm/km) , R Cable Reactance (ohm/km) at 60 Hz , X No.of parallel cables required

327.05 A 2 185 mm

= = = = Ic/Ca

453 A 0.127 0.10056 1

CU, XLPE INSULATED, ARMOURED CABLE (As per KEI catalogue) (As per KEI Catalogue) (As per KEI Catalogue)

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) / K Where, 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 Full Load Current 4320V Symmentrical SC current Isc Operating time t Required Cross sectional Area S

= = = =

1600 228.93 40000 0.16 111.89

K - Factor

XLPE Cables PVC Cables

kVA A A S Sqmm

Copper

Aluminium

143 115

94 76

Cable Voltage Drop: Transformer Running Voltage Drop (%) = = Where,

L *Sqrt(3) * IFL( R cosφr+X sin φr) (VL-L/100)* 1000 *No.of runs 0.12 %

L:Length of the cable in metres IFL: 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