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Cable sizing calculatin
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Cable Sizing Calculation – Low Voltage
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INPUT DATALoad detailsCable detailsCABLE SIZING CALCULATION CRITERIAShort circuit current capacityA=Cable cross-section area in Sq. mm
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System voltage415 VPower Rating of motor90  kWLength120 mThe minimum cross section of LV voltage cables shall be calculated based on below methods:This criteria defines the minimum cross section required for the cable.=Short circuit current
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Type of InsulationXLPESource Voltage 415  VResistance of the conductor (95mm2)0.247 ohm/km (As per Cable Catalogue)1. Short circuit current capacityK=Constant, 143 for XLPE Cu. Cable
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Type of conductorCopperEfficiency 0.9Reactance of the conductor (95mm2)0.0734 ohm/km (As per Cable Catalogue)2. Current carrying capacityt=Duration of short-circuit in seconds
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Maximum Ambient air temperature40°Power factor 0.853. Voltage drop of cable
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Ground temperature30°The fault current at 415V is 40kA and the duration t is considered to be 0.1 based on relay delay time and breaker opening time.
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Continuous temperature of the cable90°
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Maximum short circuit temperature of the cable250°
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Current carrying capacity
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Derating Factors considered for multicore cables laid in ductCorrection FactorThe next nearest available cable size will be 95 Sq.mm
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Installation method:Direct buriedDerating factor for variation in ground temperature at 30°Ck1:0.93(As per IEC 60364-5-52,Number of Runs / Phase:1
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Ground temperature:30°CTable B.52.15 )Current rating for 95 Sq.mm Cu. Cable (Duct):211(As per IEC 60502-1 )
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Soil Thermal Resistivity:2.5 K.m/WDerating factor:0.93
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Depth of Laying:75 cmDerating factor for depth of laying of cablek2:1(As per cable catalogue)Derated current carrying capacity   I(Derated):196.23
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Cable Grouping:
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No. of cables in group:1Derating factor for soil and thermal resistivityk3:1(As per IEC 60364-5-52,
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Spacing between duct centres:0 mmTable B.52.16 )
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Derating factor for cable grouping factork4:1(As per IEC 60364-5-52, Table B.52.17 )
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Overall correction factor for cables installed in Underground duct (k1 x k2 x k3 x k4):0.93
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Voltage drop of cable
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The cable has its resistance and reactance causing some voltage drop. Motor takes 6 to 12 times the full load current during starting. The voltage drop criteria has to be checked in order to have sufficient voltage to develop load torque.
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Number of Runs / Phase:1
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Resistance of the conductor (95mm2):0.247ohm/km(As per Cable catalogue)
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Reactance of the conductor (95mm2):0.0734ohm/km(As per Cable catalogue)
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Voltage:415V
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The three phase voltage drop
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L=Length of cable (m)
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R=Cable resistance at 90°C – Ohm/km
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X=Cable reactance at 50 Hz
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I=Rated current (A)
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n=No. of Parallel runsVdrop2.039877063by Formula
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cos ø=Power factor
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Ist=Starting current
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cos øst=Power factor at starting
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The voltage drop during running must be less than 3% (V(Running) < 3%)

The voltage drop during starting must be less than 10 to 15% ( V(Starting) <10-15%)

According to three major criteria, the cable size selected is 95mm2
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International standard wire size Chart (IEC 60228)
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0.75 mm²4 mm²0.5 mm²1 mm²2.5 mm²1.5 mm²
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70 mm²6 mm²25 mm²185 mm²35 mm²400 mm²
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95 mm²1000 mm²10 mm²800 mm²150 mm²50 mm²
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300 mm²120 mm²500 mm²16 mm²630 mm²240 mm²
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Electrical Cables Size & Current Rating Chart
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Cross Section (mm2)Approximate Overall Diameter (mm)Current Rating
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Single Phase (Amps)Three Phase (Amps)
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1.52.917.515.5
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2.53.532421
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44.43228
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64.684136
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105.985750
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166.957668
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258.710189
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3510.08125110
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5011.8151134
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7013.5192171
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9515.7232207
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12017.4296239
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15019.3300262
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18521.5341296
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24024.6400346
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30027.9458394
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40030.8546467
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50033.8626533
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63037.6720611
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