4. Calculate Cable Size and Voltage Drop

CALCULATE CABLE SIZE AND VOLTAGE DROP

CALCULATE CABLE SIZE AND VOLTAGE DROP:

CALCULATE VOLTAGE DROP AND SIZE OF ELECTRICAL CABLE FOR FOLLOWING DATA.

• Electrical Details: Electrical Load of 80KW, Distance between Source and Load is 200 Meter, System Voltage 415V Three Phase, Power Factor is 0.8,Permissible Voltage drop is 5%, Demand Factor is 1, * Cable Laying Detail: Cable is directed buried in Ground in trench at the depth of 1 meter. Ground Temperature is approximate 35 Deg. No of Cable per Trench is 1. No of Run of Cable is 1 Run. * Soil Details: Thermal Resistivity of Soil is not known. Nature of Soil is Damp Soil.

CALCULATION:

• Consumed Load= Total Load x Demand Factor * Consumed Load in KW= 80 x 1 =80KW * Consumed Load in KVA= KW/P.F * Consumed Load in KVA =80/0.8=100KVA * Full Load Current= (KVAx1000) / (1.732xVoltage) * Full Load Current= (100×1000) / (1.732×415) = 139Amp. * Calculating Correction Factor of Cable from following data : * Temperature Correction Factor (K1) When Cable is in Air is

Temperature Correction Factor in Air :K1

Ambient Temp©

Insulation

PVC

XLPE/EPR

10

1.22

1.15

15

1.17

1.12

20

1.12

1.08

25

1.06

1.04

35

0.94

0.96

40

0.87

0.91

45

0.79

0.87

50

0.71

0.82

55

0.61

0.76

60

0.5

0.71

65

0

0.65

70

0

0.58

75

0

0.5

80

0

0.41

• Ground Temperature Correction Factor (K2):

Ground Temperature Correction Factor:K2

Ground Temp©

Insulation

PVC

XLPE/EPR

10

1.1

1.07

15

1.05

1.04

20

0.95

0.96

25

0.89

0.93

35

0.77

0.89

40

0.71

0.85

45

0.63

0.8

50

0.55

0.76

55

0.45

0.71

60

0

0.65

65

0

0.6

70

0

0.53

75

0

0.46

80

0

0.38

• Thermal Resistance Correction Factor (K4) for Soil (When Thermal Resistance of Soil is known):

Ther.Resi Correction Factor: K4

Soil Thermal Resistivity: 2.5 KM/W

Resistivity

K3

1

1.18

1.5

1.1

2

1.05

2.5

1

3

0.96

• Soil Correction Factor(K4) of Soil (When Thermal Resistance of Soil is not known):

Soil Correction Factor:K4

Nature of Soil

K3

Very Wet Soil

1.21

Wet Soil

1.13

Damp Soil

1.05

Dry Soil

1

Very Dry Soil

0.86

• Cable Depth Correction Factor (K5):

Cable Depth Factor (K5)

Laying Depth(Meter)

Rating Factor

0.5

1.1

0.7

1.05

0.9

1.01

1

1

1.2

0.98

1.5

0.96

• Cable Distance correction Factor (K6):

Cable Distance Correction Factor(K6)

No of Circuit

Nil

cable Diameter

0.125m

0.25m

0.5m

1

1

1

1

1

1

2

0.75

0.8

0.85

0.9

0.9

3

0.65

0.7

0.75

0.8

0.85

4

0.6

0.6

0.7

0.75

0.8

5

0.55

0.55

0.65

0.7

0.8

6

0.5

0.55

0.6

0.7

0.8

• Cable Grouping Factor ( No of Tray Factor) (K7):

No of Cable/Tray

(Cable Grouping factor K7 )==No of Tray

1

2

3

4

6

8

1

1

1

1

1

1

1

2

0.84

0.8

0.78

0.77

0.76

0.75

3

0.8

0.76

0.74

0.73

0.72

0.71

4

0.78

0.74

0.72

0.71

0.7

0.69

5

0.77

0.73

0.7

0.69

0.68

0.67

6

0.75

0.71

0.7

0.68

0.68

0.66

7

0.74

0.69

0.675

0.66

0.66

0.64

8

0.73

0.69

0.68

0.67

0.66

0.64

• According to above Detail correction Factors are * Ground Temperature Correction Factor (K2) =0.89 * Soil Correction Factor (K4)=1.05 * Cable Depth Correction Factor (K5)=1.0 * Cable Distance correction Factor (K6)=1.0 * Total De rating Factor= k1x k2 x k3 x K4 x K5 x K6 x K7 * Total De rating Factor= 0.93

SELECTION OF CABLE:

• For selection of Proper Cable following Conditions should be satisfied * (1) Cable De rating Amp should be higher than Full Load Current of Load. * (2) Cable Voltage Drop should be less than Defined Voltage drop. * (3) No of Run of Cable >= (Full Load current / Cable De rating Current). * (4) Cable Short Circuit Capacity should be higher than System S.C Capacity at that Point.

SELECTION OF CABLE CASE (1):

• Let’s Select 3.5Core 70 Sq.mm cable for Single run. * Current Capacity of 70 Sq.mm cable is 170Amp,Resistance=0.57Ω/Km and Reactance=0.077 mho/Km * Total De rating Current of 70 Sq.mm Cable= 170×0.93 =159 Amp. * Voltage Drop of Cable= (1.732x Current x (RcosǾ+jsinǾ) x Cable Lengthx100) / (Line Voltage x No of Runx1000) * Voltage Drop of Cable= (1.732x139x(0.57×0.8+0.077×0.6)x200x100)/(415x1x1000)=5.8% * Voltage Drop of Cable=5.8% * Here Voltage drop for 70 Sq.mm Cable (5.8%) is higher than Define Voltage drop (5%) so either select higher size of cable or Increase no of Cable Runs. * If we Select 2 No’s of Run than Voltage drop is 2.8% which is within limit (5%) but to use 2 no’s of Run of cable of 70 Sq.mm Cable is not economical so It is necessary to use next higher size of Cable.

SELECTION OF CABLE CASE (2):

• Let’s Select 3.5Core 95 Sq.mm cable for Single run, S.C Capacity =8.2KA. * Current Capacity of 95 Sq.mm cable is 200Amp,Resistance=0.41Ω/Km and Reactance=0.074 mho/Km * Total De rating Current of 70 Sq.mm Cable= 200×0.93 =187 Amp. * Voltage Drop of Cable= (1.732x139x(0.41×0.8+0.074×0.6)x200x100)/(415x1x1000)=2.2% * Voltage D…