Calculation of Cable Sizing

CALCULATION OF CABLE SIZING 1.

General These design citeria covers choosing the conductor size of power cables for following system. a. 6,3 kV power cable system b. 0,4 kV power cable system

2.

General condition

2.1

Basic deign data : a. b. c. d.

2.2

2.3

Frequency is 50 Hz Ambient air temperature is 40 0C Ambient earth temperature is 20 0C Thermal reistivity (RHO) soil is 90 90 0C.cm/watt

Application of power cable system 6,3 kV power system

:

Up to 400 V power system

:

Insulation and jacket operation temperature a. Normal Continuous Operation

:

90 0C

b. Maximum Emergency Operation

:

130 0C

c. Maximum Short Circuit Operation

:

250 0C

3.

Design Criteria

3.1

Code and Standard a. b. c. d.

3.2

6/10 kV Cooper conductor XLPE insullated, Cooper tapes screen, and Outer sheathed PVC 0,6/1 kV Cooper conductor XLPE insulated, and Outer sheathed PVC

IEC Standard ICEA Standard IEEE Standard NFPA Standard

Cable selection For choosing the conductor size of power cable, the following factor shall be considered : a. Minimum size of conductor conductor based based on short circuit circuit capacity capacity b. Allowable continuous current rating of cable c. Allowable voltage voltage drop cable

3.3

Minimum size of conductor (1) The minimum size of conductor withstand a short circuit current shall be taken into considered following factor : a) Maximum short circuit current b) Maximum duration time of short circuit current c) Under worst conditions, the conductor temperature shall not exceed its maximum short circuit teperature of 250 OC (2) Calculation formula Formula given by the IEC 986 (Shown on below) have been used to select the minimum size of cable. It = K S ( θf + ß )/ ln (θi + ß) The above equation can be simplified as S = I √ t / 143 Where ; I = maximum short circuit current ( r.m.s over duration ) (A) t = maximum duration time of short circuit current(S) K = constant dependig on material of conductor = 226 (AS ½ / mm2) S = cross sectional area of conductor(mm2) O Θf = final temperature of conductor = 250 ( C) O Θi = initial temperature of conductor = 90 ( C) ß = reciprocal of temperature coefficient of resistance of conductor at 0 OC = 234,5 ln = log e (3) The minimum of size of cable to 0,38 kV motor circuit shall be of 2,5 mm2 (4) The minimum size of cable for equipment loads from Distribution Panel Boards ( minimum 6 A MCCB) shall be of 2,5 mm2

3.4

Allowable continuous current rating of cable (1) The allowable continuous current rating cable shall be determined in accordance with standard as follows : a. IEC 287 Calculation of continuous current rating of cables (100 % load factor) b. IEC 364 – 5 – 523 Current  –Carrying capacities (2) Cable ampacity shall be at least 100 % of name plate rating full load current for motor. And non motor loads cable ampacity shall be at least 100 % maximum continuous load current. (3) The cable is routed through several type of installation such as run via cable tray, direct burial, underground duct, the cable ampacity shall be base on the part of the cable run with the lowest current rating (4) Calculation of rated currents and starting currents.

The rated current and the starting current of a motor, or a load are based in principle, on the values which provided by the equipment manufacturers (5) If the values not available, the basic equations below shall apply. a. Rated currents In (A) DC AC 1, 2W

Direct Current 2 wire system ........... In = P / V Single phase 2 wire system ........... In = P / V.Cosθ.η

AC 2, 3W

Three phase 3 wire system ........... In = P / √3 V.Cosθ.η

AC 2, 4W

Three phase 4 wire system ........... In = P / √3 V.Cosθ.η

Where : In P V Cosθ η

, Rated Current (A) , Rated Load (kW) or (kVA) , Rated Voltage (V) , Power Factor (at rated load) , Efficiency (at rated load)

b. Starting current; Is(A) Is = Ks x In Where; Is In Ks

, Starting Currents (A) , Rated Currents (A) , Current multiplication at starting

The values for Calculations of Rated Currents and Starting Currents.

No

1

Type of Load

Medium Voltage Motor

Power Factor

Efficiency

0,85

0,95

Current multiple at Starting 7

Power factor at Starting 0,2 0,3

Low Voltage motor

0,85

0,93

6,5

3

Small motor (motor operating valve)

0,8

0,93

4,5

4

Transformer

0,85

N/A

N/A

5

Ac Single phase circuit

0,8

N/A

N/A

6

DC circuit

N/A

N/A

4.0

2

0,6 N/A N/A N/A

c. Allowable voltage drop cable The allowable voltage drop (%) of cable are ; Circuit

Allowable voltage drop Normal operation Starting of motor

Motor Circuit

2,5 %

15 %

Other circuit

3%

N/A

Allowable Voltage Drop (V) = Rated voltage x (%)/100

Formula for voltage drops are : 1. Voltage drop value at rated load : ∆Vn(V) DC

Direct Current 2 wire system AC1ф 2 W Single phase 2 wire system AC1ф 3 W Single phase 3 wire system

∆Vn = 2 . ln . L . Rdc

AC3ф 3 W Three phase 3 wire system AC3ф 4 W Three phase 4 wire system

∆Vn = √3 In . L . (R Cosθ + X Sinθ)

∆Vn = 2 . In . L . (R Cosθ + X Sinθ) ∆Vn = In . L . (R Cosθ + X Sinθ)

∆Vn = In . L . (R Cosθ + X Sinθ)

2. Voltage drop value at starting : ∆Vs(V) DC

Direct Current 2 wire system AC1ф 2 W Single phase 2 wire system

∆Vs = 2 . ls . L . Rdc

AC3ф 3 W Three phase 3 wire system AC3ф 4 W Three phase 4 wire system

∆Vs = √3 Is . L . (R Cosψ + X Sinψ)

∆Vs = 2 . Is . L . (R Cosψ + X Sinψ)

∆Vs = Is . L . (R Cosψ + X Sinψ)

Where: ∆Vn ∆Vs

In Is L

= = = = =

Voltage drop value at rated load (V) Voltage drop value at starting (V) Rated current (A) Starting current (A) Length of cable

R

= Cable resistance at maximum temperatue of cable conductor( R , AC resistance, Rdc, DC resistance) Cosθ = Power factor at motor rated load Cosψ = Power factor at motor starting

4. Cable data’s and Cable Ampacities Base on the above matters and manufacturers cable data’s, the values of cable ampacities can be found as table below;

Cable Datas and Cable Ampacities for Low Voltage

No

Voltage (kV)

Type and Size of CABLE (mm²)

IN GROUND 20 OC (Amp)

IN AIR 30 O C Amp)

Max DC Cable resistance Ampacities at 20 OC (Amp) (Ω/km)

1

0,6/1

N2XY 4 x 2,5

41

32

7,41

22,4

2

0,6/1

N2XY 4 x 4

53

41

4,61

28,7

3

0,6/1

N2XY 4 x 6

67

53

3,08

37,1

4

0,6/1

N2XY 4 x 10

89

71

1,83

49,7

5

0,6/1

N2XY 4 x 16

116

96

1,15

67,2

6

0,6/1

N2XY 4 x 25

151

130

0,727

91

7

0,6/1

N2XY 4 x 35

181

159

0,524

111,3

8

0,6/1

N2XY 4 x 50

208

193

0,387

135,1

9

0,6/1

N2XY 4 x 70

254

245

0,268

171,5

10

0,6/1

N2XY 4 x 95

305

302

0,193

211,4

11

0,6/1

N2XY 4 x 120

347

349

0,153

244,3

12

0,6/1

N2XY 4 x 150

392

400

0,124

280

13

0,6/1

N2XY 4 x 185

441

464

0,0991

324,8

14

0,6/1

N2XY 4 x 240

511

552

0,0754

386,4

15

0,6/1

N2XY 4 x 300

576

677

0,0601

473,9

Cable Datas and Cable Ampacities for Low Voltage No

Voltage (kV)

Type and Size of CABLE (mm²)

IN GROUND 20 OC (Amp)

IN AIR 30 O C Amp)

Max DC Cable resistance Ampacities at 20 OC (Amp) (Ω/km)

1

0,6/1

N2XY 3 x 2,5

41

32

7,41

22,4

2

0,6/1

N2XY 3 x 4

53

41

4,61

28,7

3

0,6/1

N2XY 3 x 6

65

52

3,08

36,4

4

0,6/1

N2XY 3 x 10

89

71

1,83

49,7

5

0,6/1

N2XY 3 x 16

117

96

1,15

67,2

6

0,6/1

N2XY 3 x 25

153

130

0,727

91

7

0,6/1

N2XY 3 x 35

184

159

0,524

111,3

8

0,6/1

N2XY 3 x 50

217

193

0,387

135,1

9

0,6/1

N2XY 3 x 70

269

245

0,268

171,5

10

0,6/1

N2XY 3 x 95

323

302

0,193

211,4

11

0,6/1

N2XY 3 x 120

367

349

0,153

244,3

12

0,6/1

N2XY 3 x 150

412

400

0,124

280

13

0,6/1

N2XY 3 x 185

470

464

0,0991

324,8

14

0,6/1

N2XY 3 x 240

546

552

0,0754

386,4

15

0,6/1

N2XY 3 x 300

632

677

0,0601

473,9

Cable Datas and Cable Ampacities for Medium Voltage Type and Size of CABLE (mm²)

IN GROU ND 20 O C (Amp)

IN AIR 30 O C Amp)

Max DC Cable resistance Ampacities at 20 OC (Amp) (Ω/km)

No

Voltage (kV)

1

6 kV/10 kV

N2XSY 1 x 25

157

162

0,708

105,3

2

6 kV/10 kV

N2XSY 1 x 35

187

195

0,514

126,75

3

6 kV/10 kV

N2XSY 1 x 50

220

234

0,379

152,1

4

6 kV/10 kV

N2XSY 1 x 70

269

292

0,262

189,8

5

6 kV/10 kV

N2XSY 1 x 95

321

354

0,189

230,1

6

6 kV/10 kV

N2XSY 1 x 120

364

407

0,150

264,55

7

6 kV/10 kV

N2XSY 1 x 150

405

460

0,122

299

8

6 kV/10 kV

N2XSY 1 x 185

457

527

0,097

342,55

9

6 kV/10 kV

N2XSY 1 x 240

528

621

0,074

403,65

10

6 kV/10 kV

N2XSY 1 x 300

593

729

0,059

473,85

11

6 kV/10 kV

N2XSY 1 x 400

665

815

0,046

529,75

Cable Datas and Cable Ampacities for Medium Voltage Type and Size of CABLE (mm²)

IN GROU ND 20 O C (Amp)

IN AIR 30 O C Amp)

Max DC Cable resistance Ampacities at 20 OC (Amp) (Ω/km)

No

Voltage (kV)

1

6 kV/10 kV

N2XSEBY 3 x 25

145

141

0,727

91,65

2

6 kV/10 kV

N2XSEBY 3 x 35

173

168

0,524

109,2

3

6 kV/10 kV

N2XSEBY 3 x 50

205

201

0,387

130,65

4

6 kV/10 kV

N2XSEBY 3 x 70

248

250

0,268

162,5

5

6 kV/10 kV

N2XSEBY 3 x 95

293

299

0,193

194,35

6

6 kV/10 kV

N2XSEBY 3 x 120

330

340

0,153

221

7

6 kV/10 kV

N2XSEBY 3 x 150

367

385

0,124

250,25

8

6 kV/10 kV

N2XSEBY 3 x 185

409

434

0,099

282,1

9

6 kV/10 kV

N2XSEBY 3 x 240

459

496

0,0754

322,4

10

6 kV/10 kV

N2XSEBY 3 x 300

503

548

0,0601

356,2

11

6 kV/10 kV

N2XSEBY 3 x 400

552

607

0,047

394,55

5. Calculation of Cable Size for Low Voltage and Medium Voltage 5. 1 Calculation of Cable Size for Low Voltage (0,6/1 kV)

CALCULATION OF CABLE SIZE (N2XY,N2XBY, 0,6/1 kV) FOR MOTORS BASE ON CABLE AMPACITY

No

Cap Motor 3ф (kW)

Current of  Motor (Im)

Ratio Im/Ia

Safety Factor (%)

Justify for Safety Factor (25%)

1

4

4 x 2,5

0,30

70

ok

2

0,33

4x4

0,33

67

ok

37,1

0,34

4x6

0,34

66

ok

18,70

37,1

0,50

4x6

0,50

50

ok

15

25,50

49,7

0,51

4 x 10

0,51

49

ok

6

18,5

31,45

49,7

0,63

4 x 10

0,63

37

ok

7

22

37,40

67,2

0,56

4 x 16

0,56

44

ok

8

25

42,50

91

0,47

4 x 25

0,47

53

ok

9

30

51,00

91

0,56

4 x 25

0,56

44

ok

10

37,5

63,75

111,3

0,57

4 x 35

0,57

43

ok

11

45

76,50

111,3

0,69

4 x 35

0,69

31

ok

12

55

93,51

135,1

0,69

4 x 50

0,69

31

ok

13

75

127,51

171,5

0,74

4 x 70

0,74

26

ok

14

95

161,51

244,3

0,66

3 x 120

0,66

34

ok

15

110

187,01

292

0,64

3 x 150

0,64

36

ok

16

132

224,41

324,8

0,69

3 x185

0,69

31

ok

17

160

272,02

386,4

0,70

3 x240

0,70

30

ok

18

200

340,02

509,54

0,67

2x3x120

0,67

33

ok

19

250

425,03

649,6

0,65

2X3x185

0,65

35

ok

20

315

535,53

772,8

0,69

2X3X240

0,69

31

ok

Ampacities of Cable( Ia)

Core size (sq.mm)

Cable size (sq.mm)

6,80

22,4

0,30

5,5

9,35

28,7

3

7,5

12,75

4

11

5

CALCULATION OF CABLE SIZE (N2XY,N2XBY, 0,6/1 kV) FOR MOTORS BASE ON CABLE AMPACITY

No

Cap Motor 3ф (kW)

Current of  Motor (Im)

Rated of  Cable( Ic)

Core size (sq.mm)

Cable size (sq.mm)

Ratio Im/Ia

Safety Factor (%)

Justify for Safety Factor (25%)

1

4

6,80

32

0,21

4 x 2,5

0,21

79

ok

2

5,5

9,35

41

0,23

4x4

0,23

77

ok

3

7,5

12,75

53

0,24

4x6

0,24

76

ok

4

11

18,70

53

0,35

4x6

0,35

65

ok

5

15

25,50

71

0,36

4 x 10

0,36

64

ok

6

18,5

31,45

71

0,44

4 x 10

0,44

56

ok

7

22

37,40

96

0,39

4 x 16

0,39

61

ok

8

25

42,50

130

0,33

4 x 25

0,33

67

ok

9

30

51,00

130

0,39

4 x 25

0,39

61

ok

10

37,5

63,75

159

0,40

4 x 35

0,40

60

ok

11

45

76,50

159

0,48

4 x 35

0,48

52

ok

12

55

93,51

193

0,48

4 x 50

0,48

52

ok

13

75

127,51

245

0,52

4 x 70

0,52

48

ok

14

95

161,51

349

0,46

3 x 120

0,46

54

ok

15

110

187,01

400

0,47

3 x 150

0,47

53

ok

16

132

224,41

464

0,48

3 x185

0,48

52

ok

17

160

272,02

552

0,49

3 x240

0,49

51

ok

18

200

340,02

698

0,49

2x3x120

0,49

51

ok

19

250

425,03

928

0,46

2X3x185

0,46

54

ok

20

315

535,53

1104

0,49

2X3X240

0,49

51

ok

5. 2 Calculation of Cable Size for Medium Voltage (6/10 kV)

CALCULATION OF CABLE SIZE (N2XSY,N2XSEBY, 6/10 kV) FOR MOTORS BASE ON AMPACITY AND RATED CURRENT OF CABLE Rated Curren t of  Cable (Ia)

Ratio Current (Ifm/Ia)

Safety Factor

N2XSEBY 3 x 25

141,00

0,12

88,09

ok

0,32

N2XSEBY 3 x 25

141,00

0,20

79,57

ok

91,25

0,42

N2XSEBY 3 x 25

141,00

0,27

72,77

ok

39,20

91,25

0,43

N2XSEBY 3 x 25

141,00

0,28

72,20

ok

5

40,00

91,25

0,44

N2XSEBY 3 x 25

141,00

0,28

71,63

ok

6

42,40

91,25

0,46

N2XSEBY 3 x 25

141,00

0,30

69,93

ok

7

57,60

91,25

0,63

N2XSEBY 3 x 25

141,00

0,41

59,15

ok

8

57,60

113,75

0,51

N2XSEBY 3 x 25

175,00

0,33

67,09

ok

9

121,60

130,65

0,93

N2XSEBY 3 x 25

201,00

0,60

39,50

ok

10

137,60

130,65

1,05

N2XSEBY 3 x 25

201,00

0,68

31,54

ok

11

183,20

264

0,69

N2XSY 3 x 1 x 120

407

0,45

54,99

ok

12

200,80

194,35

1,03

N2XSEBY 3 x 95

299,00

0,67

32,84

ok

13

211,20

194,35

1,09

N2XSEBY 3 x 95

299,00

0,71

29,36

ok

14

229,60

264

0,87

N2XSY 3 x 1 x 120

407

0,56

43,59

ok

15

275,20

267,8

1,03

N2XSY 3 x 1 x 120

412

0,67

33,20

ok

16

275,20

261,3

1,05

N2XSEBY 2 x 3 x 50

402

0,68

31,54

ok

17

275,20

327,6

0,84

N2XSEBY 3 x 3 x 35

504

0,55

45,40

ok

18

324,00

299

1,08

N2XSY 3 x 1 x 150

460

0,70

29,57

ok

19

344,00

299

1,15

N2XSY 3 x 1 x 150

460

0,75

25,22

ok

20

630,00

299

2,11

N2XSY 3 x 2 x 1 x 150

460,00

0,65

35,00

ok

21

613,60

299

2,05

N2XSY 3 x 2 x 1 x 150

460,00

0,67

33,30

ok

22

630,40

342,55

1,84

N2XSY 3 x 2 x 1 x 185

527

0,60

40,19

ok

23

739,20

342,55

2,16

N2XSY 3 x 2 x 1 x 185

527

0,70

29,87

ok

24

872,00

403

2,16

N2XSY 3 X 2 x 1 X240

621,00

0,70

29,79

ok

25

1250,00

403

3,10

N2XSY 3 X 2 x 1 X240

621,00

0,67

32,90

ok

26

1132,80

403

2,81

N2XSY 3 X 2 x 1 X240

621

0,61

39,19

ok

27

1176,00

403

2,92

N2XSY 3 X 2 x 1 X240

621,00

0,63

36,88

ok

28

1600,00

473,85

3,38

N2XSY 3 x 3 x 1 x 300

729

0,73

26,84

ok

29

1516,80

473,85

3,20

N2XSY 3 x 3 x 1 x 300

729

0,69

30,64

ok

30

2243,20

473,85

4,73

N2XSY 3 x 5 x 1 x 300

729

0,62

38,46

ok

31

3150,00

473,85

6,65

N2XSY 3 x 7 x 1 x 300

729

0,62

38,27

ok

32

3670,40

473,85

7,75

N2XSY 3 x 8 x 1 x 300

729

0,63

37,06

ok

No

Rated Current of  Feeder/Mot or (Ifm)

Ampacities of Cable ( Ia)

Core size (sq.mm)

1

16,80

91,25

0,18

2

28,80

91,25

3

38,40

4

Cable size (sq.mm)

Note, Cable No, 8,15,16, and 17 are 12/20 kV

Justify for Safety Factor (25%)