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 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 based on short circuit 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 ( f + )/ ln (i + )
The above equation can be simplified as
S = I t / 143
Where ;
I
t
K
S
f
i

= maximum short circuit current ( r.m.s over duration ) (A)

= maximum duration time of short circuit current(S)
= constant dependig on material of conductor = 226 (AS / mm2)
= cross sectional area of conductor(mm2)
= final temperature of conductor = 250 (OC)
= initial temperature of conductor = 90 (OC)
= 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.Cos.

AC 2, 3W

Three phase 3 wire system ........... In = P / 3 V.Cos.

AC 2, 4W

Three phase 4 wire system ........... In = P / 3 V.Cos.

Where :
In
P
V
Cos

, 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

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

Small motor
(motor operating
valve)

0,8

0,93

4,5

Transformer

0,85

N/A

N/A

Ac Single phase
circuit

0,8

N/A

N/A

DC circuit

N/A

N/A

4.0

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
AC1 2 W
AC1 3 W
AC3 3 W
AC3 4 W

Direct Current
2 wire system
Single phase
2 wire system
Single phase
3 wire system
Three phase
3 wire system
Three phase
4 wire system

Vn = 2 . ln . L . Rdc
Vn = 2 . In . L . (R Cos + X Sin)
Vn = In . L . (R Cos + X Sin)
Vn = 3 In . L . (R Cos + X Sin)
Vn = In . L . (R Cos + X Sin)

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

DC

Direct Current
2 wire system
AC1 2 W Single phase
2 wire system
AC3 3 W Three phase
3 wire system
AC3 4 W Three phase
4 wire system

Vs = 2 . ls . L . Rdc
Vs = 2 . Is . L . (R Cos + X Sin)
Vs = 3 Is . L . (R Cos + X Sin)
Vs = Is . L . (R Cos + X Sin)

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

= Cable resistance at maximum temperatue of cable

conductor( R , AC resistance, Rdc, DC resistance)
Cos = Power factor at motor rated load
Cos = Power factor at motor starting
4. Cable datas and Cable Ampacities
Base on the above matters and manufacturers cable datas, the values of cable
ampacities can be found as table below;

Cable Datas and Cable

Ampacities for Low Voltage
Type and
Size of
CABLE
(mm)

IN
GROUND
20 OC
(Amp)

No

Voltage
(kV)

1
2
3
4
5
6
7
8
9
10
11
12
13
14

0,6/1
0,6/1
0,6/1
0,6/1
0,6/1
0,6/1
0,6/1
0,6/1
0,6/1
0,6/1
0,6/1
0,6/1
0,6/1
0,6/1

N2XY 4
N2XY 4
N2XY 4
N2XY 4
N2XY 4
N2XY 4
N2XY 4
N2XY 4
N2XY 4
N2XY 4
N2XY 4
N2XY 4
N2XY 4
N2XY 4

x 2,5
x4
x6
x 10
x 16
x 25
x 35
x 50
x 70
x 95
x 120
x 150
x 185
x 240

41
53
67
89
116
151
181
208
254
305
347
392
441
511

15

0,6/1

N2XY 4 x 300

576

IN
Max DC
AIR
Cable
resistance
30
Ampacities
O
at 20 C
O
C
(Amp)
(/km)
Amp)
32
7,41
22,4
41
4,61
28,7
53
3,08
37,1
71
1,83
49,7
96
1,15
67,2
130
0,727
91
159
0,524
111,3
193
0,387
135,1
245
0,268
171,5
302
0,193
211,4
349
0,153
244,3
400
0,124
280
464
0,0991
324,8
552
0,0754
386,4
677

0,0601

473,9

Cable Datas and Cable

Ampacities for Low Voltage
No

1
2
3
4

Voltage
(kV)
0,6/1
0,6/1
0,6/1
0,6/1

Type and
Size of
CABLE
(mm)
N2XY 3
N2XY 3
N2XY 3
N2XY 3

x 2,5
x4
x6
x 10

IN
GROUND
20 OC
(Amp)
41
53
65
89

IN
Max DC
AIR
Cable
resistance
30
Ampacities
O
at 20 C
O
C
(Amp)
(/km)
Amp)
32
7,41
22,4
41
4,61
28,7
52
3,08
36,4
71
1,83
49,7

5
6
7
8
9
10
11
12
13
14
15

0,6/1
0,6/1
0,6/1
0,6/1
0,6/1
0,6/1
0,6/1
0,6/1
0,6/1
0,6/1
0,6/1

N2XY 3
N2XY 3
N2XY 3
N2XY 3
N2XY 3
N2XY 3
N2XY 3
N2XY 3
N2XY 3
N2XY 3
N2XY 3

x 16
x 25
x 35
x 50
x 70
x 95
x 120
x 150
x 185
x 240
x 300

117
153
184
217
269
323
367
412
470
546
632

96
130
159
193
245
302
349
400
464
552
677

1,15
0,727
0,524
0,387
0,268
0,193
0,153
0,124
0,0991
0,0754
0,0601

67,2
91
111,3
135,1
171,5
211,4
244,3
280
324,8
386,4
473,9

Cable Datas and Cable Ampacities

for Medium Voltage
No

Voltage
(kV)

1
2
3
4
5
6
7
8
9
10
11

6 kV/10 kV
6 kV/10 kV
6 kV/10 kV
6 kV/10 kV
6 kV/10 kV
6 kV/10 kV
6 kV/10 kV
6 kV/10 kV
6 kV/10 kV
6 kV/10 kV
6 kV/10 kV

Type and Size

of CABLE
(mm)
N2XSY 1
N2XSY 1
N2XSY 1
N2XSY 1
N2XSY 1
N2XSY 1
N2XSY 1
N2XSY 1
N2XSY 1
N2XSY 1
N2XSY 1

x 25
x 35
x 50
x 70
x 95
x 120
x 150
x 185
x 240
x 300
x 400

IN
GROU
ND 20
O
C
(Amp)
157
187
220
269
321
364
405
457
528
593
665

IN
Max DC
AIR
Cable
resistance
30
Ampacities
at 20 OC
O
C
(Amp)
(/km)
Amp)
162
0,708
105,3
195
0,514
126,75
234
0,379
152,1
292
0,262
189,8
354
0,189
230,1
407
0,150
264,55
460
0,122
299
527
0,097
342,55
621
0,074
403,65
729
0,059
473,85
815
0,046
529,75

Cable Datas and Cable Ampacities

for Medium Voltage
No

Voltage
(kV)

1
2
3
4
5
6

6 kV/10 kV
6 kV/10 kV
6 kV/10 kV
6 kV/10 kV
6 kV/10 kV
6 kV/10 kV

Type and Size

of CABLE
(mm)
N2XSEBY 3
N2XSEBY 3
N2XSEBY 3
N2XSEBY 3
N2XSEBY 3
N2XSEBY 3

x 25
x 35
x 50
x 70
x 95
x 120

IN
IN
Max DC
GROU AIR
Cable
resistance
ND 20
30
Ampacities
O
at 20 C
O
O
C
C
(Amp)
(/km)
(Amp) Amp)
145
141
0,727
91,65
173
168
0,524
109,2
205
201
0,387
130,65
248
250
0,268
162,5
293
299
0,193
194,35
330
340
0,153
221

7
8
9
10
11

6 kV/10 kV
6 kV/10 kV
6 kV/10 kV
6 kV/10 kV
6 kV/10 kV

N2XSEBY 3
N2XSEBY 3
N2XSEBY 3
N2XSEBY 3
N2XSEBY 3

x 150
x 185
x 240
x 300
x 400

367
409
459
503
552

385
434
496
548
607

0,124
0,099
0,0754
0,0601
0,047

250,25
282,1
322,4
356,2
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
3
(kW)

Current
of
Motor
(Im)

Ampacities
of Cable( Ia)

Core
size
(sq.mm)

Cable
size
(sq.mm)

Ratio
Im/Ia

Safety
Factor
(%)

Justify for
Safety
Factor
(25%)

1
2
3
4
5
6
7
8
9
10
11
12
13
14

4
5,5
7,5
11
15
18,5
22
25
30
37,5
45
55
75
95

6,80
9,35
12,75
18,70
25,50
31,45
37,40
42,50
51,00
63,75
76,50
93,51
127,51
161,51

22,4
28,7
37,1
37,1
49,7
49,7
67,2
91
91
111,3
111,3
135,1
171,5
244,3

0,30
0,33
0,34
0,50
0,51
0,63
0,56
0,47
0,56
0,57
0,69
0,69
0,74
0,66

4 x 2,5
4x4
4x6
4x6
4 x 10
4 x 10
4 x 16
4 x 25
4 x 25
4 x 35
4 x 35
4 x 50
4 x 70
3 x 120

0,30
0,33
0,34
0,50
0,51
0,63
0,56
0,47
0,56
0,57
0,69
0,69
0,74
0,66

70
67
66
50
49
37
44
53
44
43
31
31
26
34

ok
ok
ok
ok
ok
ok
ok
ok
ok
ok
ok
ok
ok
ok

15

110

187,01

292

0,64

3 x 150

0,64

36

ok

16
17
18
19
20

132
160
200
250
315

224,41
272,02
340,02
425,03
535,53

324,8
386,4
509,54
649,6
772,8

0,69
0,70
0,67
0,65
0,69

3 x185
3 x240
2x3x120
2X3x185
2X3X240

0,69
0,70
0,67
0,65
0,69

31
30
33
35
31

ok
ok
ok
ok
ok

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

BASE ON CABLE AMPACITY
No

Cap
Motor
3
(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
2
3
4
5
6
7
8
9
10
11
12
13
14

4
5,5
7,5
11
15
18,5
22
25
30
37,5
45
55
75
95

6,80
9,35
12,75
18,70
25,50
31,45
37,40
42,50
51,00
63,75
76,50
93,51
127,51
161,51

32
41
53
53
71
71
96
130
130
159
159
193
245
349

0,21
0,23
0,24
0,35
0,36
0,44
0,39
0,33
0,39
0,40
0,48
0,48
0,52
0,46

4 x 2,5
4x4
4x6
4x6
4 x 10
4 x 10
4 x 16
4 x 25
4 x 25
4 x 35
4 x 35
4 x 50
4 x 70
3 x 120

0,21
0,23
0,24
0,35
0,36
0,44
0,39
0,33
0,39
0,40
0,48
0,48
0,52
0,46

79
77
76
65
64
56
61
67
61
60
52
52
48
54

ok
ok
ok
ok
ok
ok
ok
ok
ok
ok
ok
ok
ok
ok

15

110

187,01

400

0,47

3 x 150

0,47

53

ok

16
17
18
19
20

132
160
200
250
315

224,41
272,02
340,02
425,03
535,53

464
552
698
928
1104

0,48
0,49
0,49
0,46
0,49

3 x185
3 x240
2x3x120
2X3x185
2X3X240

0,48
0,49
0,49
0,46
0,49

52
51
51
54
51

ok
ok
ok
ok
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

No

Rated
Current of
Feeder/Mot
or (Ifm)

Ampacities
of Cable
( Ia)

Core size
(sq.mm)

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32

16,80
28,80
38,40
39,20
40,00
42,40
57,60
57,60
121,60
137,60
183,20
200,80
211,20
229,60
275,20
275,20
275,20
324,00
344,00
630,00
613,60
630,40
739,20
872,00
1250,00
1132,80
1176,00
1600,00
1516,80
2243,20
3150,00
3670,40

91,25
91,25
91,25
91,25
91,25
91,25
91,25
113,75
130,65
130,65
264
194,35
194,35
264
267,8
261,3
327,6
299
299
299
299
342,55
342,55
403
403
403
403
473,85
473,85
473,85
473,85
473,85

0,18
0,32
0,42
0,43
0,44
0,46
0,63
0,51
0,93
1,05
0,69
1,03
1,09
0,87
1,03
1,05
0,84
1,08
1,15
2,11
2,05
1,84
2,16
2,16
3,10
2,81
2,92
3,38
3,20
4,73
6,65
7,75

Cable size
(sq.mm)

N2XSEBY 3 x 25
N2XSEBY 3 x 25
N2XSEBY 3 x 25
N2XSEBY 3 x 25
N2XSEBY 3 x 25
N2XSEBY 3 x 25
N2XSEBY 3 x 25
N2XSEBY 3 x 25
N2XSEBY 3 x 25
N2XSEBY 3 x 25
N2XSY 3 x 1 x 120
N2XSEBY 3 x 95
N2XSEBY 3 x 95
N2XSY 3 x 1 x 120
N2XSY 3 x 1 x 120
N2XSEBY 2 x 3 x 50
N2XSEBY 3 x 3 x 35
N2XSY 3 x 1 x 150
N2XSY 3 x 1 x 150
N2XSY 3 x 2 x 1 x 150
N2XSY 3 x 2 x 1 x 150
N2XSY 3 x 2 x 1 x 185
N2XSY 3 x 2 x 1 x 185
N2XSY 3 X 2 x 1 X240
N2XSY 3 X 2 x 1 X240
N2XSY 3 X 2 x 1 X240
N2XSY 3 X 2 x 1 X240
N2XSY 3 x 3 x 1 x 300
N2XSY 3 x 3 x 1 x 300
N2XSY 3 x 5 x 1 x 300
N2XSY 3 x 7 x 1 x 300
N2XSY 3 x 8 x 1 x 300

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

Rated
Curren
t of
Cable
(Ia)

Ratio
Current
(Ifm/Ia)

Safety
Factor

141,00
141,00
141,00
141,00
141,00
141,00
141,00
175,00
201,00
201,00
407
299,00
299,00
407
412
402
504
460
460
460,00
460,00
527
527
621,00
621,00
621
621,00
729
729
729
729
729

0,12
0,20
0,27
0,28
0,28
0,30
0,41
0,33
0,60
0,68
0,45
0,67
0,71
0,56
0,67
0,68
0,55
0,70
0,75
0,65
0,67
0,60
0,70
0,70
0,67
0,61
0,63
0,73
0,69
0,62
0,62
0,63

88,09
79,57
72,77
72,20
71,63
69,93
59,15
67,09
39,50
31,54
54,99
32,84
29,36
43,59
33,20
31,54
45,40
29,57
25,22
35,00
33,30
40,19
29,87
29,79
32,90
39,19
36,88
26,84
30,64
38,46
38,27
37,06

Justify
for
Safety
Factor
(25%)
ok
ok
ok
ok
ok
ok
ok
ok
ok
ok
ok
ok
ok
ok
ok
ok
ok
ok
ok
ok
ok
ok
ok
ok
ok
ok
ok
ok
ok
ok
ok
ok