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MV Cables Tabulated continuous current ratings

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Annex B
(informative)
 
Tabulated continuous current ratings for cables having extruded insulation and a rated voltage from 3,6/6 kV up to 18/30 kV
 

B.1 General

 
This annex deals solely with the steady-state continuous current ratings of single-core and three-core cables having extruded insulation. The tabulated current ratings provided in this annex have been calculated for cables having a rated voltage of 6/10 kV and constructions as detailed in Clause B.2.
 
These ratings can be applied to cables of similar constructions in the voltage range of 3,6/6 kV to 18/30 kV.
 
Some parameters such as screen cross-sectional area and oversheath thickness have an influence on the rating of large cables. In addition, the method of screen bonding has to be taken into account in the rating of single-core cables.
 
The tabulated current ratings have been calculated using the methods set out in IEC 60287.
 
NOTE 1 For cyclic current ratings, see IEC 60853.
NOTE 2 For short-circuit temperature limits, see IEC 60986.
 
 
B.2 Cable constructions
 
The cable constructions and dimensions for which current ratings have been tabulated are based on those given in this standard. The constructions and dimensions used are not related to specific national designs but reflect different model cables. Armoured three-core cables are assumed to have flat wire armour and single-core cables are assumed to be unarmoured. All the cables have copper tape core screens except the single-core XLPE insulated cable, which has a copper wire screen. The nominal cross-sectional areas of the screens for the model cables is given in Table B.1.
 
 
Table B.1 – Nominal screen cross-sectional areas
 
Nominal area of conductor, mm²
 
16
 
25
 
35
 
50
 
70
 
95
 
120
 
150
 
185
 
240
 
300
 
400
Nominal cross-sectional area of screen, per core, mm²
EPR insulated cable
3
3
4
4
4
5
5
5
6
6
7
8
XLPE insulated cable
16
16
16
16
16
16
16
25
25
25
25
35
 
The oversheath is taken to be polyethylene for the single core cables and PVC for the three- core cables.
 
 
 
 B.3 Temperatures
 
The maximum conductor temperature for which the tabulated cable ratings have been calculated is 90 °C.
 
The reference ambient temperatures assumed are as follows:
 
– for cables in air: 30 °C
– for buried cables, either directly in the soil
or in ducts in the ground: 20 °C
Correction factors for other ambient temperatures are given in Tables B.10 and B.11.
 
The current ratings for cables in air do not take account of the increase, if any, due to solar or other infra-red radiation. Where the cables are subject to such radiation, the current rating should be derived by the methods specified in IEC 60287.
 
 
B.4 Soil thermal resistivity
 
 
The tabulated current ratings for cables in ducts or direct in the ground relate to a soil thermal resistivity of 1,5 K.m/W. Information on the likely soil thermal resistivity in various
countries is given in IEC 60287-3-1. Correction factors for other values of thermal resistivity are given in Tables B.14 to B.17.
 
It is assumed that the soil properties are uniform, no allowance has been made for the possibility of moisture migration which can lead to a region of high thermal resistivity around the cable. If partial drying-out of the soil is foreseen, the permissible current rating should be derived by the methods specified in IEC 60287.
 
 
B.5 Methods of installation
 
Current ratings are tabulated for cables installed in the following conditions.
 
 
B.5.1 Single-core cables in air
 
The cables are assumed to be spaced at least 0,5 times the cable diameter from any vertical surface and installed on brackets or ladder racks as follows:
 
a) three cables in trefoil formation touching throughout their length;
b) three cables in horizontal flat formation touching throughout their length;
c) three cables in horizontal flat formation with a clearance of one cable diameter.
 
 
where De is the external diameter of the cable.
 
Figure B.1 – Single-core cables in air
  
 
B.5.2 Single-core cables buried direct
 
Current ratings are given for cables buried direct in the ground at a depth of 0,8 m under the following conditions:
 
a) three cables in trefoil formation touching throughout their length;
b) three cables in horizontal flat formation with a clearance of one cable diameter, De. a) b)
 

 

 
Figure B.2 – Single-core cables buried direct
 
 
 The cable depth is measured to the cable axis or centre of the trefoil group.
 
 
B.5.3 Single-core cables in earthenw are ducts
 
Current ratings are given for cables in earthenware ducts buried at a depth of 0,8 m with one cable per duct as follows:
 
a) three cables in trefoil ducts touching throughout their length;
b) three cables in horizontal flat formation, ducts touching throughout their length. a) b)
 
 
 
 
Figure B.3 – Single-core cables in earthenw are ducts
 
 
The ducts are assumed to be earthenware having an inside diameter of 1,5 times the outside diameter of the cable and a wall thickness equal to 6 % of the duct inside diameter. The ratings are based on the assumption that the ducts are air filled. If the ducts have been filled with a material such as Bentonite, then it is usual to adopt the current ratings for cables buried direct.
 
The tabulated ratings may be applied to cables in ducts having an inside diameter of between
1,2 and 2 times the outside diameter of the cables. For this range of diameters the variation in the rating is less than 2 % of the tabulated value.
 
 
B.5.4 Three-core cables
 
Current ratings are given for three-core cables installed under the following conditions:
 
a) single cable in air spaced at least 0,3 times the cable diameter from any vertical surface;
b) single cable buried direct in the ground at a depth of 0,8 m;
c) single cable in a buried earthenware duct having dimensions calculated in the same manner as for the single-core cables in ducts. The depth of burial of the duct is 0,8 m.
a) b) c)
 
 
 
Figure B.4 – Three-core cables
 
 
 
B.6 Screen bonding
 
All the tabulated ratings for single-core cables assume that the cable screens are solidly bonded, i.e. bonded at both ends of the cables.
 
 
B.7 Cable loading
 
The tabulated ratings relate to circuits carrying a balanced three-phase load at a rated frequency of 50 Hz.
 
 
B.8 Rating factors for grouped circuits
 
The tabulated current ratings apply to a set of three single-core cables or one three-core cable forming a three-phase circuit. When a number of circuits are installed in close proximity the rating should be reduced by the appropriate factor from Tables B.18 to B.23.
 
These rating factors should also be applied to groups of parallel cables forming the same circuit. In such cases, attention should also be given to the arrangement of the cables to ensure that the load current is shared equally between the parallel cables.
 
 
B.9 Correction factors
 
The correction factors given in Tables B.10 to B.23 for temperature, installation conditions and grouping are averages over a range of conductor sizes and cable types. For particular cases, the correction factor may be calculated using the methods in IEC 60287-2-1.
 
 
 
 
Table B.2 – Current ratings for single-core cables w ith XLPE insulation – Rated voltage 3,6/6 kV to 18/30 kV * –
Copper conductor
 
 
 
 
 
Nominal area of
conductor
Buried direct
in the ground
In single-w ay ducts
In air
 
Trefoil
 
Flat spaced
 
Trefoil ducts
Flat
touching ducts
 
Trefoil
 
Flat
touching
 
Flat spaced
           
 
 
mm2
A
A
A
A
A
A
A
16
 
25
 
35
 
 
50
 
70
 
95
 
 
120
 
150
 
185
 
 
240
 
300
 
400
109
 
140
 
166
 
 
196
 
239
 
285
 
 
323
 
361
 
406
 
 
469
 
526
 
590
113
 
144
 
172
 
 
203
 
246
 
293
 
 
332
 
366
 
410
 
 
470
 
524
 
572
103
 
132
 
157
 
 
186
 
227
 
271
 
 
308
 
343
 
387
 
 
447
 
504
 
564
104
 
133
 
159
 
 
188
 
229
 
274
 
 
311
 
347
 
391
 
 
453
 
510
 
571
125
 
163
 
198
 
 
238
 
296
 
361
 
 
417
 
473
 
543
 
 
641
 
735
 
845
128
 
167
 
203
 
 
243
 
303
 
369
 
 
426
 
481
 
550
 
 
647
 
739
 
837
150
 
196
 
238
 
 
286
 
356
 
434
 
 
500
 
559
 
637
 
 
745
 
846
 
938
Maximum conductor temperature 90 °C
Ambient air temperature 30 °C Ground temperature 20 °C Depth of laying 0,8 m
Thermal resistivity of soil 1,5 K.m/W
Thermal resistivity of earthenware ducts 1,2 K.m/W Screens bonded at both ends.
* Current rating calculated for cables having a rated voltage of 6/10 kV.
 
 
 
Table B.3 – Current ratings for single-core cables w ith XLPE insulation – Rated voltage 3,6/6 kV to 18/30 kV * –
Aluminium conductor
 
 
 
 
 
Nominal area of
conductor
Buried direct in the ground
 
In single-w ay ducts
 
In air
 
Trefoil
 
Flat spaced
 
Trefoil ducts
Flat
touching ducts
 
Trefoil
 
Flat
touching
 
Flat spaced
         
 
 
 
 
mm2
A
A
A
A
A
A
A
16
 
25
 
35
 
 
50
 
70
 
95
 
 
120
 
150
 
185
 
 
240
 
300
 
400
84
 
108
 
129
 
 
152
 
186
 
221
 
 
252
 
281
 
317
 
 
367
 
414
 
470
88
 
112
 
134
 
 
157
 
192
 
229
 
 
260
 
288
 
324
 
 
373
 
419
 
466
80
 
102
 
122
 
 
144
 
176
 
210
 
 
240
 
267
 
303
 
 
351
 
397
 
451
81
 
103
 
123
 
 
146
 
178
 
213
 
 
242
 
271
 
307
 
 
356
 
402
 
457
97
 
127
 
154
 
 
184
 
230
 
280
 
 
324
 
368
 
424
 
 
502
 
577
 
673
99
 
130
 
157
 
 
189
 
236
 
287
 
 
332
 
376
 
432
 
 
511
 
586
 
676
116
 
153
 
185
 
 
222
 
278
 
338
 
 
391
 
440
 
504
 
 
593
 
677
 
769
Maximum conductor temperature 90 °C
Ambient air temperature 30 °C Ground temperature 20 °C Depth of laying 0,8 m
Thermal resistivity of soil 1,5 K.m/W
Thermal resistivity of earthenware ducts 1,2 K.m/W Screens bonded at both ends.
* Current rating calculated for cables having a rated voltage of 6/10 kV.
 
 
 
 
Table B.4 – Current ratings for single-core cables w ith EPR insulation – Rated voltage 3,6/6 kV to 18/30 kV * –
Copper conductor
 
 
 
 
 
Nominal area of
conductor
Buried direct in the ground
 
In single-w ay ducts
 
In air
 
Trefoil
 
Flat spaced
 
Trefoil ducts
Flat
touching ducts
 
Trefoil
 
Flat
touching
 
Flat spaced
             
mm2
A
A
A
A
A
A
A
16
 
25
 
35
 
 
50
 
70
 
95
 
 
120
 
150
 
185
 
 
240
 
300
 
400
106
 
136
 
162
 
 
192
 
234
 
280
 
 
319
 
357
 
403
 
 
467
 
526
 
597
109
 
140
 
167
 
 
198
 
242
 
289
 
 
329
 
369
 
417
 
 
484
 
545
 
618
99
 
128
 
153
 
 
181
 
222
 
266
 
 
303
 
341
 
386
 
 
449
 
509
 
580
100
 
129
 
154
 
 
183
 
224
 
269
 
 
306
 
344
 
390
 
 
454
 
515
 
588
116
 
153
 
186
 
 
224
 
280
 
343
 
 
398
 
454
 
522
 
 
619
 
712
 
825
119
 
156
 
190
 
 
229
 
287
 
352
 
 
407
 
465
 
534
 
 
634
 
728
 
843
138
 
181
 
221
 
 
266
 
334
 
409
 
 
474
 
540
 
621
 
 
736
 
843
 
977
Maximum conductor temperature 90 °C
Ambient air temperature 30 °C Ground temperature 20 °C Depth of laying 0,8 m
Thermal resistivity of soil 1,5 K.m/W
Thermal resistivity of earthenware ducts 1,2 K.m/W Screens bonded at both ends.
* Current rating calculated for cables having a rated voltage of 6/10 kV.
 
 
Table B.5 – Current ratings for single-core cables w ith EPR insulation – Rated voltage 3,6/6 kV to 18/30 kV * –
Aluminium conductor
 
 
 
 
 
Nominal area of
conductor
Buried direct in the ground
 
In single-w ay ducts
 
In air
 
Trefoil
 
Flat spaced
 
Trefoil ducts
Flat
touching ducts
 
Trefoil
 
Flat
touching
 
Flat spaced
             
mm2
A
A
A
A
A
A
A
16
 
25
 
35
 
 
50
 
70
 
95
 
 
120
 
150
 
185
 
 
240
 
300
 
400
82
 
105
 
126
 
 
149
 
182
 
217
 
 
247
 
277
 
314
 
 
364
 
411
 
471
84
 
109
 
130
 
 
153
 
188
 
224
 
 
256
 
287
 
325
 
 
377
 
426
 
487
77
 
99
 
118
 
 
140
 
172
 
206
 
 
235
 
264
 
300
 
 
350
 
397
 
456
78
 
100
 
120
 
 
142
 
174
 
208
 
 
238
 
267
 
303
 
 
354
 
401
 
462
90
 
119
 
144
 
 
174
 
218
 
266
 
 
309
 
352
 
406
 
 
483
 
556
 
651
92
 
121
 
147
 
 
178
 
223
 
273
 
 
317
 
361
 
417
 
 
495
 
570
 
667
107
 
141
 
171
 
 
207
 
259
 
317
 
 
368
 
419
 
484
 
 
575
 
659
 
770
Maximum conductor temperature 90 °C Ambient air temperature 30 °C Ground temperature 20 °C Depth of laying 0,8 m
Thermal resistivity of soil 1,5 K.m/W
Thermal resistivity of earthenware ducts 1,2 K.m/W Screens bonded at both ends.
* Current rating calculated for cables having a rated voltage of 6/10 kV.
 
 
Table B.6 – Current rating for three-core XLPE insulated cables – Rated voltage 3,6/6 kV to 18/30 kV * –
Copper conductor, armoured and unarmoured
 
 
 
 
 
Nominal area of conductor
Unarmoured
Armoured
Buried direct in ground
In a buried duct
 
In air
Buried direct in ground
In a buried duct
 
In air
           
mm2
A
A
A
A
A
A
16
 
25
 
35
 
50
 
 
70
 
95
 
120
 
150
 
 
185
 
240
 
300
 
400
101
 
129
 
153
 
181
 
 
221
 
262
 
298
 
334
 
 
377
 
434
 
489
 
553
87
 
112
 
133
 
158
 
 
193
 
231
 
264
 
297
 
 
336
 
390
 
441
 
501
109
 
142
 
170
 
204
 
 
253
 
304
 
351
 
398
 
 
455
 
531
 
606
 
696
101
 
129
 
154
 
181
 
 
220
 
263
 
298
 
332
 
 
374
 
431
 
482
 
541
88
 
112
 
134
 
158
 
 
194
 
232
 
264
 
296
 
 
335
 
387
 
435
 
492
110
 
143
 
172
 
205
 
 
253
 
307
 
352
 
397
 
 
453
 
529
 
599
 
683
Maximum conductor temperature 90 °C
Ambient air temperature 30 °C Ground temperature 20 °C Depth of laying 0,8 m
Thermal resistivity of soil 1,5 K.m/W
Thermal resistivity of earthenware ducts 1,2 K.m/W
* Current rating calculated for cables having a rated voltage of 6/10 kV.
 
 
 
Table B.7 – Current rating for three-core XLPE insulated cables – Rated voltage 3,6/6 kV to 18/30 kV * –
Aluminium conductor, armoured and unarmoured
 
 
 
 
 
Nominal area of conductor
Unarmoured
Armoured
Buried direct in ground
In a buried duct
 
In air
Buried direct in ground
In a buried duct
 
In air
           
mm2
A
A
A
A
A
A
16
 
25
 
35
 
50
 
 
70
 
95
 
120
 
150
 
 
185
 
240
 
300
 
400
78
 
100
 
119
 
140
 
 
171
 
203
 
232
 
260
 
 
294
 
340
 
384
 
438
67
 
87
 
103
 
122
 
 
150
 
179
 
205
 
231
 
 
262
 
305
 
346
 
398
84
 
110
 
132
 
158
 
 
196
 
236
 
273
 
309
 
 
355
 
415
 
475
 
552
78
 
100
 
119
 
140
 
 
171
 
204
 
232
 
259
 
 
293
 
338
 
380
 
432
68
 
87
 
104
 
123
 
 
150
 
180
 
206
 
231
 
 
262
 
304
 
343
 
393
85
 
111
 
133
 
159
 
 
196
 
238
 
274
 
309
 
 
354
 
415
 
472
 
545
Maximum conductor temperature 90 °C
Ambient air temperature 30 °C Ground temperature 20 °C Depth of laying 0,8 m
Thermal resistivity of soil 1,5 K.m/W
Thermal resistivity of earthenware ducts 1,2 K.m/W
* Current rating calculated for cables having a rated voltage of 6/10 kV.
 
 
 
Table B.8 – Current rating for three-core EPR insulated cables – Rated voltage 3,6/6 kV to 18/30 kV * –
Copper conductor, armoured and unarmoured
 
 
 
 
 
Nominal area of conductor
Unarmoured
Armoured
Buried direct in ground
In a buried duct
 
In air
Buried direct in ground
In a buried duct
 
In air
   
 
     
mm2
A
A
A
A
A
A
16
 
25
 
35
 
50
 
 
70
 
95
 
120
 
150
 
 
185
 
240
 
300
 
400
98
 
125
 
150
 
176
 
 
216
 
258
 
292
 
328
 
 
371
 
429
 
482
 
545
84
 
109
 
130
 
154
 
 
189
 
227
 
258
 
291
 
 
330
 
384
 
434
 
494
104
 
135
 
164
 
195
 
 
243
 
296
 
339
 
385
 
 
441
 
519
 
590
 
678
98
 
125
 
150
 
177
 
 
216
 
257
 
292
 
327
 
 
368
 
424
 
475
 
534
85
 
109
 
131
 
155
 
 
190
 
227
 
259
 
291
 
 
328
 
381
 
429
 
485
104
 
136
 
164
 
197
 
 
244
 
296
 
339
 
385
 
 
439
 
513
 
583
 
666
Maximum conductor temperature 90 °C
Ambient air temperature 30 °C Ground temperature 20 °C Depth of laying 0,8 m
Thermal resistivity of soil 1,5 K.m/W
Thermal resistivity of earthenware ducts 1,2 K.m/W
* Current rating calculated for cables having a rated voltage of 6/10 kV.
 
 
Table B.9 – Current rating for three-core EPR insulated cables – Rated voltage 3,6/6 kV to 18/30 kV * –
Aluminium conductor, armoured and unarmoured
 
 
 
 
 
Nominal area of conductor
Unarmoured
Armoured
Buried direct in ground
In a buried duct
 
In air
Buried direct in ground
In a buried duct
 
In air
           
mm2
A
A
A
A
A
A
16
 
25
 
35
 
50
 
 
70
 
95
 
120
 
150
 
 
185
 
240
 
300
 
400
76
 
97
 
116
 
137
 
 
167
 
200
 
227
 
255
 
 
289
 
335
 
378
 
432
65
 
84
 
101
 
119
 
 
147
 
176
 
201
 
226
 
 
257
 
300
 
340
 
392
80
 
105
 
127
 
151
 
 
189
 
229
 
263
 
299
 
 
343
 
406
 
462
 
538
76
 
97
 
116
 
137
 
 
168
 
200
 
227
 
254
 
 
288
 
332
 
374
 
426
66
 
85
 
101
 
120
 
 
147
 
176
 
201
 
226
 
 
257
 
299
 
338
 
387
81
 
105
 
127
 
153
 
 
190
 
230
 
264
 
300
 
 
343
 
402
 
459
 
530
Température maximale de l’âme 90 °C
Température ambiante 30 °C Température du sol 20 °C Profondeur de pose 0,8 m
Résistivité thermique du sol 1,5 K.m/W
Résistivité thermique du conduit en terre cuite 1,2 K.m/W
* Current rating calculated for cables having a rated voltage of 6/10 kV.
 
 
 
Table B.10 – Correction factors for ambient air temperatures other than 30 °C
 
Maximum conductor temperature
°C
Ambient air temperature
°C
20
25
35
40
45
50
55
60
90
1,08
1,04
0,96
0,91
0,87
0,82
0,76
0,71
 
 
 
Table B.11 – Correction factors for ambient ground temperatures other than 20 °C
 
Maximum conductor temperature
°C
Ambient ground temperature
°C
10
15
25
30
35
40
45
50
90
1,07
1,04
0,96
0,93
0,89
0,85
0,80
0,76
 
 
Table B.12 – Correction factors for depths of laying other than 0,8 m for direct buried cables
 
 
 
Depth of laying
m
Single-core cables
 
 
Three-core cables
Nominal conductor size
mm2
185 mm2
>185 mm2
0,5
 
0,6
 
1
 
1,25
 
1,5
 
1,75
 
2
 
2,5
 
3
1,04
 
1,02
 
0,98
 
0,96
 
0,95
 
0,94
 
0,93
 
0,91
 
0,90
1,06
 
1,04
 
0,97
 
0,95
 
0,93
 
0,91
 
0,90
 
0,88
 
0,86
1,04
 
1,03
 
0,98
 
0,96
 
0,95
 
0,94
 
0,93
 
0,91
 
0,90
 
 
 
 
Table B.13 – Correction factors for depths of laying other than 0,8 m for cables in ducts
 
 
 
Depth of laying
m
Single-core cables
 
Three-core cable
Nominal conductor size
mm2
185 mm2
>185 mm2
0,5
 
0,6
 
1
 
1,25
 
1,5
 
1,75
 
2
 
2,5
 
3
1,04
 
1,02
 
0,98
 
0,96
 
0,95
 
0,94
 
0,93
 
0,91
 
0,90
1,05
 
1,03
 
0,97
 
0,95
 
0,93
 
0,92
 
0,91
 
0,89
 
0,88
1,03
 
1,02
 
0,99
 
0,97
 
0,96
 
0,95
 
0,94
 
0,93
 
0,92
 
 
Table B.14 – Correction factors for soil thermal resistivities other than 1,5 K.m/W for direct buried single-core cables
 
Nominal area of
conductor
 
mm2
Values of soil thermal resistivity
  1. K.m/W
 
0,7
 
0,8
 
0,9
 
1
 
2
 
2,5
 
3
16
 
25
 
35
 
50
 
70
 
95
 
120
 
150
 
185
 
240
 
300
 
400
1,29
 
1,30
 
1,30
 
1,32
 
1,33
 
1,34
 
1,34
 
1,35
 
1,35
 
1,36
 
1,36
 
1,37
1,24
 
1,25
 
1,25
 
1,26
 
1,27
 
1,28
 
1,28
 
1,28
 
1,29
 
1,29
 
1,30
 
1,30
1,19
 
1,20
 
1,21
 
1,21
 
1,22
 
1,22
 
1,22
 
1,23
 
1,23
 
1,23
 
1,24
 
1,24
1,15
 
1,16
 
1,16
 
1,16
 
1,17
 
1,18
 
1,18
 
1,18
 
1,18
 
1,18
 
1,19
 
1,19
0,89
 
0,89
 
0,89
 
0,89
 
0,89
 
0,89
 
0,88
 
0,88
 
0,88
 
0,88
 
0,88
 
0,88
0,82
 
0,81
 
0,81
 
0,81
 
0,81
 
0,80
 
0,80
 
0,80
 
0,80
 
0,80
 
0,80
 
0,79
0,75
 
0,75
 
0,75
 
0,74
 
0,74
 
0,74
 
0,74
 
0,74
 
0,74
 
0,73
 
0,73
 
0,73
 
 
Table B.15 – Correction factors for soil thermal resistivities other than 1,5 K.m/W single-core cables in buried ducts
 
Nominal area of
conductor
 
mm2
Values of soil thermal resistivity
  1. K.m/W
 
0,7
 
0,8
 
0,9
 
1
 
2
 
2,5
 
3
16
 
25
 
35
 
50
 
70
 
95
 
120
 
150
 
185
 
240
 
300
 
400
1,20
 
1,21
 
1,21
 
1,21
 
1,22
 
1,23
 
1,23
 
1,24
 
1,24
 
1,25
 
1,25
 
1,25
1,17
 
1,17
 
1,18
 
1,18
 
1,19
 
1,19
 
1,20
 
1,20
 
1,20
 
1,21
 
1,21
 
1,21
1,14
 
1,14
 
1,15
 
1,15
 
1,15
 
1,16
 
1,16
 
1,16
 
1,17
 
1,17
 
1,17
 
1,17
1,11
 
1,12
 
1,12
 
1,12
 
1,12
 
1,13
 
1,13
 
1,13
 
1,13
 
1,14
 
1,14
 
1,14
0,92
 
0,91
 
0,91
 
0,91
 
0,91
 
0,91
 
0,91
 
0,91
 
0,91
 
0,90
 
0,90
 
0,90
0,85
 
0,85
 
0,84
 
0,84
 
0,84
 
0,84
 
0,84
 
0,83
 
0,83
 
0,83
 
0,83
 
0,83
0,79
 
0,79
 
0,79
 
0,78
 
0,78
 
0,78
 
0,78
 
0,78
 
0,78
 
0,77
 
0,77
 
0,77
 
 
 
Table B.16 – Correction factors for soil thermal resistivities other than 1,5 K.m/W for direct buried three-core cables
 
Nominal area of conductor
mm2
Values of soil thermal resistivity
Km/W
0,7
0,8
0,9
1
2
2,5
3
16
 
25
 
35
 
50
 
70
 
95
 
120
 
150
 
185
 
240
 
300
 
400
1,23
 
1,24
 
1,25
 
1,25
 
1,26
 
1,26
 
1,26
 
1,27
 
1,27
 
1,28
 
1,28
 
1,28
1,19
 
1,20
 
1,21
 
1,21
 
1,21
 
1,22
 
1,22
 
1,22
 
1,23
 
1,23
 
1,23
 
1,23
1,16
 
1,16
 
1,17
 
1,17
 
1,18
 
1,18
 
1,18
 
1,18
 
1,18
 
1,19
 
1,19
 
1,19
1,13
 
1,13
 
1,13
 
1,14
 
1,14
 
1,14
 
1,14
 
1,15
 
1,15
 
1,15
 
1,15
 
1,15
0,91
 
0,91
 
0,91
 
0,91
 
0,90
 
0,90
 
0,90
 
0,90
 
0,90
 
0,90
 
0,90
 
0,90
0,84
 
0,84
 
0,83
 
0,83
 
0,83
 
0,83
 
0,83
 
0,83
 
0,83
 
0,83
 
0,82
 
0,82
0,78
 
0,78
 
0,78
 
0,77
 
0,77
 
0,77
 
0,77
 
0,77
 
0,77
 
0,77
 
0,77
 
0,76
 
 
Table B.17 – Correction factors for soil thermal resistivities other than 1,5 K•m/W for three-core cables in ducts
 
Nominal area of
conductor
 
Values of soil thermal resistivity
Km/W
mm2
0,7
0,8
0,9
1
2
2,5
3
16
 
25
 
35
 
50
 
70
 
95
 
120
 
150
 
185
 
240
 
300
 
400
1,12
 
1,14
 
1,14
 
1,14
 
1,15
 
1,15
 
1,15
 
1,16
 
1,16
 
1,16
 
1,17
 
1,17
1,11
 
1,12
 
1,12
 
1,12
 
1,13
 
1,13
 
1,13
 
1,13
 
1,14
 
1,14
 
1,14
 
1,14
1,09
 
1,10
 
1,10
 
1,10
 
1,11
 
1,11
 
1,11
 
1,11
 
1,11
 
1,12
 
1,12
 
1,12
1,08
 
1,08
 
1,08
 
1,08
 
1,09
 
1,09
 
1,09
 
1,09
 
1,09
 
1,10
 
1,10
 
1,10
0,94
 
0,94
 
0,94
 
0,94
 
0,94
 
0,94
 
0,93
 
0,93
 
0,93
 
0,93
 
0,93
 
0,92
0,89
 
0,89
 
0,88
 
0,88
 
0,88
 
0,88
 
0,88
 
0,88
 
0,87
 
0,87
 
0,87
 
0,86
0,84
 
0,84
 
0,84
 
0,84
 
0,83
 
0,83
 
0,83
 
0,83
 
0,83
 
0,82
 
0,82
 
0,81
 
 
 
Table B.18 – Correction factors for groups of three-core cables in horizontal formation laid direct in the ground
 
 
Number of cables in group
Spacing betw een cable centres
mm
Touching
200
400
600
800
2
 
3
 
4
 
5
 
6
 
7
 
8
 
9
 
10
 
11
 
12
0,80
 
0,69
 
0,62
 
0,57
 
0,54
 
0,51
 
0,49
 
0,47
 
0,46
 
0,45
 
0,43
0,86
 
0,77
 
0,72
 
0,68
 
0,65
 
0,63
 
0,61
 
0,60
 
0,59
 
0,57
 
0,56
0,90
 
0,82
 
0,79
 
0,76
 
0,74
 
0,72
 
0,71
 
0,70
 
0,69
 
0,69
 
0,68
0,92
 
0,86
 
0,83
 
0,81
 
0,80
 
0,78
 
0,78
 
0,77
 
-
 
-
 
-
0,94
 
0,89
 
0,87
 
0,85
 
0,84
 
0,83
 
 
-
 
-
 
-
 
-
 
 
 
Table B.19 – Correction factors for groups of three-phase circuits of single-core cables laid direct in the ground
 
Number of cables in group
Spacing betw een group centres
mm
Touching
200
400
600
800
2
 
3
 
4
 
5
 
6
 
 
7
 
8
 
9
 
10
 
11
 
12
0,73
 
0,60
 
0,54
 
0,49
 
0,46
 
 
0,43
 
0,41
 
0,39
 
0,37
 
0,36
 
0,35
0,83
 
>0,73
 
0,68
 
0,63
 
0,61
 
 
0,58
 
0,57
 
0,55
 
0,54
 
0,53
 
0,52
0,88
 
0,79
 
0,75
 
0,72
 
0,70
 
 
0,68
 
0,67
 
0,66
 
0,65
 
0,64
 
0,64
0,90
 
0,83
 
0,80
 
0,78
 
0,76
 
 
0,75
 
0,74
 
0,73
 
-
 
-
 
-
0,92
 
0,86
 
0,84
 
0,82
 
0,81
 
 
0,80
 
-
 
-
 
-
 
-
 
-
 
 
Table B.20 – Correction factors for groups of three-core cables in single w ay ducts in horizontal formation
 
 
Number of cables in group
Spacing betw een duct centres
mm
Touching
200
400
600
800
2
 
3
 
4
 
5
 
6
 
 
7
 
8
 
9
 
10
 
11
 
12
0,85
 
0,75
 
0,69
 
0,65
 
0,62
 
 
0,59
 
0,57
 
0,55
 
0,54
 
0,52
 
0,51
0,88
 
0,80
 
0,75
 
0,72
 
0,69
 
 
0,67
 
0,65
 
0,64
 
0,63
 
0,62
 
0,61
0,92
 
0,85
 
0,82
 
0,79
 
0,77
 
 
0,76
 
0,75
 
0,74
 
0,73
 
0,73
 
0,72
0,94
 
0,88
 
0,86
 
0,84
 
0,83
 
 
0,82
 
0,81
 
0,80
 
-
 
-
 
-
0,95
 
0,91
 
0,89
 
0,87
 
0,87
 
 
0,86
 
-
 
-
 
-
 
-
 
-
 
 
 
Table B.21 – Correction factors for groups of three-phase circuits of single-core cables in single-w ay ducts
 
 
Number of cables in group
Spacing betw een duct group centres
mm
Touching
200
400
600
800
2
 
3
 
4
 
5
 
6
 
 
7
 
8
 
9
 
10
 
11
 
12
0,78
 
0,66
 
0,59
 
0,55
 
0,51
 
 
0,48
 
0,46
 
0,44
 
0,43
 
0,42
 
0,40
0,85
 
0,75
 
0,70
 
0,66
 
0,64
 
 
0,61
 
0,60
 
0,58
 
0,57
 
0,56
 
0,55
0,89
 
0,81
 
0,77
 
0,74
 
0,72
 
 
0,71
 
0,70
 
0,69
 
0,68
 
0,67
 
0,67
0,91
 
0,85
 
0,82
 
0,80
 
0,78
 
 
0,77
 
0,76
 
0,76
 
-
 
-
 
-
0,93
 
0,88
 
0,86
 
0,84
 
0,83
 
 
0,82
 
-
 
-
 
-
 
-
 
-
 
 
 
Table B.22 – Reduction factors for groups of more than one multi-core cable in air – To be applied to the current-carrying capacity for one multi-core cable in free air
 
 
Method of installation
 
Number of trays
Number of cables
1
2
3
4
6
9
 
 
 
 
 
 
 
Cables on
perforated trays
 
1
 
 
2
 
 
3
1,00
 
 
1,00
 
 
1,00
0,88
 
 
0,87
 
 
0,86
0,82
 
 
0,80
 
 
0,79
0,79
 
 
0,77
 
 
0,76
0,76
 
 
0,73
 
 
0,71
0,73
 
 
0,68
 
 
0,66
 
 
1
 
 
2
 
 
3
 
1,00
 
 
1,00
 
 
1,00
 
1,00
 
 
0,99
 
 
0,98
 
0,98
 
 
0,96
 
 
0,95
 
0,95
 
 
0,92
 
 
0,91
 
0,91
 
 
0,87
 
 
0,85
 
-
 
 
-
 
 
-
 
 
 
 
 
 
 
Cables on vertical perforated trays
 
 
 
 
1
 
 
2
 
 
1,00
 
 
1,00
 
 
0,88
 
 
0,88
 
 
0,82
 
 
0,81
 
 
0,78
 
 
0,76
 
 
0,73
 
 
0,71
 
 
0,72
 
 
0,70
 
 
 
1
 
 
2
 
 
1,00
 
 
1,00
 
 
0,91
 
 
0,91
 
 
0,89
 
 
0,88
 
 
0,88
 
 
0,87
 
 
0,87
 
 
0,85
 
 
-
 
 
-
 
 
 
 
 
 
 
Cables on ladder supports, cleats, etc.
 
1
 
 
2
 
 
3
1,00
 
 
1,00
 
 
1,00
0,87
 
 
0,86
 
 
0,85
0,82
 
 
0,80
 
 
0,79
0,80
 
 
0,78
 
 
0,76
0,79
 
 
0,76
 
 
0,73
0,78
 
 
0,73
 
 
0,70
 
1
 
 
2
 
 
3
1,00
 
 
1,00
 
 
1,00
1,00
 
 
0,99
 
 
0,98
1,00
 
 
0,98
 
 
0,97
1,00
 
 
0,97
 
 
0,96
1,00
 
 
0,96
 
 
0,93
-
 
 
-
 
 
-
NOTE 1 Values given are averages for the cable types and range of conductor sizes considered. The spread of values is generally less than 5 %.
 
NOTE 2 Factors apply to single layer groups of cables as shown above and do not apply when cables are installed in more than one layer touching each other. Values for such installations may be significantly lower and must be determined by an appropriate method.
 
NOTE 3 Values are given for vertical spacings between trays of 300 mm and at least 20 mm between trays and wall. For closer spacing, the factors should be reduced.
 
NOTE 4 Values are given for horizontal spacing between trays of 225 mm with trays mounted back to back. For closer spacing, the factors should be reduced.
 
 
 
Table B.23 – Reduction factors for groups of more than one circuit of single-core cables (Note 2)
To be applied to the current-carrying capacity for one circuit of single-core cables in free air
 
 
 
Method of installation
 
Number of trays
Number of three-phase circuits (Note 5)
 
Use as a
multiplier to rating for
1
2
3
 
 
Perforated trays
 
(Note 3)
 
1
 
 
2
 
 
3
0,98
 
 
0,96
 
 
0,95
0,91
 
 
0,87
 
 
0,85
0,87
 
 
0,81
 
 
0,78
 
 
Three cables in horizontal
formation
 
 
Ladder
supports, cleats etc.
 
(Note 3)
 
1
 
 
2
 
 
3
1,00
 
 
0,98
 
 
0,97
0,97
 
 
0,93
 
 
0,90
0,96
 
 
0,89
 
 
0,86
 
 
 
Three cables in horizontal
formation
 
 
Perforated trays
 
(Note 3)
 
1
 
 
2
 
 
3
1,00
 
 
0,97
 
 
0,96
0,98
 
 
0,93
 
 
0,92
0,96
 
 
0,89
 
 
0,86
 
 
 
 
 
 
 
 
 
 
Three cables in trefoil formation
 
 
Vertical
perforated trays
 
(Note 4)
 
 
 
1
 
 
2
 
 
1,00
 
 
1,00
 
 
0,91
 
 
0,90
 
 
0,89
 
 
0,86
 
 
Ladder
supports,
cleats, etc.
 
(Note 3)
 
1
 
 
2
 
 
3
1,00
 
 
0,97
 
 
0,96
1,00
 
 
0,95
 
 
0,94
1,00
 
 
0,93
 
 
0,90
NOTE 1 Values given are averages for the cable types and range of conductor sizes considered. The spread of values is generally less than 5 %.
 
NOTE 2 Factors are given for single layers of cables (or trefoil groups) as shown in the table and do not
apply when cables are installed in more than one layer touching each other. Values for such installations may be significantly lower and should be determined by an appropriate method.
 
NOTE 3 Values are given for vertical spacings between trays of 300 mm. For closer spacing, the factors should be reduced.
 
NOTE 4 Values are given for horizontal spacing between trays of 225 mm with trays mounted back to back. For closer spacing, the factors should be reduced.
 
NOTE 5 For circuits having more than one cable in parallel per phase, each three phase set of conductors should be considered as a circuit for the purpose of this table.