MV Cables Tabulated continuous current ratings

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
Maximum conductor 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
Maximum conductor 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 K.m/W
Nominal area of conductor mm2	0,7	0,8	0,9	1	2	2,5	3
16 25 35 50 70 95 120 150 185 240 300 400	1,29 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 K.m/W
Nominal area of conductor mm2	0,7	0,8	0,9	1	2	2,5	3
16 25 35 50 70 95 120 150 185 240 300 400	1,20 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 K•m/W
Nominal area of conductor mm2	0,7	0,8	0,9	1	2	2,5	3
16 25 35 50 70 95 120 150 185 240 300 400	1,23 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 K•m/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
Number of cables in group	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
Number of cables in group	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
Number of cables in group	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
Number of cables in group	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
Method of installation	Number of trays	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
Cables on perforated trays	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
Cables on vertical perforated trays	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
Cables on ladder supports, cleats, etc.	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
Method of installation	Number of trays	1	2	3	Use as a multiplier to rating for
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.

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