# Isc at the receiving end of a feeder as a function of the Isc at its sending end

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The network shown in Figure G41 typifies a case for the application of Figure G42 , derived by the "method of composition" (mentioned in chapter Protection against electric shocks and electrical fires). These tables give a rapid and sufficiently accurate value of short-circuit current at a point in a network, knowing:

• The value of short-circuit current upstream of the point considered
• The length and composition of the circuit between the point at which the short-circuit current level is known, and the point at which the level is to be determined
Fig. G41 – Determination of downstream short-circuit current level Isc

It is then sufficient to select a circuit-breaker with an appropriate short-circuit fault rating immediately above that indicated in the tables.

If more precise values are required, it is possible to make a detailed calculation or to use a software package, such as Ecodial. In such a case, moreover, the possibility of using the cascading technique should be considered, in which the use of a current limiting circuit-breaker at the upstream position would allow all circuit-breakers downstream of the limiter to have a short-circuit current rating much lower than would otherwise be necessary (See Coordination between circuit-breakers).

## Method

Select the c.s.a. of the conductor in the column for copper conductors (in this example the c.s.a. is 50 mm2).

Search along the row corresponding to 50 mm2 for the length of conductor equal to that of the circuit concerned (or the nearest possible on the low side). Descend vertically the column in which the length is located, and stop at a row in the middle section (of the 3 sections of the Figure) corresponding to the known fault-current level (or the nearest to it on the high side).

In this case 30 kA is the nearest to 28 kA on the high side. The value of short-circuit current at the downstream end of the 20 metre circuit is given at the intersection of the vertical column in which the length is located, and the horizontal row corresponding to the upstream Isc (or nearest to it on the high side).

This value in the example is seen to be 14.7 kA.

The procedure for aluminium conductors is similar, but the vertical column must be ascended into the middle section of the table.

In consequence, a DIN-rail-mounted circuit-breaker rated at 63 A and Isc of 25 kA (such as a NG 125N unit) can be used for the 55 A circuit in Figure G41.

A Compact rated at 160 A with an Isc capacity of 25 kA (such as a NSX160 unit) can be used to protect the 160 A circuit.

Fig. G42 – Isc at a point downstream, as a function of a known upstream fault-current value and the length and c.s.a. of the intervening conductors, in a 230/400 V 3-phase system
Copper 230 V / 400 V
c.s.a.of phase conductors (mm2) Length of circuit (in metres)
1.5 1.3 1.8 2.6 3.6 5.2 7.3 10.3 14.6 21
2.5 1.1 1.5 2.1 3.0 4.3 6.1 8.6 12.1 17.2 24 34
4 1.2 1.7 2.4 3.4 4.9 6.9 9.7 13.7 19.4 27 39 55
6 1.8 2.6 3.6 5.2 7.3 10.3 14.6 21 29 41 58 82
10 2.2 3.0 4.3 6.1 8.6 12.2 17.2 24 34 49 69 97 137
16 1.7 2.4 3.4 4.9 6.9 9.7 13.8 19.4 27 39 55 78 110 155 220
25 1.3 1.9 2.7 3.8 5.4 7.6 10.8 15.2 21 30 43 61 86 121 172 243 343
35 1.9 2.7 3.8 5.3 7.5 10.6 15.1 21 30 43 60 85 120 170 240 340 480
50[a] 1.8 2.6 3.6 5.1 7.2 10.2 14.4 20 29 41 58 82 115 163 231 326 461
70 2.7 3.8 5.3 7.5 10.7 15.1 21 30 43 60 85 120 170 240 340
95 2.6 3.6 5.1 7.2 10.2 14.5 20 29 41 58 82 115 163 231 326 461
120 1.6 2.3 3.2 4.6 6.5 9.1 12.9 18.3 26 37 52 73 103 146 206 291 412
150 1.2 1.8 2.5 3.5 5.0 7.0 9.9 14.0 19.8 28 40 56 79 112 159 224 317 448
185 1.5 2.1 2.9 4.2 5.9 8.3 11.7 16.6 23 33 47 66 94 133 187 265 374 529
240 1.8 2.6 3.7 5.2 7.3 10.3 4.6 21 29 41 58 83 117 165 233 330 466 659
300 2.2 3.1 4.4 6.2 8.8 12.4 17.6 25 35 50 70 99 140 198 280 396 561
2x120 2.3 3.2 4.6 6.5 9.1 12.9 18.3 26 37 52 73 103 146 206 292 412 583
2x150 2.5 3.5 5.0 7.0 9.9 14.0 20 28 40 56 79 112 159 224 317 448 634
2x185 2.9 4.2 5.9 8.3 11.7 16.6 23 33 47 66 94 133 187 265 375 530 749
3x120 3.4 4.9 6.9 9.7 13.7 19.4 27 39 55 77 110 155 219 309 438 619
3x150 3.7 5.3 7.5 10.5 14.9 21 30 42 60 84 119 168 238 336 476 672
3x185 4.4 6.2 8.8 12.5 17.6 25 35 50 70 100 141 199 281 398 562
Isc upstream (in kA) Isc downstream (in kA)
100 93 90 87 82 77 70 62 54 45 37 29 22 17.0 12.6 9.3 6.7 4.9 3.5 2.5 1.8 1.3 0.9
90 84 82 79 75 71 65 58 51 43 35 28 22 16.7 12.5 9.2 6.7 4.8 3.5. 2.5 1.8 1.3 0.9
80 75 74 71 68 64 59 54 47 40 34 27 21 16.3 12.2 9.1 6.6 4.8 3.5 2.5 1.8 1.3 0.9
70 66 65 63 61 58 54 49 44 38 32 26 20 15.8 12.0 8.9 6.6 4.8 3.4 2.5 1.8 1.3 0.9
60 57 56 55 53 51 48 44 39 35 29 24 20 15.2 11.6 8.7 6.5 4.7 3.4 2.5 1.8 1.3 0.9
50 48 47 46 45 43 41 38 35 31 27 22 18.3 14.5 11.2 8.5 6.3 4.6 3.4 2.4 1.7 1.2 0.9
40 39 38 38 37 36 34 32 30 27 24 20 16.8 13.5 10.6 8.1 6.1 4.5 3.3 2.4 1.7 1.2 0.9
35 34 34 33 33 32 30 29 27 24 22 18.8 15.8 12.9 10.2 7.9 6.0 4.5 3.3 2.4 1.7 1.2 0.9
30 29 29 29 28 27 27 25 24 22 20 17.3 14.7 12.2 9.8 7.6 5.8 4.4 3.2 2.4 1.7 1.2 0.9
25 25 24 24 24 23 23 22 21 19.1 17.4 15.5 13.4 11.2 9.2 7.3 5.6 4.2 3.2 2.3 1.7 1.2 0.9
20 20 20 19.4 19.2 18.8 18.4 17.8 17.0 16.1 14.9 13.4 11.8 10.1 8.4 6.8 5.3 4.1 3.1 2.3 1.7 1.2 0.9
15 14.8 41.8 14.7 14.5 14.3 14.1 13.7 3.3 12.7 11.9 11.0 9.9 8.7 7.4 6.1 4.9 3.8 2.9 2.2 1.6 1.2 0.9
10 9.9 9.9 9.8 9.8 9.7 9.6 9.4 9.2 8.9 8.5 8.0 7.4 6.7 5.9 5.1 4.2 3.4 2.7 2.0 1.5 1.1 0.8
7 7.0 6.9 6.9 6.9 6.9 6.8 6.7 6.6 6.4 6.2 6.0 5.6 5.2 4.7 4.2 3.6 3.0 2.4 1.9 1.4 1.1 0.8
5 5.0 5.0 5.0 4.9 4.9 4.9 4.9 4.8 4.7 4.6 4.5 4.3 4.0 3.7 3.4 3.0 2.5 2.1 1.7 1.3 1.0 0.8
4 4.0 4.0 4.0 4.0 4.0 3.9 3.9 3.9 3.8 3.7 3.6 3.5 3.3 3.1 2.9 2.6 2.2 1.9 1.6 1.2 1.0 0.7
3 3.0 3.0 3.0 3.0 3.0 3.0 2.9 2.9 2.9 2.9 2.8 2.7 2.6 2.5 2.3 2.1 1.9 1.6 1.4 1.1 0.9 0.7
2 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 12.0 1.9 1.9 1.9 1.8 1.8 1.7 1.6 1.4 1.3 1.1 1.0 0.8 0.6
1 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.9 0.9 0.9 0.8 0.8 0.7 0.6 0.6 0.5
Aluminium 230 V / 400 V
c.s.a. of phase conductors(mm2) Length of circuit (in metres)
2.5 1.4 1.9 2.7 3.8 5.4 7.6 10.8 15.3 22
4 1.1 1.5 2.2 3.1 4.3 6.1 8.6 12.2 17.3 24 35
6 1.6 2.3 3.2 4.6 6.5 9.2 13.0 18.3 26 37 52
10 1.9 2.7 3.8 5.4 7.7 10.8 51.3 22 31 43 61 86
16 2.2 3.1 4.3 6.1 8.7 12.2 17.3 24 35 49 69 98 138
25 17.1 2.4 3.4 4.8 6.8 9.6 13.5 19.1 27 38 54 76 108 153 216
35 1.7 2.4 3.4 4.7 6.7 9.5 13.4 18.9 27 38 54 76 107 151 214 302
50[a] 1.6 2.3 3.2 4.6 6.4 9.1 12.9 18.2 26 36 51 73 103 148 205 290 410
70 2.4 3.4 4.7 6.7 9.5 13.4 19.0 27 38 54 76 1.7 151 214 303 428
95 2.3 3.2 4.6 6.4 9.1 12.9 18.2 26 36 51 73 103 145 205 290 411
120 2.9 4.1 5.8 8.1 11.5 16.3 23 32 46 65 92 130 184 259 367
150 3.1 4.4 6.3 8.8 12.5 17.7 25 35 50 71 100 141 199 282 399
185 2.6 3.7 5.2 7.4 10.4 14.8 21 30 42 59 83 118 167 236 333 471
240 1.2 1.6 2.3 3.3 4.6 6.5 9.2 13.0 18.4 26 37 52 73 104 147 208 294 415
300 1.4 2.0 2.8 3.9 5.5 7.8 11.1 15.6 22 31 34 62 88 125 177 250 353 499
2x120 1.4 2.0 2.9 4.1 5.8 8.1 11.5 16.3 23 33 46 65 92 130 184 260 367 519
2x150 1.6 2.2 3.1 4.4 6.3 8.8 12.5 17.7 25 35 50 71 100 141 200 280 399
2x185 1.9 2.6 2.7 5.2 7.4 10.5 14.8 21 13 42 59 83 118 167 236 334 472
2x240 2.3 3.3 4.6 6.5 9.2 13.0 18.4 26 37 52 74 104 147 208 294 415 587
3x120 2.2 3.1 4.3 6.1 8.6 12.2 17.3 24 34 49 69 97 138 195 275 389 551
3x150 2.3 3.3 4.7 6.6 9.4 13.3 18.8 27 37 53 75 106 150 212 299 423 598
3x185 2.8 3.9 5.5 7.8 11.1 13.7 22 31 44 63 89 125 177 250 354 500 707
3x240 3.5 4.9 6.9 9.8 13.8 19.5 28 39 55 78 110 156 220 312 441 623
• Note: for a 3-phase system having 230 V between phases, divide the above lengths by √3
1. ^ 1 2 for 50 mm² cables, the actual cross-section used for calculation is 47.5 mm². For more information, see note in Figure G53