Services
Practical advice
Maximum lengths of busways
in the TN system
Maximum length in metres of busways in the
TN system(1) protected against indirect contacts by
circuit breakers
Correction factor to be applied to the lengths given
400 V networks(3)
Phase-to-phase
(2)
m = Sphases
Spe
1
2
3
4
Copper cable
Aluminium cable
1
0.62
0.67
0.41
0.50
0.31
0.40
0.25
P25M
Three-phase network in 400 V, copper cable,
Sph = Spe , Ul = 50 V,
in the TN system.
Sphases Calibre (A)
mm2
0.16 0.24 0.4
1.5
2.5
4
5 (2 x 2.5)
0.6
730
1
426
710
1.6
255
425
681
851
2.4
170
284
454
568
4
102
170
272
340
6
68
113
181
227
10
42
71
113
142
16
27
44
71
89
20
21
35
56
71
25
17
28
45
56
B curve C60, C120 and NG125
Three-phase network in 400 V, copper cable,
Sph = Spe , Ul = 50 V,
in the TN system.
Sphases Rating (A)
mm2
2
3
4
6
10 13
1.5
613 409 307 204 123 94
2.5
4
6
10
16
25
35
50
16
77
681 511 341 204 157 128
1090 818 545 327 252 204
818 491 377 307
818 629 511
818
20
61
102
164
245
409
654
25
49
82
131
196
327
523
818
32
38
64
102
153
256
409
639
894
40
31
51
82
123
204
327
511
716
50
25
41
65
98
164
262
409
572
818
63
19
32
52
78
130
208
325
454
649
80
15
28
41
61
102
164
258
358
511
100
12
20
33
49
82
131
204
288
409
125
10
25
25
41
65
98
164
262
409
572
818
32
19
32
51
77
128
204
319
447
639
40
15
26
41
61
102
164
256
358
511
50
12
20
33
49
82
131
204
286
409
63
10
16
26
39
65
104
162
227
325
80
8
13
20
31
51
82
128
179
258
100
6
10
16
25
41
65
102
143
204
125
5
16
26
39
65
105
164
229
311
C curve C60, C120 and NG125
Three-phase network in 400 V, copper cable,
Sph = Spe , Ul = 50 V,
in the TN system.
Sphases Rating (A)
mm2
2
3
4
1.5
307 204 153
2.5
511 341 256
4
818 545 409
6
818 613
10
16
25
35
50
6
10
102 61
13
47
16
38
20
31
170 102 79 64 51
273 164 126 102 82
409 245 189 153 123
681 409 315 256
654 503 409
786 639
894
204
327
511
716
8
13
20
33
52
82
114
156
(1)b IT system with distributed neutral, apply a coefficient of 0.50
b IT system with non-distributed neutral, apply a coefficient of 0.72
(2)b Sphases : phase conductor cross-section in mm2.
b Spe : protective conductor cross-section in mm2.
(3)b for phase-to-phase 237 V networks, also apply a coefficient of 0.57
b for single-phase 237 V networks (between phase and neutral), do not apply this additional
coefficient.
version: 3.1
92504E.indd
Maximum lengths of busways
in the TN system
Services
Practical advice
D curve C60, C120 and NG125
K curve C60
Three-phase network in 400 V, copper cable,
Sph = Spe , Ul = 50 V,
in the TN system.
Sphases Rating (A)
mm2
1
2
3
1.5
438 219 146
2.5
730 365 243
4
584 389
6
876 584
10
974
16
25
35
50
4
6
110 73
10
44
13
34
16
27
20
22
37
183
292
456
639
913
146
234
365
511
730
183 122 73 56 46
292 195 117 90 73 58
438 292 175 135 110 88
730 487 192 225
779 467 359
730 562
786
25
18
29
47
70
117
187
292
409
584
32
14
23
37
55
91
146
228
319
456
40
11
18
29
44
73
117
183
258
365
50
9
15
23
35
58
93
146
204
292
63
7
12
19
28
46
74
116
162
232
80
5
9
14
21
35
58
88
123
178
100
4
7
12
18
29
47
73
102
146
125
2
4
7
10
16
26
41
57
78
MA curve C60, NG125
Three-phase network in 400 V, copper cable,
Sph = Spe , Ul = 50 V,
in the TN system.
Sphases Rating (A)
mm2
1,6
2,5
1.5
274
175
2.5
456
292
4
730
467
6
701
10
16
25
35
50
4
110
183
292
438
730
6,3
70
116
186
279
465
743
10
44
73
117
175
292
467
730
12,5
35
58
93
140
234
374
584
818
16
27
46
73
110
183
292
456
639
913
25
18
29
47
70
117
187
292
409
584
40
11
18
29
44
73
117
183
256
365
63
7
12
19
28
46
74
116
162
232
80
5
9
14
21
35
58
88
123
178
Conventional calculation method
In most cases this method will be sufficient and results in limiting the length of the
various electrical circuits
B
PE
A
A
Imagn
Id
SPE
L
C
Sph
Principle
This consists of applying Ohm’s law only to the feeder concerned by the fault,
assuming that the voltage between the faulty phase and the PE (or PEN) continues
to be greater than 80 % of nominal phase voltage.
This coefficient globally takes account of all the upstream impedances.
In LV, when the protective conductor is routed alongside the corresponding phase
conductors, conductor reactance can be ignored compared with resistance: this
approximation is considered acceptable up to cross-sections of 120 mm2.
Beyond this value, resistance is increased as follows:
Cross-section (mm2)
Calculation of L max in the TN system using
the conventional method
150
185
240
Resistance
R + 15 %
R + 20 %
R + 25 %
The maximum length of a circuit in the TN system is given by the following formula:
L max = 0,8 x Uo x Sph
ρ x (1 + m) x la
In which:
b L max : maximum length in metres
b Uo: phase voltage:
v 230 V for 230/400 V network
b ρ: resistivity at normal operating temperature:
v for copper: 22.5 103 Ω x mm2/m
v for aluminium: 36 103 Ω x mm2/m
b Ia is equal to the current (A):
v for operation of the circuit breaker magnetic trip unit
v ensuring fuse blowing within the specified time
b m: Sph/Spe
b Sph: phase cross-section in mm2
b Spe : protective conductor cross-section in mm2
92504E.indd
version: 3.1