Required Cable Cross-Sections For Line And Motor Connections; Grounding And Pe Conductor Cross-Section - Siemens SINAMICS G130 Engineering Manual

Required cable cross-sections for line and motor connections

It is always advisable to use shielded, symmetrical, 3-wire three-phase cables or to connect several cables of this
type in parallel if necessary. There are basically two reasons for this choice of cable:
This is the only way in which the high IP55 degree of protection can be achieved for the motor terminal box without
problems because the cables enter the terminal box via glands and the number of possible glands is limited by the
geometry of the terminal box. Therefore single cables are less suitable.
With symmetrical, 3-wire, three-phase cables, the summed ampere-turns over the cable outer diameter are equal to
zero and they can be routed in conductive, metal cable ducts or racks without any significant currents (ground current
or leakage current) being induced in these conductive, metal connections. The danger of induced leakage currents
and thus of increased cable shield losses increases with single-wire cables.
The required cable cross-section depends on the amperage which flows through the cable. The permissible current
loading of cables is defined e.g. in DIN VDE 0298 Part 2/DIN VDE 0276-1000. It depends on ambient conditions such
as the temperature, but also on the routing method. It depends whether cables are routed singly and therefore
relatively well ventilated, or whether groups of cables are routed together. In the latter instance, the cables heat one
another and are therefore far less well ventilated. Reference should be made to the corresponding reduction factors
for such conditions as specified in DIN VDE 0298 Part 2 / DIN VDE 0276-1000. With an ambient temperature of
40 °C, the cross-sections of copper cables can be based on the following table.
Cross-section With single
of 3-wire cables routing
2
mm
50 138
70 176
95 212
120 245
150 282
185 323
240 380
300 418
Current-carrying capacity according to DIN VDE 0298 Part 2 at 40° C
With higher amperages, cables must be connected in parallel.
Note:
The recommendations for the North American market in AWG or MCM must be taken from the appropriate NEC
(National Electrical Code)/CEC (Canadian Electrical Code) standards.

Grounding and PE conductor cross-section

The PE conductor must be dimensioned to meet the following requirements:
• In the case of a ground fault, no impermissibly high contact voltages resulting from voltage drops on the PE
conductor caused by the ground fault current may occur (< 50 VAC or < 120 VDC, EN 50 178 Subsection 5.3.2.2,
IEC 60 364, IEC 60 543).
• The PE conductor should not be excessively loaded by any ground fault current it carries.
• If it is possible for continuous currents to flow through the PE conductor when a fault occurs in accordance with
EN 50 78 Subsection 8.3.3.4, the PE conductor cross-section must be dimensioned for this continuous current.
• The PE conductor cross-section should be selected according to EN 60 204-1, EN 60 439-1, IEC 60 364.
Cross-section of phase
conductor
2
mm
Up to 16
16 to 35
35 and above
With several cables on a
common cable rack
A A
95
121
146
169
194
222
261
289
Minimum cross-section of the external PE
conductor
Minimum phase conductor cross-section
Minimum half the phase conductor cross-section
2
mm
16
SINAMICS Engineering Manual - May 2008
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