Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 169)

The selection of conductors with a higher insulation class does not affect the rate of heat-flow
out of the devices. For this reason, their cross-section should be the same as those of conduc-
tors with a 70 °C limiting temperature.
In the case of busbars it should be noted that, for the same reasons, the load capacity of
busbars that are connected to devices is lower than the load capacity of busbars that are
exclusively serving for power distribution. The corresponding tables can be found in the annex
to IEC 60890.
6.1.4.4 Conductor length
As shown in Section 6.1.2 in the type tests for devices comparatively long connecting lines to
the terminals are used and these help to radiate a substantial amount of heat from the devices.
With short connections this does not occur. As a consequence the temperatures of the termi-
nals, the device interiors and the conductors themselves rise even if the load remains un-
changed. For this reason, with compact device assemblies such as for example motor starters
consisting of a circuit breaker and a contactor, type tests of the complete starter including the
connecting (power wiring) components are performed. The power wiring modules have a higher
temperature withstand than normal wiring material and the tests ensure that the temperature-
rise limits for all the components are observed.
Fig. 6.1-5
With compact motor starters with power wiring modules, the type test ensures that the limit temperatures
of all components are observed.
With short connections in individually wired installations, compliance with the temperature limits
should be ensured by load reduction and/or forced cooling. The selection of larger connecting
cross-sections increases the heat exchange of mutually connected devices and reduces the
amount of heat dissipation in the conductor itself. Therefore the rate of heat-flow to the outside
is not improved.
6.1.4.5 Tightening torques
In the catalogues and on the devices themselves, often ranges for the tightening torques of the
terminals are stated. These then relate to all current ranges and the respective wire sizes for a
frame size. From the point of view of device heating it is a good idea to always use a value in
the upper part of the torque range as this will have a positive effect on the electrical and thermal
transition resistance and hence the heat generation and flow. See
Fig. 6.1-6 . The upper range
limit should not be exceeded so that the mechanical strength of the terminals is not unaccepta-
bly stressed.
6.1.4.6 Line ducting
As can be seen from the relevant tables (see also RALVET) for conductor selection, the method
of installation (in the open, on tracks, in cable ducts etc.) and the accumulation of conductors
have a large influence on their load-carrying capacity. The more heat-flow to the surrounding air
is prevented, the lower is the load capacity or, in other words, the greater is the required cross-
section for a given current. For technical reasons therefore, the lines should be laid as loosely
as possible. Lines that are routed into a cable duct only a short distance from the connection
LVSAM-WP001A-EN-P - April 2009 6-5