The current flow in a connector and in its cable generates heat, thus a temperature rise. Any connector or cable insulating material has a maximum safety temperature that should never be exceeded to ensure the stability of their mechanical properties.
After verifying the insulating material quality and the working ambient air temperature, the maximum current-carrying capacity permitted in the connector can be calculated in function of the terminals size and the conductors section.
Table 2 on page http://www.shield.net/files/catalogue/support/2.2.pdf shows the maximum current values for a pvc cable (quality 70°C) with 2 or 3 wires without metal sheath, with copper conductors, employed for normal use and inserted into cable tray. For open air applications a current about 10% higher is admitted. The ambient air temperature is measured around the cable, for example inside the cabinet or close to the machine interspaces. The right ambient air temperature is found after the normal functioning of the machine, under full load for a sufficient period of time, without taking into account the variations of the temperature due to heating of the cable. If the use does not require a higher temperature project, the conventional value of 40°C (IEC 60204-1) must be considered.
The table takes into account that in most of the machines not all cables are used simultaneously for long periods. In special cases (many wires used at the same time or high ambient air temperatures), a reduction factor of between 0,3 and 0,9 must be used to reflect the overheating of cables in the same group, otherwise verify that the temperature remains within the limits allowed. Particular attention must also be paid to vertical and limited ventilation routes.
Values and comments refer to IEC 60204-1 (Machinery Safety, edition October 2005) and IEC 60364-5-52 (Wiring Systems Security).
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