#Current-carrying #Capacity. Ambient Air and Cable Temperature

How to choose the #cable #section.
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).

#Current-carrying #Capacity. Cable Voltage Drop

Cable choice.
An important element of assessment is the length of the cable. Each meter of the cable causes a voltage drop proportional to the value of the current that runs through it, if cosf = 1 we have: V = IxR.
V = voltage drop, I = current, R = resistance of the cable. In the case of inductive loads (solenoid valves, motors, etc.) or with variable currents, the higher current peak must be considered to avoid even temporary loss of power, which could hinder the proper functioning of the equipment. The longer is the cable, the greater the #voltage #drop, the IEC allows maximum values drop from 3% to 5% of rated voltage. Greater values are admitted only for the current peaks (e.g.: engines starting), if they do not create functioning problems. To minimize the voltage drop the cable section should be increased. Here are some tables that can help to easily calculate the minimum theoretical cable section. See tables 3 and 4 on the page http://www.shield.net/files/catalogue/support/2.2.pdf.
For example, an average continuous current 1 A with section 0,34 mm2 and cable length 10 m generates a voltage drop of 1,3 V, which is approximately 5% of 24 V.

#Cable Maximum Voltage Rating and #Mechanical Strength

Tips to choose a cable.
The maximum voltage depends on the material characteristics, the conductor insulation thickness and the outer sheath thickness. The insulation must be sufficiently resistant to prevent even accidental crushing situations, so different uses can have different requirements of project. Norms as UL and CSA (American), IEC (International) and EN (European) set values which are not always equal to each other and sometimes refer only to heavy employment, such as cables for power transmission.

For the transmission of signals inside industrial machinery, it is frequent to use reduced thickness which refers to commercial standards which are not supported by norms, such as cables LiYY and FROR. The advantage of using reduced thickness is particularly evident in the smaller volume occupied by bundles of cables. The reference nominal voltage is generally at least 300 V. On the market there are also low-cost cables in PVC, with extremely reduced thickness, which may not be compatible with the safety standards for specific applications; it is advisable to carefully evaluate the characteristics of the products.
The norm IEC 60204-1 (#Machinery #Safety) recommends the minimum section of the cables according to the application as well, see table 1 on page http://www.shield.net/files/catalogue/support/2.2.pdf. The norm allows smaller sections only if the mechanical protection is sufficiently guaranteed, for example with conduits.