Cross-section calculation

A correct cross-section calculation considers both the required amperage and derating factors. Why? The proximity of other energy cables and installation factors influence the heating of the cable. Therefore, the rated load values must be corrected by the applicable derating factors to adapt the maximum load of the cable to the actual situation. Please note that depending on the specific installation conditions, several derating factors may apply.

What is the rated current of a cable?

The permissible maximum current - also designated by Iz or Imax - stated on the technical data sheets applies to the following standard conditions:

Laying underground:

• Ground temperature: 20°C
• Thermal resistivity of ground: 100 K.cm/W
• Laying depth: 70 cm
• Number of cables/system: 1, without other cables in the proximity.
• Trefoil laying for single core systems.

Laying in air:

• Ambient temperature: 30°C
• Installation on an open and ventilated cable duct
• Number of cables/system: 1, without other cables in the proximity (not in a bundle or layer)
• For YMz1K, YMvK and YMvKas single cores: based on 3 conductors in trefoil
• For H07Z1-U & -R and H07V-U & -R based on 3 conductors in conduit, integrated in a wall.

Derating factors, also called correction factors, must be applied in all other installation conditions. Use the tables for this.

How to apply the derating factors? An example

You need the tables with the derating factors for this. 

Consider this situation:

• Required current = 125 A
• 3 phases + earth
• Voltage: 400 V
• Cable type: YMz1K Cca
• Length = 15 m
• Special conditions: the cable is installed on a well-ventilated cable ladder, on which there are also 2 other power cables. The cable is installed in a space where the ambient temperature regularly rises to 40°C.

Application of the derating factors:

The datasheet of the YMZ1K Cca 4G25 mm² shows it has an Iz of 127 A in normalised conditions. But is it sufficient to transfer the required current of 125 A in these deviating installation conditions? You now need to divide the required current by the derating factors that apply to the specific installation conditions:

A. In case the cables are laid side-to-side (in a bundle):

• derating because of the increased ambient temperature: 125 A / 0.91 = 137.4 A
• derating because of cables laid side-to-side: 137.4 A / 0.82 = 167.56 A
• Conclusion: you should take a section whose Iz is higher than this 167.56 A or in this case an YMz1K Cca 4G50 mm² which has an Iz = 192 A.

B. In case the cables are spaced (not in a bundle or layer):

• derating because of the increased ambient temperature: 125 A / 0.91 = 137.4 A
• derating due to nearby cables that are sufficiently spaced: 137.4 A / 1.0 = 137.4 A
• Conclusion: you should take a section whose Iz is higher than this 137.4 A or in this case an YMz1K Cca 4G35 which has an Iz = 158 A.