Calculate the derating for 5 parallel circuits in one conduit

Modified on Mon, 26 Jul, 2021 at 8:20 AM

This example calculation is based on that from Example 1 in Appendix A of AS/NZS 3008.1.1:2017.


An underground 1450 A three-phase circuit is to be made up of parallel 400 mm2 V-75 single core cables with copper conductors.  All circuits will be installed in one conduit.


Voltage drop is acceptable and will not be checked.

No derating factors for:

Ambient soil temperature (standard 25 degrees)

Depth of laying (standard 0.5 m from the ground surface)

Thermal resistivity of soil (standard 1.2 K.m/W)


There are two parts to the solution of this special case:

1. Determine derating factor for 5 circuits bunched in an enclosure (Table 22) which is equal to 0.6.

2. Using Cable Pro Web software enter the following:

  • Load current = 1450 A
  • Derating factor = 0.6
  • Installation = Buried enclosure Combined

Also set the Active size (solution) to MANUAL with 400 mm2 No of circuits as 5.

The result is a total spare capacity of 26 A.

Was this article helpful?

That’s Great!

Thank you for your feedback

Sorry! We couldn't be helpful

Thank you for your feedback

Let us know how can we improve this article!

Select at least one of the reasons
CAPTCHA verification is required.

Feedback sent

We appreciate your effort and will try to fix the article