Calculate the derating for 5 parallel circuits in one conduit

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

Problem

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.

Assumptions

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)

Solution

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.