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.