UPS Installation – 400V, 10kVA Load via 15kVA Transformer: Design Review Request

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Kyllie016

New User
Location
Multan
Occupation
Employ
Hello everyone,


I'm working on a UPS installation project and would like to verify my approach with experienced professionals here.


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  • UPS: 10kVA, 400V (3-phase)
  • Supply Source: 480V panel
  • Transformer: 15kVA, 480V to 400V (Delta/Wye)
  • Upstream Breaker: 25A 3P
  • Cable from panel to transformer: 3-core 10mm² Cu
  • Cable from transformer to UPS: 3-core 10mm² + 4mm² ground
  • Intermediate disconnect switch: 25A


  • Confirming if the cable sizing is appropriate for the expected load
  • Ensuring the 25A breaker provides enough inrush tolerance
  • Grounding best practices for the wye secondary (plan is to ground the neutral at the panel)

The expected current is ~14.5A (assuming PF ~1), so there’s margin, but I want to be sure it meets best practices. The cable runs are relatively short, but voltage drop is still a consideration.


Would love to hear your thoughts or corrections.


Thanks in advance,
[FAISAL NADEEM]
 
We don't have much experience with non-NEC installations on this discussion board; I have no experience with the rules that apply in Pakistan. With that caveat:

10mm2 conductors are bigger than US 8 AWG, so I see no problems with your expected or peak current. Voltage drop won't be a problem unless the run gets into the 500 foot range.

The nominal full load input current for your transformer is 18A; so a 25A breaker will likely hold on inrush, but there is no guarantee. Under the US NEC the breaker is permitted to be larger, but you would be required to have a suitable secondary breaker.

In the US, grounding the neutral point of the wye would be the standard approach.

-Jonathan
 
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