KVA sanity check.

[pavement]

Hello,
Me and some of the colleagues are having a discussion about requesting electricty supplies from the Distribution Network Operator and stating the KVA rating of the supply.

The application is for street lighting, with the supply into a three phase feeder pillar, with circuits being supplied of individual phases.

Some of us are saying that the KVA rating should be calculated using the total current load from the three phases, and others are saying it should be done using the worse case loading on a phase?!

There doesn't seem to be a majority vote, so any help would be much appreciated to put our minds at rest.


Many Thanks.

 

[Skogsgurra]

Current in a three-phase system is always, with no exception, defined as current in one of the phases. So it seems very reasonable to use current in the heaviest loaded phase.

Gunnar Englund

http://www.gke.org

 

[waross]

Agreed skogsgurra
And the current in the heaviest loaded phase is what will trip the circuit breaker, determine the wire size, and heat the transformer on that phase. The demand will be the total KVA, but use the heaviest phase loading for sizing the wire, the main breaker, and the transformers. I would hope that the lighting circuits can be balanced well enough that the question is acedemic.