KVA (Roughly Speaking)

[sameulepapi]

Hello Clever Electrical People,

 

We have just bought a barn of about 500sqm. We  plan to build a house of a similar footprint. It currently has no power but an overhead cable runs in the grounds.

 

UKPN have come out and said no issue with the cable, pretty easy job. HOWEVER, the transformer is small and already feeding a few places. 

 

The guy ran a test and said we can get max 3-phase with 69KVA - anything over and it is a new transformer

 

I understand from my reading that we most 4/5 bed houses still only need a 100amp single phase (23kva) and I also understand that 100 amp per phase 3-phase is 72kva ish

 

We are not at the point we have a spec yet so we will get them exact KVA in the future. For now though, I want to just understand if that transformer is likely need to be changed (and thus big extra cost)

 

The plan is to be passivhaus with mechanical ventilation, GSHP, Electric Car Charger and PV to some degree. 5 beds and we do not plan on having a pool or anything but maybe a hot tub. 

 

My questions are:

 

1. Does anyone have a similar barn / house size and spec and have a rough KVA number?

2. On a 3-phase supply could you have 2 x 100amp and 1 x something else?

 

Thanks in advance and yes I realise there is no firm answer here until we have done the plans!

 

Thank you

 

 

[ProDave]

100A single phase is 23KVA

 

100A 3 phase is 69KVA

 

So they are pretty much saying you can have a normal 100A supply either single or 3 phase.

 

You won't need anything like that, so no need for a new transformer, which is good.  Probably worth getting 3 phase if available as it won't cost much more and you don't have to use all 3 if you have no need.

[Jilly]

We had 3 phase put into a kiosk at the edge of our property ahead of time, which the electrician connected into when we are ready. We only used 1 phase for the house and have the other two ready for unspecified use in the future. 

[SteamyTea]

Worth working out your energy usage. Ideally you want to minimise usage, then the problem goes away.

Also worth considering car charging. We are heading to a BEV world, so may be better to fit 3P now.

You can easily connect up more PV with 3P as well.

 

Don't worry about the system being balanced, that is the electrician's job.

Edited November 12, 2022 by SteamyTea

[sameulepapi]

Amazing thank you - this is exactly what I needed to know for now. You 3 are great

[IanR]

Sounds like you won't need to upgrade the transformer. I needed to pay for an upgrade whether I went 1 or 3 phase, and it wasn't much more for 3 phase. I was charged £2,819.60 to upgrade the pole transformer from 50kVA to 100kVA. I'd asked for a 45kVA connection, but they put 100A on each phase so I assume my connection is at 69kVA.

Sounds like you are doing a knock down and rebuild, mine's a conversion of the same size foot-print as yours. Built to PassivHaus standards (un-certified), my 12kW ASHP is overkill. I can't see you need to go to the expense of a GSHP, unless you are doing so for other reasons.

I wanted 3 phase for the workshop, the house doesn't really need it, although it does offer you other options for PV and car charging.