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Large house
lyledunn
390 Posts
Question
The standard supply for a new  house seems to be circa 18KVA single-phase. I have a client who needs to make application for a supply for a new build house at 400m2 with the only certain information being that an air source heat pump is to be used for heating. The client simply does not know what the electrical loading might be. The house is only coming out of the foundations but the DNO supply may take many months as the house is in a relatively remote rural location, thus loading details are required before the DNO will consider progressing the proposal. It is a big house, but that is what it is, just a big house. I am inclined to advise a standard supply but 3-phase as with a large AS heat pump, maybe shower, cooking and perhaps an EV or two, 18KVA wouldn't be long being gobbled up. As we move towards a higher reliance on electric as a power source for some heavy current-using equipment, it might be that 18KVA is no longer a realistic standard supply for even the smaller property. 
7 Replies
Sparkingchip
3120 Posts
However they might be hit with a high quote that includes upgrading the local distribution network, which I am sure you have already considered.
Chris Pearson
1739 Posts
18 kVA presumably after diversity else why not 23 kVA? But it does seem to be the norm here in SSE-land.

I wouldn't (didn't) think twice about getting three-phase. OK, you need a three-phase main DB, but the cost is a drop in the ocean of a 400 m² build. Never mind heat pumps, etc., don't you want fast EV charging?

ETA: a three-phase supply avoids smart metering. 😁
AJJewsbury
1849 Posts
How thermally efficient is this house going to be? If it's something approaching PassivHaus it might only need 2 or 3kW to keep warm even on the coldest days - which probably wouldn't be so much of a worry. Even something that only meets current building regs should be a lot easier to heat that something of the same size from the 1970s or earlier.

   - Andy.
broadgage
379 Posts
Two not very fast EV chargers and one electric shower will use about 100 amps in total, without any cooking, space heating, or other loads.
That to me suggests the likely need for a three phase supply of 100 amps, or at the very least split phase at 100 amps.

Even with good insulation 400 square meters would probably require at least a 24Kw gas boiler, or say 12 Kw of electricity for a heat pump.

A guestimate of the larger loads could include
Space heating=12 Kw. No diversity allowable.
Cooker= 5Kw Very little diversity allowable, since it has already been applied to the cooker rating.
Shower=9Kw
A couple of EV chargers=14 Kw. No diversity allowed in my view, both could run at full load for hours.
Water heating=3 Kw. No diversity allowable.

Consider the possibility, even a probability of a second shower, a larger cooker, and a load of portable heaters when the heat pump breaks.
Small appliances are unknown, but consider perhaps 4 ring finals with an assumption that one will be fully loaded and the other three much less so.
AJJewsbury
1849 Posts
Even with good insulation 400 square meters would probably require at least a 24Kw gas boiler, or say 12 Kw of electricity for a heat pump.
I reckon that's rather pessimistic. I've done some very rough heat loss calculations for a 400m² building (with a lot of assumptions and guesses of course) based on some L1A U values (0.3 for walls, 2 for windows etc), and come out with a figure of less than 6.4kW total for -5 degrees outside. If it's a self-build for someone interested in keeping their running costs down, insulating to something closer to PasivHaus standard could half that figure. That's just space heating so add a bit for DHW of course - but it's still a lot less than 12kW never mind 24kW.

   - Andy.
lyledunn
390 Posts
Apparently it is a 12kw unit with a 3.61kw Electrical input so not huge demand. A preliminary meeting with client has concluded that 2no 22kw 3phase EV points would be desirable. I have applied for a 100A TPN service. Will be interesting to see costs. 
broadgage
379 Posts
Some of the figures given for heating demand sound rather low to me, considering not just just the steady state demand, but also heating from cold after a winter holiday.
Neighbours of mine have a house of just under 500 square meters and a 26 Kw oil boiler is only just sufficient and that is with an estimated heat input of another 5Kw from an oil burning cooker. The house is about 30 years old but was built with insulation far in excess of that then required.

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