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.
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. ?
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.
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.
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.
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.
I also think these heat requirements are far too low. Ventilation of this property of 1 air change per hour will need far more than Andy has allowed, and zero changes per hour is generally fatal for anyone with lung problems. I think a 3ph 100A supply is quite reasonable, and if it is never all used, well thats that. I see a COP of 4 is being used for the heat pump, this is too high too, At -5 to +25 it is unlikely to make 3 for an air excahnge unit (including the fans necessary to make it work properly). I love the idea of never opening a door or window, you can loose 1kWh in a few seconds on a cold day because the postman came, and it takes at least 20 minutes of the described system to get the temperature back to that required. -5 is not the English outside temperature either, it is more like -20 when it snows and the Wind gets going. This is severely underdesigned. If in upland Scotland it is criminal!
Even a 3 phase 100 amp supply might be marginal with a couple of 22Kw vehicle chargers.
The two EV chargers and the heat pump will use in total about 70 amps per phase. No diversity should be allowed in my view as both the vehicle chargers and the heat pump could run for hours simultaneously.
That leaves only 30 amps per phase for everything else.
A 40 amp shower will overload the phase to which it is connected.
A 30 amp cooker, and a lightly loaded ring final will overload the phase to which they are connected.
A heavily loaded ring final and another lightly loaded ring final will overload the phase to which they are connected.
And to which phase is a second shower, an immersion heater, or another cooker to be connected ? remembering that all three are already potentially overloaded.
And yes I know that the DNO cut out fuse will probably survive such overloads, but it is poor practice to design a new installation on such a basis.
A single phase 300 amp supply would IN THEORY be preferable as diversity works better, but in practice single phase services of that size are almost unknown, and 22Kw vehicle chargers are 3 phase.
It looks to me as though EITHER a three phase 150 amp supply will ne needed, or that the EV chargers should be ones that throttle the charging rate when the rest of the installation is heavily loaded.
Strictly speaking Broadgauge I would not call that an overload. A shower for 15 minutes assuming that the load of everything else is at maximum, will not cause any problems.It is unlikely that electric heating will be needed in this house if the heat pump is working, and the odd kettle is nothing. Most houses with a 90A maximum demand get on very well with a 60A main fuse as you well know. Going above 100A is very expensive in switchgear and everything else. The supply charge will be much larger than 100A supply as CT metering is needed, and the head etc. is much larger, not really domestic at all. I am sure they will be fine with a 75kW supply, even with 2 car charging points. Imagine the supply transformer for a street full of these houses, every tenth one will have to have a substation in the garden! That is the real cost of these electric vehicles. Every street will have a pylon close by too, not too good for the radio buffs.
