kva for a flat with all electric heating

[Gus Beare]

Hi everyone

I am tearing my hair out here. I have trying to get UK Power Networks to quote for new electrical supplies in a property development I am managing. The property is a conversion into four one bed flats.

I have been asked by UKPN to provide the After Diversity Max Demand in kVA for each unit. I have asked an electrician and he said 20kVA. I asked another and he said 24 kVA.

But UKPN said this:

"UK Power Networks would in general expect the (ADMD - after diversity maximum demand) for a gas heated flat/house to be between 1.5kVA and 3.0kVA and an electric heated flat/house to be between 5kVA & 10kVA unless abnormal high usage appliances are being installed."

These units will have electric wall heaters and will be very well insulated.
They will also each have 125l unvented cylinders for water heating. They will each have an oven/grill and washing machine.

I have asked UKPN how they arrived at this figure and they pointed me to their web site (which only confuses matters and fails to show how this is calculated):

UK Power Networks - What are Watts (W), Kilowatts (kW) and Kilovolt Amps (kVA)? - https://www.ukpowernetworks.co.uk/internet/en/help-and-advice/need-help/what-are-watts.html

But on their site it says this:

" If you’re unsure and don’t tell us what you need then we would assume 15kVA but it’s important you get this right. "

I cannot seem to get them to explain. Either they are clueless or I am clueless or both.

Can anyone help?

thanks
Gus

 

[Leesparkykent]

Do you have a list of circuits and loadings for each circuit for a flat? If so post them up and and I’m sure some one will be able to tell you the typical max demand after diversity.

 

[mhar]

I think the first figures UKPN are indicating is what they allow on their network diversity calculations. We had a 300A (maybe less) fuse go in the substation the other month and we lost a phase, it was quite suprising how many properties were covered by this fuse. 1.5-3kVA per gas property would be the figure they are applying for their network.

15kVA is the normal supply agreement where I live (15,000/230 = 65A being the maximum demanded by the loads in the property without any power factor considerations).

 

[Gus Beare]

Thanks for the help.

Each flat will have the following.

3kw water cylinder
5kw elec cooker
Sockets and LED lights
Washing Machine
Fridge/Freezer
3 or 4 1.5kw to 2kw heaters.
Kettle, toaster & Microwave

I have written back to UKPN asking them to clarify how they work out kVA in the light of the difference between what they have told me and what is written on their web site.

For now I am as confused as ever.

 

[Wilko]

UKPN said this: "UK Power Networks would in general expect the (ADMD - after diversity maximum demand) for a gas heated flat/house to be between 1.5kVA and 3.0kVA and an electric heated flat/house to be between 5kVA & 10kVA unless abnormal high usage appliances are being installed." ... I have asked UKPN how they arrived at this figure and they pointed me to their web site (which only confuses matters and fails to show how this is calculated) ... I cannot seem to get them to explain.

Hi Gus - welcome to the Forum
It looks like they've rounded their Design & Planning figures and fed them to you. These are likely to be a good guide for the planning of electrical distribution networks. But I think the values will be subject to the rule of averages. Some of the actual demands will be less, some will be more and the result is the table of values.

Their table seems to assume E7 to provide heating as off peak demand and that may make a big difference if your plans for heating are not off peak (?).

 

[Gus Beare]

thanks @Wilko

The flats will have normal meters (not E7 or E10) and wall mounted convector heaters.

So what should I tell them? They have said to me 5-10kVA for an electrically heated house but their web site suggests 15kVA.

 

[Wilko]

Well if folks come home in winter and turn on 8kW of heating and put on dinner to cook while they take a shower the flat will likely draw more than 10kW for a while. The room and hot water heating are heavy loads, so I'd be asking for their max of 15kW, say a 60A fuse. Having said that, if they used 40kW for 4 flats it might not be far out as not all folks arrive home and do the same thing at the same time. Don't forget to ask for something for common areas supply.

 

[Gus Beare]

Well if folks come home in winter and turn on 8kW of heating and put on dinner to cook while they take a shower the flat will likely draw more than 10kW for a while. The room and hot water heating are heavy loads, so I'd be asking for their max of 15kW, say a 60A fuse. Having said that, if they used 40kW for 4 flats it might not be far out as not all folks arrive home and do the same thing at the same time. Don't forget to ask for something for common areas supply.

thanks Wilko

It's confusing that in their emails to me they keep insisting on 5-10 but their web site says 15.

 

[Wilko]

I found this new (to me) emoji that might be relevant here ...

 

[Gus Beare]

I made a long and careful list of all of the devices that would likely be used in each flat.
I added up the ratings of these appliances and came up with 25.85kw.

Of course, it is extremely unlikely that all of the devices would be on at the same time in each flat. However, I spoke to an electrician that did some work for me at home and he said this:

"I would definitely say 100 amp per flat and 23kva is what i worked it out at. 100 amps x 230volts divided by 1000 = 23kva per flat. yes you take in diversity but if its an all electric flat with no gas . somebody could turn dishwasher heating cooker shower on all at same time so i wouldn't want anything less than 100amp incoming fuse."

UKPN told me that there was a supply nearby which offered 69kva but that works out at 17.25kVA per flat. They said they would have to take the supply from somewhere further down the road if I wanted more.
(No prizes for guessing that will cost a lot more.)

I am not comfortable with 17.25 per flat after what my electrician and several others have suggested. Should I press for the 23kVA?

What do you guys think?

 

[Gus Beare]

It's not inconceivable that we'll need charging for electric cars in the near future. There are already points on the same street. But those are public ones. Slow chargers are only about 3kw but fast ones can be 50kw and above.

 

[Wilko]

Hi - with 4 flats in the block there will be a statistical reduction in max demand as not everybody does the same thing at the same time. So if heating and hot water were fully looked after at 8+3 = 11kW then their new max of 17kW sounds plausible to me. However if you want EV capability that would tip me over into spending more, as EV would likely be charging at night when folks are home with heating on, showering and cooking tea etc. Perhaps have a conversation with UKPN that you are planning to have resident EV charging and see if that changes things. Just my thoughts .

 

[Gus Beare]

Thanks Wilko. I am going to call them this morning and see what they say.

 

[davesparks]

. somebody could turn dishwasher heating cooker shower on all at same time so i wouldn't want anything less than 100amp incoming fuse."

Yes someone could run round the house turning every appliance on at once, but how likely is this to happen?

With a large un vented hot water cylinder you presumably won't be having electric showers? In which case the showers won't come in to it.

Everything else will not use a constant amount of current, the thermostats will turn heating elements on and off to achieve the set temperatures so the actual power used will be a lot less than the result of adding up all of the ratings.

 

[Gus Beare]

Yes indeed. There will be no electric showers. They will run on mains pressure from the tanks.
The thing is what happens if the load is exceeded? The woman told me a fuse would blow at the supply in the street.
But is that correct? Would the domestic trip flip instead?
If that's the case its not an issue.