Minimum Kva/Amps for a new build

[gc100]

Hi,

first post- I'm not an electrician and have spoken to my local one about this, but he didn't offer much opinion so I thought I would ask here. I'm converting a barn in open countryside and its about a 600meter run from the nearest transformer. If we go with a 100amp single phase (I'm told the voltage drop and earth loop impendence would be ok) or 40amps over each 3 phase, he thinks I'll need 95mm or 120mm or 150mm 4 core xlpe swa depending on the configuration. It seems for this amount of cable its going to be 16K to 22K + VAT just for the cable !!! .

So I'm wondering if we really need 100amps for a house. Its a new build/conversion so will be insulated very well. I was looking at doing ground source for the heating but could switch to LPG. I was also wanting to to 2x4K solar array on the roof plus Telsa powerwall to store and excess. 3 bed house, 4 person family. 

I was wondering if we can get away with 60amps ? To reach that, we'd needed on at the same time, oven 32A, dryer 11A, washing machine 10A + misc all on at the same time. I could loose the oven completely and go gas (botttle) with that.

Any real world experience of this?  

Many thanks

 

[ProDave]

You need to discuss the options with the DNO.  I would be surprised if they ran an LV cable 600 metres. More likely would be extend the HV overhead and fit a transformer closer to you.  I don't think asking for a lower rating supply would make them fit a smaller cable, loop impedance would be an issue so I doubt there is any saving to be made that way.

In our case I took what was offered, which was a 12KVA supply.  We are now the 9th house sharing one transformer, and the implication was if I wanted more than 12KVA I would have to contribute to upgrading that for a bigger transformer.  The reality is I have the same size supply cable and same 100A fuse so could draw a lot more than 12KVA, just as long as we don't all do so at the same time.

As it happens we are building a very low energy house, almost to passive house standards so the energy use will be low and 12KVA would be plenty. We are all electric except LPG for the hob.

I chose an Air Source Heat Pump. When you start costing it, the cost of ground loops and a not insignificant cost of the brine to fill them with, makes a GSHP very expensive for only a small improvement in eficciency compared to an ASHP.

 

[gc100]

You need to discuss the options with the DNO.  I would be surprised if they tan an LV cable 600 metres. More likely would be extend the HV overhead and fit a transformer closer to you.

In out case I took what was offered, which was a 12KVA supply.  We are now the 9th house sharing one transformer, and the implication was if I wanted more than 12KVA I woudl have to contribute to upgrading that for a bigger transformer.  The reality is I have the same size supply cable and same 100A fuse so could draw a lot more than 12KVA, just as long as we don't all do so at the same time.

As it happens we are building a very low energy house, almost to passive house standards so the energy use will be low and 12KVA would be plenty. We are all electric except LPG for the hob.

Thanks.

I have spoken to the DNO - they suggested this route. Their quote was £40K for a 40kVa which is too much both in power and cost!. The nearest transformer is 600 odd meters away, and its only a 16KvA transformer feeding an existing customer. They have told me they will need to upgrade that transformer to a 100kvA no matter what power I choose (and pay the percentage cost of that upgrade). They did not offer HV overhead in any form unfortunately.

 

[Tony S]

If I were you, I’d find a proper electrician, one that can do diversity calculations.

Have the DNO actually said you can have a 3Ø supply? If they have a 4 core cable would be doubtful.

 

[Risteard]

12kVA is the Standard domestic supply here. 16kVA is offered as an "Enhanced" supply.

 

[binky]

My house is on a 60 Amp main fuse, so that should be sufficient given that you will also be installing solar panels to generate at source.  The unfortunate truth is that it is a long run, so cable has to be large to cope with voltage drop. Almost worth contemplating going off grid and installing larger battery system??? Not a fan of Tesla - firking big heavy things and not as good as the sales blurb suggests they are.

Depending on how energy intensive you are (incidentally a 32A cooker rarely draws more than 10 A for any period of time) you could probably reduce the required supply to around 40A, ie enough to run an electric shower without any issues. I would certainly be looking at installing a hot water tank to take load off electric supply, install wood burmers and few other gadgets like that to offer alternatives for cooking and heating for mid winter. A couple of solar thermal panels would also probably work well for you. I would also be looking at TT earthing so you need less cores in the supply cable. Worth sticking numbers into the Doncaster Cable calculator and possibly buying cables direct from them. You can also use DNO approved contractors rather than the DNO themselves - should be cheaper.

 

[ProDave]

How close does your land go to the available supply, that is the edge of your land, not where you are siting the house?

If part of your land is significantly closer, you might consider getting the supply terminated in a kiosk on the land boundary so what the DNO has to supply is a shorter run.  The cable from the supply kiosk to the house is then your responsibility but it might give you more flexibility in choosing max demand and cable sizes. Still have to comply with BS7671 voltage drop limits.

 

[apprentice87]

Firstly, 60A is MORE than enough, Secondly, if you have single phase, you only need a 2 core cable. thirdly, where are you??

I make it a 400mm csa cable ??????

Cannot see that being possible..... I think you will need to go with the DNO.. Still, the rules of ohms law apply to them too, no way will they have a service cable that big for one house.. Ask them what they propose to do??

john..

 

[ProDave]

^^ which is why I think they will extend the HV line and install a new transformer.

 

[binky]

according to Doncaster cables cables would be 95mm for 3 phase or 135mm single phase in xlpe buried direct - ruddy big cable for a house. DNOs tend to discount costs if you do  a load of work for them like dig trench, erect poles etc etc. HV can be buried I believe -, but cable is still expensive - cheapest I've got to so far is £10k for the cable alone, but may well be much cheaper if bought direct from the likes of Doncaster?

 

[apprentice87]

^^ which is why I think they will extend the HV line and install a new transformer.

I would definitely agree with that one..!!

john..

 

[apprentice87]

according to Doncaster cables cables would be 95mm for 3 phase or 135mm single phase in xlpe buried direct - ruddy big cable for a house. DNOs tend to discount costs if you do  a load of work for them like dig trench, erect poles etc etc. HV can be buried I believe -, but cable is still expensive - cheapest I've got to so far is £10k for the cable alone, but may well be much cheaper if bought direct from the likes of Doncaster?

Would have to be a LOT bigger than that. Just working out the volt drop i came up with 400mm [and that was for 60A]

I have just found what seems to me to be a really good cable calculator thingy, it came up with 300mm Still massive!!

Here is a link to the calculator thing!!

https://myelectrical.com/tools/cable-sizing-calculator

john..

 

[binky]

think that was at  60 Amp supply total...try it yourself http://www.doncastercables.com/technical-help/

 

[apprentice87]

Hi Binky,

I tried the thing, and it did indeed come up with 185mm but that was with 5% volt drop. You need a maximum of 3% as i assume the bloke will have lights in his house..

I tried with 3% and the only cable it came up with was a 600mm flexible cord.

They have cable impedance tables in the regs book. it is given in mV/A/M.

The maximum drop you can have is 6900mV at 60A over the run of 600 metres

So 6900mV divided by 60A/600M = 0.1912

So from doncasters own chart here;

http://www.doncastercables.com/technical-help/6

400mm 2 core is given at 0.190

400mm it is..

john..

 

[binky]

That's an interesting point - as a general power supply, do you say 'he's got lights in the house so must be 3%' ?  

 

[apprentice87]

That's an interesting point - as a general power supply, do you say 'he's got lights in the house so must be 3%' ?  

Hi Binky, I would imagine so.. I know the rules say 5% for general and 3% for lighting, so i suppose for a submain it would be 3%. Massive cable what ever!!!

john..

 

[binky]

It's never been clear to me what a submain constitutes. I think the phrase for 3% is a 'dedicated lighting cct' so I've always regarded sub mains as general power... It certainly isn't a small cable either way.

 

[apprentice87]

It's never been clear to me what a submain constitutes. I think the phrase for 3% is a 'dedicated lighting cct' so I've always regarded sub mains as general power... It certainly isn't a small cable either way.

Hi Binky, I am no expert, but as far as i know, the dno service cable terminates at the cutout. Then you have the tails to the meter and CU. You cannot have these longer than 3 metres, so, if the CU is, say, 10 metres away, you stick some sort of OCPD at the origin, [the cutout] and then your cable to the CU is your submain. I suppose you could see it as a privately owned service cable.

Volt drop before the cutout caused by supply impedance, is not for us to worry about. It is after that point that the 5% and 3% come in. The idea of the max 3% for lighting is so that when you turn on a large load, elsewhere in the installation, say a shower, the lights do not dim excessively.

From this idea, you can see that the 3% would apply to the entire installation if it feeds lights at all, as, if it just applied to the lighting final circuit, the lights would dim regardless when the shower came on.

That is my understanding anyway!! [and i might be miles out!!]

john..

 

[Tony S]

Just to throw a pebble in to the pool.

Assuming 230V is actually 248V* at the point of supply, what do you base your calculations on?

* I’ve just checked my supply.

 

[Sharpend]

I do believe we are frowned upon for using the actual measured voltage as the book says we are on 230v now, although we know nothing has changed. And of course there is their own tolerance % that is permitted.  

So to ask again what voltage should we use for calculations?????