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3 phase connection question asked by SSE


BigBuilder

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Hi

i am at the start of my new house build, I’m applying for a new 3phase supply from SSE, I’ve filled out the forms and they are now asking the following ……..

Important information: Please confirm Size of Generation required in kW or MW per phase.

I don’t have an electrician onboard yet but I want to get this going ASAP due to their apparent ‘up to 7 month’ lead in!? ……..my question is what would be an appropriate figure to give them for each phase to answer this question? it doesn’t need to be accurate at the moment, just plausible.
A bit of background, 4500m2 house, will have 2 fast car chargers, PV , ASHP, 2 dish washer etc etc , my chosen electrician will work out the final loads and sizing when they are on board , also I need a good supply to the garage for woodwork machinery etc in future.

Thanks

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Dumb question but to check this isn't asking about export allowance?

"Size of Generation required" seems a weird way to describe supply or import but maybe that's just me. 

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The DNOs seem to delight in asking these questions in a stupid way ... or maybe they ask the same question whether you have a 3 bed house or are putting in a new blast furnace.

 

The options in the real world (for residential) are likely 63A or 100A fuses (or maybe 80A as ProDave says). Which means 63A x 230V ~= 15 kW or 80A x 230V = 18 kW or 100A x 230V ~= 25 kW per phase.

 

The smaller is probably fine - that's a lot of power. I have 63A 3-phase.

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3 hours ago, joth said:

Dumb question but to check this isn't asking about export allowance?

"Size of Generation required" seems a weird way to describe supply or import but maybe that's just me. 

yes I thought it was a strange way to phrase the question .

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4 hours ago, ProDave said:

I would aim high, and quote say 80A per phase which is about 19 kW per phase or 57kW

 

If they come back with a silly price, then talk about reducing the power needed.

great, thanks Dave for input, ill try replying with that 

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23 hours ago, ProDave said:

I would aim high, and quote say 80A per phase

Is that the sort of capacity that would allow everything on at the same time and a bit to spare?

Is there any sort of rule of thumb? Base level plus x/m2? Factor for new build or old?

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It's at this point I usually remind people that the point is to use less energy rather than more energy.

 

Maybe add the max power of your car chargers and the heat pump and add 20%. It could be that the DNO doesn't care much anyway and there's no change in cost. Or you could ask for 100A and see what they'll give you. They won't change the size of the cable their planning to use, so it'll just be up to the marketing department. It's a bit like deciding between a 3L engine and a 5L. Yeah, more is better right? All your single phase mates will be on 25kW or less anyway. At 63A per phase you'll have twice that.

Edited by Alan Ambrose
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Doing commercial and education projects , where our company took over from the clients' consultants, our electrician usually suggested that the mains supply be reduced, typically halved. The consultants apparently didn't want to hear of the project again, so estimated way to high. Other peoples' money.

 

1 hour ago, Alan Ambrose said:

won't change the size of the cable they're planning to use,

 

I didn't know that. Isn't a 3 phase cable rather different and more expensive?

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7 minutes ago, saveasteading said:

Doing commercial and education projects , where our company took over from the clients' consultants, our electrician usually suggested that the mains supply be reduced, typically halved. The consultants apparently didn't want to hear of the project again, so estimated way to high. Other peoples' money.

 

 

I didn't know that. Isn't a 3 phase cable rather different and more expensive?

3 phase cable is probably twice the cost (4 core cable vs 2 core cable)

 

Another issue though is what infrastructure is in place already.  We are near the end of a single phase 11KV spur.  If I really really really wanted, or had to have 3 phase, then about half a mile of overhead 11KV line would need to be upgraded from single to 3 phase, and the transformer feeding our house would need replacing with a 3 phase one.  You can imagine the cost of that would be completely stupid.

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  • 2 months later...
On 24/01/2024 at 17:19, ProDave said:

3 phase cable is probably twice the cost (4 core cable vs 2 core cable)

 

Another issue though is what infrastructure is in place already.  We are near the end of a single phase 11KV spur.  If I really really really wanted, or had to have 3 phase, then about half a mile of overhead 11KV line would need to be upgraded from single to 3 phase, and the transformer feeding our house would need replacing with a 3 phase one.  You can imagine the cost of that would be completely stupid.

Just had my quote through from SSE - £70k to get 3-phase to us. Clearly not going to pay anything near that.

 

Anyone got any tips on how to get that reduced, or is it so prohibitively high that there’s no point going down that road…?

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7 minutes ago, AppleDown said:

Just had my quote through from SSE - £70k to get 3-phase to us. Clearly not going to pay anything near that.

 

Anyone got any tips on how to get that reduced, or is it so prohibitively high that there’s no point going down that road…?

 

Do you really need three phase? My quote between single phase and three phase wasn't that different, so went three phase though don't need it.

 

Have they quoted for the trench work from the road to the house  as iirc its quite a distance.

Edited by Moonshine
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10 minutes ago, AppleDown said:

Just had my quote through from SSE - £70k to get 3-phase to us. Clearly not going to pay anything near that.

 

Anyone got any tips on how to get that reduced, or is it so prohibitively high that there’s no point going down that road…?

It will depend on what the breakdown of the quote is. Can you provide that? Also, what did you ask for?

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7 minutes ago, Moonshine said:

 

Do you really need three phase? My quote between single phase and three phase wasn't that different, so went three phase though don't need it.

 

Have they quoted for the trench work from the road to the house  as iirc its quite a distance.

Yeah that’s correct - from the main road and then down the lane.

 

We don’t particularly need it - we were looking at as much solar as possible, but from what I can see it may be an issue with a higher capacity generated and not have 3-phase…

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4 minutes ago, AppleDown said:

Yeah that’s correct - from the main road and then down the lane.

 

We don’t particularly need it - we were looking at as much solar as possible, but from what I can see it may be an issue with a higher capacity generated and not have 3-phase…

 

I would ask them to revise the quote based on you doing the trench work on your land.

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2 minutes ago, Moonshine said:

 

I would ask them to revise the quote based on you doing the trench work on your land.

That quote is with us doing the trench work 🙃

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