HELP! Electrical supply - advice sought! How do I calculate the power requirement for our build?

[Great_scot_selfbuild]

It's in the title really, and I'm short on time to scour the internet for how I calculate the total load.

 

We don't have everything specified yet, but to give an idea:

4 bed house, high-performance timber build, ASHP (6kW).

It will also have MVHR, a 7.5kV EV charger.

I'm sure we could run off single phase power if that's all they had on the street (the power company doesn't know until they do a full application and in-person survey?!?), however, we want them to quote us for 3-phase power so that we have it installed as future-proofing, or rather so we at least know what the cost difference is and whether it is even routed nearby. If we put the demands in for single phase, I just don't want them to only quote for single phase.

 

Hopefully that makes sense?

 

Grateful if one of you would offer some advice.

 

Many thanks

 

[JohnMo]

Bit of conflicting requirements - you want to calculate but you also just want 3 phase.  Why bother calculating? Although no idea why a domestic house needs 3 phase!

 

12 minutes ago, Great_scot_selfbuild said:

4 bed house, high-performance timber build, ASHP (6kW)

If as stated, it could get away with 4kW, would need to do the calcs.

 

We have all you state above, except car charger, our highest 30 mins demand over the last few days including driving immersion (3kW) and heat pump on, MVHR, well pump, treatment plant compressor etc at that point was just over 1.8kWh in 30 mins, so just under 16 amps from a 100 amp supply.

[Great_scot_selfbuild]

2 minutes ago, JohnMo said:

Bit of conflicting requirements - you want to calculate but you also just want 3 phase.  Why bother calculating? Although no idea why a domestic house needs 3 phase!

 

If as stated, it could get away with 4kW, would need to do the calcs.

 

We have all you state above, except car charger, our highest 30 mins demand over the last few days including driving immersion (3kW) and heat pump on, MVHR, well pump, treatment plant compressor etc at that point was just over 1.8kWh in 30 mins, so just under 16 amps from a 100 amp supply.

The electricity company form appears to want you to calculate it / show figures. There I was just wanting a quote for single phase or 3-phase.

[nod]

When you call to book your quote 

Tell them you intend installing a HP

The young lady with months of experience will tell Oooo you definitely need three phase 

Three quotes All wanted to sell us a Three phase supply 

Just tell them no thanks 

 

 

[SteamyTea]

There is a thing called 'diversity'.

This is what governs how much power each circuit is likely to take, depending on the type of load and devices fitted i.e. a cooker has a diversity of 1, only thing on that circuit.

Bedrooms will be different as the largest loads will be vacuum cleaner, and that is only run for a few minutes.

 

So Google diversity and you will find formula.

Or ask @ProDave, though he is working under scotch regs.

[Conor]

There's usually a simple form that you fill out the rating of each appliance and calculates it for you. Unless you have a lot of PV, I'd be surprised if it'll dictate a three phase when you add up. Our house with two ovens, induction hob, EV charger, heatpump etc only came to 50amps or so. Well under the 80amp max.

[SteamyTea]

13 minutes ago, Conor said:

dictate a three phase when you add up.

My DNO fits 3phase by default, other DNOs may well do the same.

[Bramco]

AFAIK, what's down the road is 3 phase and in the past, they tapped a single phase off for each house, having a balance of houses on each of the phases.

 

Again AFAIK, all new builds have 3 phase connected to the meter box, future proofing things for when everyone is all electric. So the supply to the meter box is 3 phase.

 

Then, for a normal house, they tap off a single phase with a 100A rating, i.e. fuse. This is then your supply.

 

Our region is National Grid. They did a site visit - it's more to work out how much cable is needed, where to tap into on the road and to sketch out where their cable will run. So don't worry about the site visit, unless you're kms away from the nearest road.

 

We load our batteries, run the ASHP and have the DHW immersion running at the same time on the cheap night rate. Sometimes the dishwasher will also be on and the highest I've ever seen the load is about 12kW, which is well within the 100A rating of the supply. It's normally about 9 or 10kW while everything is on.

 

I guess, with an EV charger pulling 7.5kW it would take you close with absolutely everything on.

Edited November 14 by Bramco

[saveasteading]

9 hours ago, JohnMo said:

As stated, it could get away with 4kW,

A kettle and a toaster? Have to turn the ashp off during breakfast.

[Russdl]

How many EV’s are you likely to have on the drive in 10/20 years time?  3 or 4? You may want something faster than 7.5kW charger. 
 

I’d go 3 phase if it’s not prohibitively expensive. 

[saveasteading]

Just now, Russdl said:

 3 phase if it’s not prohibitively expensive

I was very surprised to see how little extra it costs. That's assuming you don't have to pay for a transformer.

 

But single phase seems to be plenty for a typical house so why pay more?

[Russdl]

We have a fully electric house and could get close to blowing our 80 amp main fuse if everything was on at once, of course it’s not - not normally but on the current Octopus smart tariff I want everything on from 23:30 at night.
 

We use a Willis heater for the underfloor heating (with the ability to use 2 if it’s really cold). If both of those came on at 23:30, plus the Sunamp and the house battery started charging plus the electric UFH and towel rails plus the dishwasher plus the washing machine plus the car started charging then the fuse would go. On the Octopus tariff I don’t have a huge amount of control over when the car starts charging so because of that I’ve restricted it’s charge rate to 4kW (I’m probably starting to use the wrong units now but hopefully you’ll understand). 
 

I know not to have everything come on at 23:30 but the next owner won’t (unless they read the Haynes style owner manual I’m writing). 
 

In the winter months I probably couldn’t charge a second EV over night, I should have gone 3 phase but I didn’t understand all of this before I made that decision. 

Edited November 14 by Russdl

[SteamyTea]

15 minutes ago, Russdl said:

How many EV’s are you likely to have on the drive in 10/20 years time?  3 or 4? You may want something faster than 7.5kW charger

If you have more cars to charge, the chargers will throttle back so that they don't cause too much voltage drop.

I think car chargers have to be 'smart' already.

[Bramco]

4 hours ago, Russdl said:

I’d go 3 phase if it’s not prohibitively expensive

 

There's been quite a bit on here about 3 phase smart meters being a pain to get hold of and (iirc) being difficult to re connection to the suppliers.

Given a new build will have 3 phase to the meter box, you can have a single phase meter connected for the supply and then at a later date trade up to a 3 phase if needed.

 

[Bramco]

3 hours ago, Russdl said:

I know not to have everything come on at 23:30 but the next owner won’t (unless they read the Haynes style owner manual I’m writing). 

 

I wonder how many of these there are.....    Maybe we need a new section for folks to upload their manuals (duly redacted) so that other folks can glean useful help/ideas, re headings, cross referencing etc.  Even whole sections could be plagiarized, for example one on Sunamps...

[Russdl]

@Bramco great idea. Mines still very much a work in progress but the more obscure stuff (Sunamp, Willis Heaters) is covered. 
 

 

And now looking at that photo I realise the house nickname on the spine text is upside down. Damn, back to the drawing board!

[LnP]

You could try playing around with this demand calculator.

I don't have an EV, but assume that if I did, I'd want something faster than a 7 kW charger. I'm guessing I'd want a 22 kW charger?

[Alan Ambrose]

Unless there’s some local power restrictions you want 100A single phase or 66 or 80A per phase (I forget which) 3P. That’s it. If there are local problems then you will still want those numbers but you may not get them.

 

p.s. I found Octopus best for 3P meter installation. Allow 3 or 4 months though.

Edited November 15 by Alan Ambrose

[LnP]

I was with Eon when I wanted my 3P meter installed and they were hopeless. I switched to Octopus and they got it sorted.

[trialuser]

19 hours ago, LnP said:

You could try playing around with this demand calculator.

I don't have an EV, but assume that if I did, I'd want something faster than a 7 kW charger. I'm guessing I'd want a 22 kW charger?

Currently not that many cars can accept a 22kw home ac charge. Tesla model Y for eg is 11 kW max. Don't buy a Tesla by the way :-)

[SteamyTea]

14 minutes ago, trialuser said:

Don't buy a Tesla by the way

That is a big statement, what are the reasons?

[Mr Blobby]

For our connection application the DNO wanted to know the big appliances, so car chargers, ASHP, oven, and list the kVA for each.

 

For us the 3-phase cost was almost the same as single phase but that is because of our location I think, with three phases running down the street outside.

Out primary reason for 3-phase was to get more solar PV on the roof and the 3-phase car charging. 

 

We have, I think 80A per phase (still to a temoprary cabinet) which is more than we need, but the DNO has a fixed price for up to 80A per phase and is quite happy to connect at that level almost as standard practice. 

The DNO was more concerned about startup current for the (7KW!) heat pump, so I had to send them the technical specifcation.

 

If you do get a 3-phase connection, remeber you need a bigger permali box and hockey stick.  Our DNO is a PITA about this stuff.

Edited November 15 by Mr Blobby

[trialuser]

4 hours ago, SteamyTea said:

That is a big statement, what are the reasons?

3 main reasons

Phantom braking

phantom braking

phantom braking

and then, minorly,  auto windscreen wipers which dont work, unpleasantly hard suspension, very expensive insurance, over intrusive corrective steering applied for my safety, massive depreciation, all contact with Tesla service has to be done via an app with very long waits for appointments, very few physical buttons for controls like wipers heating and lights, crappy voice recognition and a few other thing which will make me sound even more petty.

Good points are charging network and nippy to drive.

I knew most of the above before I bought one and thought I could live with them, but they are starting to niggle, and the phantom braking is terrifying.

[Mr Blobby]

16 minutes ago, trialuser said:

3 main reasons

Phantom braking

phantom braking

phantom braking

and then, minorly,  auto windscreen wipers which dont work, unpleasantly hard suspension, very expensive insurance, over intrusive corrective steering applied for my safety, massive depreciation, all contact with Tesla service has to be done via an app with very long waits for appointments, very few physical buttons for controls like wipers heating and lights, crappy voice recognition and a few other thing which will make me sound even more petty.

Good points are charging network and nippy to drive.

I knew most of the above before I bought one and thought I could live with them, but they are starting to niggle, and the phantom braking is terrifying.

 

Phantom braking, insurance : Agree with both of these, although I don't do long long motorway drives very often so the phantom braking is not a big deal for me. 

 

The rest are all things I like.  I like the app-service-system, and the suspension feels just right, and I like no-buttons.  I hired a Mercedes eq recently and hated the sea of buttons.

I'm on my second model 3 and very happy.  Although with Musk turning to the dark side this may be my last.

 

[SteamyTea]

29 minutes ago, trialuser said:

Phantom braking

Is that when it 'sees' a speed restriction sign on the back of a lorry?