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Domestic 3 phase metering


dnb
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I am being steered towards 3 phase in my new house. There seems little chance of escaping this so I may as well embace it (while wearing a pair of those long sleeve rubber gloves ? ). It gives opportunities later, and negates the need to beg quite so hard for permission for my desired amount of PV.

 

But I am wondering if there is a definite answer as to how the metering works in a domestic application. My experience of 3 phase billing is limited to my Dad's work where reactive loads were worried about (a half remembered conversation when I was 10 years old), and my old London flat where the supplier would send an engineer round every year when we complained about the £9000 bill because one of the 3 spinning disc meters was going backwards again... (not surprising - fluorescent lights and a lift from the 1950s all wired in the most unbalanced way possible will tend to do that...). These days my 3 phase experience is only using it at work to power my "death ray" toys. I don't pay the bills, but sometimes the man that does gets a bit upset and talks about Triads (apparently not gangs in this context) and it getting expensive for me to play, so I have to switch the toys off and perhaps I would like to go home to the family so he can save on the wage bill too?

 

I am hoping therefore that 3 phase in a house is going to be much more straight forward. I have found some nice 3 phase inverters that claim to evenly output to all phases, and I will look to have a 3 phase immersion heater so I can easily make use of my PV. The snag comes if I try to get the house to achieve zero consumption. Do I need to balance each phase separately, or do I just need to achieve net zero across the 3 phases? The latter is obviously much easier than the former. Does the "borrowing" of small amounts of power below the meter resolution still apply to 3 phase metering, enabling the trick to control the immersion heater load?

 

Or, in practice does it really matter? I have a near teenage daughter and a wife who can empty a hot water tank in minutes between them, so all the solar is therefore spoken for...

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A lot of 3 phase meters measure KVA That's thousand Volts.Amps   rather than Watts.

 

The difference is, if the load is not in phase (i.e power factor less than 1)  you get charged for the amps consumed but they translate to less actual power used.

 

I can't see it is going to be a big concern for the average domestic property, though it would be interesting to find out what the power factor of say a heat pump is.

 

3 phase will let you have three 16A PV systems without prior notification.  The downside is the house loads may not be on the same phase as the PV to use that power so you have to think carefully how to spread your loads for best effect.

 

What is the reason that is driving you towards 3 phase?

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My main concern with 3 phase in a  house would be how the phases are safely split across loads.  It could be potentially iffy to have some outlets on one phase and some outlets on another, for example.  Keeping the 3 phase supply for specific 3 phase loads seems the best approach, with the house domestic loads (lighting, outlets etc) all coming from the same single phase.  Balancing doesn't usually matter with domestic loads, as they won't be massive and there is no means of properly balancing existing domestic supplies, anyway.

 

The sort of loads that could run from a dedicated 3 phase supply might be three PV systems of up to 16 A each, maybe a 3 phase car charge point (probably not something that's going to be around in the longer term for AC charging, though), 3 phase garage equipment, perhaps, or maybe a 3 phase heat pump (have to be a pretty massive one to need a 3 phase supply, though).

 

Power factor is not yet something that the DNOs seem to bother about for domestic loads, but that seems likely to change sooner or later, as the new metering standard has provision for kVA billing.  Right now there has been a statement that there is no intention to roll out kVA billing to domestic customers, but that then begs the question as to why it was included as an option in new meters.  Some household loads have a pretty poor power factor, as things like switched mode power supplies and inverter drives have become more popular.  Sooner or later I suspect that there may be a need to consider PF correction on domestic supplies.

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You could ask the DNO how large a PV installation could be fitted to 1 phase, they may allow more than the usual 16A limit.  Then hang the appropriate loads off that circuit.

8 hours ago, dnb said:

These days my 3 phase experience is only using it at work to power my "death ray" toys. I don't pay the bills, but sometimes the man that does gets a bit upset and talks about Triads

Sound like an accountant wants to turn off the things (are these lasers) that make the company money.

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3 hours ago, SteamyTea said:

accountant wants to turn off the things (are these lasers) that make the company money

Not lasers, but it seems you've met accountants too. I work with radar.

 

4 hours ago, JSHarris said:

It could be potentially iffy to have some outlets on one phase and some outlets on another

I was worried by this too, but strangely the new blue book is rather less concerned than I am. I got accused of being old! Anyway, to satisfy myself I have a reasonable design worked out with a work colleague to mitigate most 415v issues.

 

5 hours ago, ProDave said:

What is the reason that is driving you towards 3 phase?

Building control, the location of the house on the grid and the amount of PV I would like all drive towards 3 phase. For some odd reason building control equate the floor area with power consumption. I would like at least a 10kW peak PV system, so given I am out in the sticks, SSEN are strongly hinting at 3 phase. It's an electric house (no mains gas and no desire to burn things in the house) so I can see their point. My sums say I am close to the limit when the immersion heaters, hob, oven and A/C are all on together on a dark and cloudy day, and I should allow some headroom for the future.

 

5 hours ago, JSHarris said:

3 phase garage equipment, perhaps, or maybe a 3 phase heat pump

3 phase garage equipment certainly. I have my eye on a couple of things. But the 3 phase heat pump is overkill for anything I am doing.

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SSE are our DNO, and we're out in the sticks too.  When I asked about getting a G59 approval (now G99) they wrote back and gave me approval for up to 10 kWp of generation on our single phase supply.  We're at the end of a fairly long length of 3 phase 95mm² cable, that only supplies our house and one further up the lane, though, so we're both on different phases.  The option to have 3 phase was there, but I worked out that we'd not get close to needing it, even though we're all electric, as after accounting for diversity we weren't likely to ever need more than about 60 A from a single phase supply.   The highest load is my car charger on full power, at ~ 30 A, as although the hob has a similar power rating, allowing for diversity brings this down to 20 A. 

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2 minutes ago, JSHarris said:

We're at the end of a fairly long length of 3 phase 95mm² cable, that only supplies our house and one further up the lane

A bit different here - there are 15 large houses all on multi acre sites with quite a lot of solar PV, all on the end of a long line of (low tension) 3 phase poles. This of course is only one input driving the 3 phase argument.

 

I need a bit more than 60A, but still less than 100A.

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It does vary widely.  We only have single phase here from a single phase 11KV overhead line that is at least half a mile from the nearest 3 phase network, so you could imagine the upgrade costs if we tried to get 3 phase.  But if it is available and you think you might be close to the limit, it is worth getting it.

 

Where we are the network is close to bursting, we were only offered a 12KVA supply which as it happens is plenty.  And when I registered my G83 solar PV (now G98) they initially miss read it and thought it was 4KW not 3.68 and were about to quote me for the required network upgrade. So we have no hope of more than 3.68Kw without paying for something to be upgraded.

 

I believe this is all because a number of houses share a 100KVA pole mounted transformer and in the last 15 years about 4 new houses have been added to that same transformer and they are just looking for someone to charge to upgrade it to something bigger.

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1 hour ago, dnb said:

It's an electric house (no mains gas and no desire to burn things in the house) so I can see their point. My sums say I am close to the limit when the immersion heaters, hob, oven and A/C are all on together on a dark and cloudy day, and I should allow some headroom for the future.

Exactly the reasons why I went 3 phase - I was dancing on the edge if only went single!

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I remember staying in a place in Germany years ago that had a 3 phase supply, from what I can gather this is normal practice there.  The phases seemed to be randomly distributed within the board, with no consumer unit as we'd have, just a load of rail mounted MCBs (all 2 pole, IIRC) all neatly wired up with individual L and N wires on both sides.  Didn't look to me as if there was any logic as to what was fed from each phase, and everything looked to be wired as radials as far as I could tell (I was just being nosey and was curious to see how things were wired).

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I went and checked his after a previous discussion.

 

We have 5kw of PV split equally across 3 phases.

 

Our electricians have split the circuits across the phases, but we have a 3 phase car charger and the pool dehumidifier which is probably the biggest user of electricity in the house is also 3 phase. I have taken some pictures to show how this works.

 

We also have an Elster meter, an A1140 which measures kWh across all three phases so basically works like any other meter.

 

Currently you cannot have a three phase smart meter, none have been approved yet.

 

 

IMG_7632.JPG

IMG_7634.JPG

Edited by AliG
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14 hours ago, dnb said:

The snag comes if I try to get the house to achieve zero consumption. Do I need to balance each phase separately, or do I just need to achieve net zero across the 3 phases?

I did some digging into this over here: 

 

As best I can find, for domestic 3 phase meters will report the net-exporting state (within any given moment in time / 1Wh meter bucket), not the per-phase import/export state. This seems fairly well specified in the SMETS2 polyphase meter spec Part C, although as no one is installing polyphase SMETS2 meters yet so it'll be up to your own experimentation to confirm that on any other 3ph meters.  Looks like @AliG has happily confirmed his A1140  import meter records records net usage across the phases- this is very helpful to know.

 

Whatever you decide, you might as well have the DNO install 3 phase supply to the meter head/cut out. You're then perfectly entitled to only request a single phase meter is installed and used off of that, but it'll give you a much easier upgrade path if you decide the 3ph is useful in future.

 

 

 

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  • 2 years later...

Slightly off topic here but what are they quoting for fitting a three phase supply  dnb?? I take it is reasonable to install with you?

 

My current house ( ex doctors surgery) had split phase fitted, however a couple of winters ago one of the lines broke at the pole insulators on a pole that crossed our car parking area and a cable ended up lying over the top of one of our cars. Long story short, SSEN said we'll do a quick fix for now but will put the supply underground to your house. I asked for the same set up i.e. run in three phase supply cable and use existing three phase meter (just use the two phases like the split phase so no big changes in my consumer unit cupboard) but they point blank refused. I was told pay a £3k connection fee and we'll reinstate it, the three phase connection is less than thirty feet from my house on the other side of my fence and they had dug the track for it already through my car park. I thought that was a bit steep so told them stuff it, just put single phase in.

 

I also asked about three phase for our planned new build but was told the transformer and multiple poles would need upgraded and was advised it would be many 10's of thousands to fit it so that went out the window straight away!

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I think the issue is that they are pricing that like a new connection.

 

We just paid £2800 for a single phase connection for my parents. Most of this was the cost of digging an 11m trench.

 

It looks like the 3-phase connection for our house cost £3200, so just a few hundred more (In 2016). I think less digging was required, the builder organised it.

 

I suspect that they will price up 3-phase like you are asking for a new connection and not care that they are already there fixing your current connection. You could get the quote and if it includes the trench etc ask for it to be adjusted as hey have already dug the trench.

 

If there is not capacity to ad it the cost does increase exponentially.

 

 

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  • 1 month later...
On 30/11/2021 at 00:15, acormack said:

Slightly off topic here but what are they quoting for fitting a three phase supply  dnb?? I take it is reasonable to install with you?

 

Just in case people are searching for this, attached is the quote to upgrade our current supply to 3ph (but essentially they treat it as a new connection)

 

If it wasn't for the road closure, it's pretty reasonable.... Unfortunately no real way around the road closure. The most annoying thing is that the road was closed for 2 weeks in 2020 for drainage works, if we'd known/thought to do it back then, would've been a bargain.

 

873270948_3phfees.thumb.png.c32b1173b4d2a2b7c03248fff7a14458.png

 

3ph map.png

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