3 phase distribution question

You will need to provide overcurrent protection for the "submains" to the different buildings.

I see basically two options.

1. A single 3 phase DB in the outbuilding, this is potentially the neatest soloution, but the issue is that most "normal" 3 phase boards are designed for outgoing devices up to 63A, which may be a bit on the low side if you ever plan to have electric heating in the houses. Incorporating metering may also be tricky.
2. Henley blocks and switch-fuses, While less tidy I think this is probablly the best option.

robinbanks said:

Fairly unusual to have a 3 phase domestic AC charger ...

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Indeed, but which is the chicken and which is the egg?

Even though mine does, I would say that It is at least fairly unusual (probably very unusual) for a UK domestic property to have a 3-phase supply, so the market for 3-phase domestic charges is, presumably, currently very small.most could not have a 3-ophase charger (o.

However, that might well all change in the fullness of time, As I recently reported, if one tells the SSE on-line 'Demand Calculator' that one has an electric cooker, one electric shower and a 7 kW EV charger (and no other 'large loads'), it says that a single-phase supply is not adequate. If that sort of view results in a marked increase in the number of domestic properties with 3-phase supplies, it would seem likely that there will be a corresponding move to domestic chargers >7kW, wouldn't it? ... and don't forget, it's 'the future' that the OP is thinking about

Kind Regards, John

plugwash said:

You will need to provide overcurrent protection for the "submains" to the different buildings.

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Yes, I should have added that.

I personally agree that your Option 2 (Henley's & Switch Fuses)( is probably the preferable approach. I'm probably sounding a bit like a broken record but, yet again, that's how it's done in my house.

If it were a TT installation (presumably not, but mine is) the distribution circuits would also need RCD protection (to provide fault protection for the distribution circuits).

plugwash said:

You will need to provide overcurrent protection for the "submains" to the different buildings.

I see basically two options.

1. A single 3 phase DB in the outbuilding, this is potentially the neatest soloution, but the issue is that most "normal" 3 phase boards are designed for outgoing devices up to 63A, which may be a bit on the low side if you ever plan to have electric heating in the houses. Incorporating metering may also be tricky.
2. Henley blocks and switch-fuses, While less tidy I think this is probablly the best option.

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1. If you envisage more than 63A feeds being needed, then best option is what the manufacturers often call a 'hybrid board' Hager and MEM do them, where you get a couple of TP ways at the bottom that are rated to 125A and taking physically larger breakers (each pole is 27mm rather than 18mm).

2. Personally I don't think one should be planning a new job with a load of switchfuses looped out of Henley blocks if one can help it, the proper solution is a busbar chamber, but once you have factored in the cost and the labour to mount all the switchfuses, MCCB panel boards start to look the better option!

If it were me, I'd be going with the hybrid board option, its a bit more expensive than a standard TPN board (partly because they are a varient of 250A boards, and obviously you've also got the cost of the incommer which is usually MCCB style, rather than a DIn rail device, but its tidy, one unit and you can feed local final circuits from it too

Adam_151 said:

2. Personally I don't think one should be planning a new job with a load of switchfuses looped out of Henley blocks if one can help it,

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I'm personally not sure that I'd see that as a problem in a domestic installation it's what I have ). Three is not really 'loads', but if it's the number of 'boxes' that concerns you, 3-pole switch-fuses are available (and I imagine probably appreciably cheaper than the approaches you are suggesting).

JohnW2 said:

As I recently reported, if one tells the SSE on-line 'Demand Calculator' that one has an electric cooker, one electric shower and a 7 kW EV charger (and no other 'large loads'), it says that a single-phase supply is not adequate

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Parroting the nonsense spewed by such a badly designed tool will do your credibility no favours

In Germany 11 kW chargers are the norm as far as I know, Type 3 charge leads.

Where you have "some sort of distribution", I would have that as a 3 phase distribution board. A 3 phase breaker for the heat pump/battery storage/large equipment, and three single pole MCBs for the three houses.
Arrange for the metering to be installed where required.
You have taken balancing into consideration. All seems fine.

robinbanks said:

Parroting the nonsense spewed by such a badly designed tool will do your credibility no favours

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{t's not as simple as that, since it appears that the figures produced by that tool are what are used by SSE to make its 'policy decisions'.

I don't think the tool is 'badly designed', since it appears to do satisfactorily what it is designed to do (estimate the maximum possible instantaneous load, with minimal consideration of any sort of diversity - the only lip services to diversity it plays are (a) that it estimates the first shower as representing an 8 kVA load, but the second and subsequent ones only as 4 kVA and b) that they regard "electric cooker = 'Yes' " as representing a load of 3 kVA. . What is, I would say, questionable is whether that estimate is a reasonable basis for their 'policy decisions'

I became aware of this because a friend of mine applied to SSEN or provision of an electricity supply to a house he is having built. On the application form he had to provide information about the loads, and he specified what I mentioned in my previous post - an electric cooker (for which only yes/.no is asked), one electric shower (again, on;y yes/no and 'how many), one 7 kW EV charger, and no other specific 'significant loads'

As a result of his application he was told that he would need a 3-phase supply, because his estimated 'maximum (instantaneous' demand was too high or single phase (and SSEN do not provide new 'split-phase' supplies).

He asked for further clarification/inspection, and they indicated that they base their decision about the type of supply required on an estimate of 'maximum (instantaneous) demand', based on information provided by the customer, and told him that the on-line calculator they provided uses identical algorithm to the calculation they undertake. For the information provided (as above) that resulted in the following figures:

'Assumed standard 'background' maximum demand in a domestic property = 6.25 kVA
Electric cooker =3 kVA
One electric shower = 8 kVA
One 7 kW EV charger = 7 kVA

That's an estimated total of 24.25 kVA (105.44 A at 230 V) and, since it is greater than 100A, results in this ...



My friend is currently negotiating with them,but on the face of it, the situation would appear to be that, on the basis of the 'load information'; he has provided, they are not prepared to install a single-phase supply (whereas he is arguing that it would be ';adequate' if one takes a sensible amount of diversity into account).

I am therefore far from convinced that my credibility will suffer much as a result of my reporting ('parroting') this situation

I could perhaps have added that it's not as if they don't 'understand' the concept of diversity, since they apply it very sensibly when designing their network. The whole picture changes dramatically if there is EV charging since, even for network purposes, they appear apply no diversity to EV charger(s) - but, in the absence of EV charging, the 'network design' load they consider a property to represent is only a very small proportion of the 'maximum possible (instantaneous) demand'.

As the following shows, taking the example of a property with just an electric cooker and two electric showers (plus assumed 'standard background loads') the 'maximum (instantaneous demand' is estimated as 21.25 kVA, but this is only considered to be 3 kVA for network design purposes ...

Hi all. Sorry, I stopped getting notifications for new posts to this topic for some reason. Thanks for the additional contributions.

If I can ask a semi-related question for the same project - downstream of the 3 phase distribution I have the supplies running to the two separate houses. These houses are very old, with metre thick stone walls and evidently had their electricity put in at a time when that was a thing to be proud of, as all the circuits run back to a consumer unit right next to/above the front door in both properties. Probably a bragging point at the time but considered a bit ugly these days.

Anyway, I now need to get the two feeds from the distribution in to these consumer units, and my electrician has indicated that he has to use large cross section SWA which will be very inflexible. That means that we're going to have to do some complex and careful channeling to get the cables in through the walls with the bend radius lost in the wall so that the cables come out in the correct location to terminate at the consumer unit.

Does anyone see any other alternative? Is he correct about the spec of cable he needs? It sounds logical to me but I thought I'd ask in case anyone has worked on something like this before.

If, for example, there was a permissible way to terminate the SWA cable in the building but not directly in to the consumer unit, that might open up other options.

Many thanks (again)

PS. Both properties have modern and safe wiring now - but the layout of the circuits definitely seems to have a historical influence.

Whilst your running your cables for your new supplies, consider also running several CAT6 or above internet cables, do not rely on wifi.

Consumer units can be moved, extending the cable in a large junction box with a DIN rail (the same metal bit the devices in a consumer unit are clicked onto). A new CU could then be installed in a location more convenient for a new supply.

Other than that I believe your options are limited.