3 phase domestic job

[The Mains Man]

I fully accept that having swapped trades half way through my life I don't have the years of experience that many of you guys have and maybe I should have put my time in as an apprentice, but that isn't how it all turned out for me!

I am a qualified electrician with Stroma, but find their technical support patronising and unhelpful to say the least.

I do a lot of standard domestic work but I haven't done a lot of 3 phase. I have a friend who has a domestic 3phase installation which needs re-designing as they are upgrading their old night storage as well as getting new immersion heaters and aga etc. An extension is pending too!

I really want to help them out with project as this as the work lies awkwardly between your average domestic and commercial and they can't find anyone who can deal with it. It would be interesting and push my knowledge but it wouldn't be something I would be prepared to take on without having someone more experienced check my plans.

It would be quite a mission to put it all in a post so I was hoping maybe someone experienced would be willing to chat on the phone or meet up for an hour to talk it through. Happy to pay for the time.

 

[Spoon]

We like long posts...
Also you would get more options on how to do a job if you get more input from the forum, rather than just one person.
We will be gentle..... with a little bit of pee taking (It's compulsory..)
Just give us your thought on the installation and we can advice.. That way you learn stuff as well.

 

[LankyWill]

What’s the service intake? Is there a three phase supply present and what’s it’s current loading?

 

[westward10]

Doesn't lie between commercial or domestic at all, it is domestic.

 

[static zap]

........ it is domestic.

Just the kind of domestic , who had servants in the past !

 

[The Ghost]

It is likely that they are only using one of the phases have you checked? Quite a few houses have three phase but only use one usually

 

[Gavin John Hyde]

It is likely that they are only using one of the phases have you checked? Quite a few houses have three phase but only use one usually

I came across an empty chapel/small church in Bristol around 18 months ago, that was 3 phase. They had a meter on one phase that was never used! no tails out the bottom. but all the electrics were on one of the other phases sans meter straight to the CU.. had fun explaining that to the potential buyer in the eicr.. Not sure if he still bought it or not as he wanted to convert it into some fancy home. Makes you wonder if the church were fleecing the electric or not all them years.

 

[The Ghost]

I'm in Bristol if you want to PM me.

 

[Adam W]

Doesn't lie between commercial or domestic at all, it is domestic.

You forgot to finish with "end of", meaning your opinion counts for bubkes.

 

[The Mains Man]

Thanks guys.. ok a "little" more here then...

Firstly, when I say it falls between domestic and commercial... I only meant in terms of the kind of electrician who would deal with it. Too complicated for a lot of domestic only electricians (including me probably) and not big enough for most commercial oriented companies.

It is currently economy 10, 3 phase with one 3phase meter in the DNO cut-out - 3x 80A fuses (I believe although I can't check). 5 Tails go through the wall to an internal "cutout" (it isn't really, but I will call it that for ease) There are the three lines and neutral which go to henley blocks, then from there two outs;

1 -> 30mA RCD switch ->contactor (which I have replaced when it shorted & fried) which has the off peak switched line from the meter -> off peak 3phase board (DB-A) around 8m away (one of 5 DBs in understairs cupboard) via two T&E cables 16mm2 I think although I need to check. This feeds the main heating demand for 7 night storage heaters + the aga.

2 -> seperate 30mA 3ph RCD -> DB-E via 16mm SWA .
This currently feeds permanent live feed to the 7 storage heaters + the aga... As well as (I suspect**) the other 3 single phase boards

Understairs electrical cupboard we have 3 more single phase boards with various sockets & lighting. These 5 boards are mounted on a large piece of ply with cabling behind so not easy to see without dismantling.

** So far I have not assessed the details of the current wiring as that will be a fair chunk of time to strip down and figure out exactly what goes where.

This all started when the contactor controlling the heating shorted out and went crispy blowing the main fuse twice. They then called me and after I identified the smoking switchgear was told "Ah yes, I did smell burning but my husband said it was nothing". Anyway. this was in the winter so I sourced and replaced the 90A contactor and it all seems to be fine again.
This incident (and their £1200 electricity bill) caused them to rethink everything and the plan now (after a little encouragement) is to come off economy 10, replace 7 old inefficient night storage heaters with new electric rads, replace old aga with newer more efficient one. Replace water heater with two new immersion tanks (2x6kw) which will actually be more but they want the extra capacity. They also want a new 32A induction hob fitted. They also want to lay in provision for a sizable 'garage conversion' which hasn't got to the planning stage yet.

I will replace all the 5 consumer units with one (or maybe 2 if required) - these will run all lighting & sockets as well as the storage heaters and aga, as well as a new c/u in the garage.

I need to do a full assessment of the current C/Us sockets & lights but also trying to start a rough plan of how to distribute things.

Not having had a whole lot of experience designing 3-phase installations like this I am keen to get input.

I am thinking use 16mm 3-phase XLPE SWA from 30mA RCD straight to new boards in understairs cupboard. Take one phase from understairs cupboard to new garage consumer unit using SWA which will run outside. This will power the 2x6kw immersions plus extension sockets & lights. It would be physically easier to power the new induction hob from here too, but worried about load. They haven't as yet confirmed requirements for extension but I'm assuming at least 1 final ring circuit and 2 lighting.

The new radiators total actual potential draw is about 85A, but having each rad on a breaker will mean a nominal breaker total of 110A - it seems to depend on who you ask as to how to come up with max load / diversity, however you look at it, that is the best part of a whole phase, leaving around 80A for the sockets, lights & aga

I would like opinion on any of this, but specifically how to distribute the load - I don't really want to buy new 3phase boards - the two that are already there seem fairly new and adequate. I would like to run single phase to the garage to save putting a 3-phase board there and to save running 5-core out there, but my gut says easier to balance the phases if there is a 3-ph board in the garage too.

OK - this post is clearly long enough. I'm sure there will be questions which I will do my best to answer.

Thanks for making it to the end!

I think I am prepared for the torrent of question, humour and hopefully useful suggestions to come!...

 

[UKMeterman]

The customer might apreciate a couple of these fitted to key circuits Rayleigh Instruments RI-F384 1 & 3 Phase Meter RS485 Modbus RTU & Pulse Output - https://www.rapidonline.com/rayleigh-instruments-ri-f384-1-3-phase-meter-rs485-modbus-rtu-pulse-output-23-1534 so the sub circuits can be metered, these will need external current transformers.

 

[mhar]

Do you have any 3 phase equipment to run? If not why not install an rec4 at the origin and run 3 single phase distribution circuits (maybe via swich fuses depending on distance) to convenient locations and have 3 single phase consumer units

 

[The Mains Man]

Do you have any 3 phase equipment to run? If not why not install an rec4 at the origin and run 3 single phase distribution circuits (maybe via swich fuses depending on distance) to convenient locations and have 3 single phase consumer units

Do you have any 3 phase equipment to run? If not why not install an rec4 at the origin and run 3 single phase distribution circuits (maybe via swich fuses depending on distance) to convenient locations and have 3 single phase consumer units

Yes, that us definitely an option. No 3ph equipment.

 

[davesparks]

the plan now (after a little encouragement) is to come off economy 10, replace 7 old inefficient night storage heaters with new electric rads, replace old aga with newer more efficient one.
Replace water heater with two new immersion tanks (2x6kw) which will actually be more but they want the extra capacity.

I am thinking use 16mm 3-phase XLPE SWA from 30mA RCD straight to new boards in understairs cupboard. Take one phase from understairs cupboard to new garage consumer unit using SWA which will run outside. This will power the 2x6kw immersions plus extension sockets & lights. It would be physically easier to power the new induction hob from here too, but worried about load. They haven't as yet confirmed requirements for extension but I'm assuming at least 1 final ring circuit and 2 lighting.

The new radiators total actual potential draw is about 85A, but having each rad on a breaker will mean a nominal breaker total of 110A - it seems to depend on who you ask as to how to come up with max load / diversity, however you look at it, that is the best part of a whole phase, leaving around 80A for the sockets, lights & aga

Coming off of the economy tariff and replacing storage heaters with electric radiators will increased their electricity bill, not reduce it.

Why are you selecting a cable size of 16mm when you don’t yet know the load? And why do you want to put a 30mA RCD on this circuit? This sounds like a terrible design to me.

What is a ‘nominal breaker potential’? That total heating load is not the best part of a whole phase because you balance the load across the phases of a three phase supply.

 

[Pete999]

Coming off of the economy tariff and replacing storage heaters with electric radiators will increased their electricity bill, not reduce it.

Why are you selecting a cable size of 16mm when you don’t yet know the load? And why do you want to put a 30mA RCD on this circuit? This sounds like a terrible design to me.

What is a ‘nominal breaker potential’? That total heating load is not the best part of a whole phase because you balance the load across the phases of a three phase supply.

Not sure if I'm understanding the heater question, are you intending to run 1 circuit as say, a radial for all the heaters? Probably misunderstood your statement

 

[The Mains Man]

Currently the

Not sure if I'm understanding the heater question, are you intending to run 1 circuit as say, a radial for all the heaters? Probably misunderstood your statement

No, even I know that is a bad idea they are all on separate mcbs, currently distributed across the three phases and I would maintain that. I was considering the merit of splitting the three phases by location as that suggested or whether I run all three phases to both locations to aid balancing and possibly flexibility.

Sorry if my explanation and choice of words isn't clear.

 

[The Mains Man]

Currently the

Not sure if I'm understanding the heater question, are you intending to run 1 circuit as say, a radial for all the heaters? Probably misunderstood your statement

No, even I know that is a bad idea they are all on separate mcbs, currently distributed across the three phases and I would maintain that. I was considering the merit of splitting the three phases by location as that suggested or whether I run all three phases to both locations to aid balancing and possibly flexibility.

Sorry if my explanation and choice of words isn't clear.

Coming off of the economy tariff and replacing storage heaters with electric radiators will increased their electricity bill, not reduce it.

Why are you selecting a cable size of 16mm when you don’t yet know the load? And why do you want to put a 30mA RCD on this circuit? This sounds like a terrible design to me.

What is a ‘nominal breaker potential’? That total heating load is not the best part of a whole phase because you balance the load across the phases of a three phase supply.

30mA RCD is already there, presumably to protect the distribution circuit. I would use a 100mA RCD, i don't know why it isn't.

This was one of the points I was questioning... See reply to previous question above.

Maximum breaker potential is the kind of nonsense I come up with when trying to explain something I am finding challenging at 11:30pm after a long day. I guess I meant the sum of Ib for heating circuits.

 

[Andy78]

Why would you want to put an rcd of any type on a distribution circuit ?
To work out diversity it would be best to start with total connected loads allowing a sensible amount for socket circuits. Load division across phases also needs balancing as best as possible.
The electric heating plan sounds very costly to run. Have they no gas ?

 

[westward10]

Could be incorrect but that rcd setup says TT to me.

 

[The Mains Man]

Could be incorrect but that rcd setup says TT to me.

I see your thinking, but no. TN, TNCS I believe. The RCD is before the contactor and did turn out to be handy when there was a short before on the contactor connections!