Hi Folks

I'm a newly qualified electrician with very little 3 phase experience. I'm currently doing a rewire of a 4 bed house which was to include providing an additional CU for later use, as the garage was to be knocked down and rebuilt plus an extension built where the conservatory currently stands. I was originally told the extension was going to be two additional rooms but it now turns out the client plans to turn the house into a 9 bed with 4 shower rooms and a bathroom with shower, so 5 showers!! I tried to talk him into using power showers, but he's adamant he doesn't want that. On the upside, for him, there's an old 3 phase service head, which the DNO are willing to utilise and they will install a 3 phase meter. Incidentally, everything is currently disconnected as I'm taking out all of the old cabling and chasing out, but there was no Earthing arrangement as such, only an Earth via the gas meter or water pipes, the DNO are fitting a PME for me on Monday, so it will be TN-C-S. The DNO also fitted an isolator just after the meter on the single phase, so I'll be getting them to do that for the 3 phase instead.

My plan, before all this happened, was to put the new CUs under the stairs but now I'm not sure what I should plan to do. If the consensus is that more experience is required, I'm considering subbing it out but, before I do that, if I can do it myself, I'd like to do it as it would be good experience.

Any advice on how you guys with experience would tackle this, would be of interest to me? I'd be grateful for your advice?

Is there anywhere else the CU can go? Personally speaking I wouldn't put a CU under a stairs. (Assuming a normal under stairs type space)

Unfortunately not. The under stairs cupboard is solely going to be used for equipment, I'm also installing an alarm panel and CCTV recording equipment in there and it will be lockable.

Cool.

If you don't do it you'll never learn

If there's room in the cupboard and it's lockable, put it there.

Mike

Sounds large. What's the physical layout of the building like? Would it make sense to bring all the final circuits back to the intake position (under the stairs?) or would it be better to scatter a few CUs around the building to serve different areas and run submains back to the origin?

If the former then one big TPN distribution board might be neat and convenient - and it'd make it easy to swap circuits between phases later on if you had problems balancing the loads. Not easy for a few circuits to share an RCCB though (split-load CU style), so you'd be looking at RCBOs for each circuit that needed them, which might be costly. Make sure you can get B type MCBs and RCBOs if you're using T&E cabling. Another option would be just to have three conventional CUs next to each other and just treat the system as three separate single-phase supplies.

They tend to have a lot more 3-phase domestic on the continent - if you look through suppliers' catalogues you'll often see CUs with 3 rows of devices in them and people then build their own split-load arrangements as needed - a sort of half-way house between a TPN DB and a CU. Some of the French units especially are huge and often contain phone/data/tv systems as well as mains.

- Andy.

Presuming that no 3 phase loads are involved, then the installation is not that different to a more typical single phase job.

I would be inclined to fit 3 boards, each single phase.
You may need a Henly block for the 3 neutrals, but the 3 phase conductors can run directly from the 3 phase isolator to the consumer units if nearby.

What size is the 3 phase supply ?
For 5 showers you will need 100 amps, and even that is marginal.
Many showers are now 40 amps, two of those on one phase of a 100 amp service does not give much headroom for other loads.

Just a few thoughts
1/ Is the property going to be used for purely residential purposes?
2/ 3-phase is not uncommon in large houses but separation of phases and non-ambiguous labeling would be advantageous unless 3-phase equipment is going to be used
3/ Why does the client, being, one supposes, a non-competent person wants solely electrical showers. I would have thought that energy saving systems and components would be a priority as well as complying with current regulations.
4/ A switch room with main-intake protection and isolation under the stairs is not what I would consider a good design move. particularly if it is a residential property....
5/ With the plans for this property you should be able to design a suitable management system, use of service ducting, sub-main routes etc

Legh

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Why do we need Vernier Calipers when we have container ships?

http://www.leghrichardson.co.uk

"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."

dont forget your pfc will be higher on a 3 phase installation.

--putting aside the "inexperienced and 3 phase" stuff, I always advise
checking with the DNO the service position.
as our poster has mentioned getting a PME, this is probably an elderly
3 phase supply, in fact a "TT" system, O/H or U/G, to be upgraded.
my concern therefore is the size of the fuses, possibly 60amp.
9 bedrooms? 5 showers? how many televisions, computers etc.
and the garage, and of course this project will need a sizable kitchen.
does our poster understand the PME bonding requirements?
the DNO may not be able to upgrade an old main to 100amp.
Ive seen so many of these jobs where an inexperienced spark ends
up with 200volts in the middle of february when this kind of load is
switched on.
its not enough for our poster to connect an excessive M/D to an
isolater (if we fit one) and hoping for the best.
Its a good project, but perhaps a little care and guidance needed.

Regards.

Unlikely to get a pme connection at this stage either. It sounds as though the OP is still chasing and wiring so a TBS is in order which unless the rules have changed whilst I've been busy saving the world as still that it needs to be TT.

That said the quality of some of the suppliers meter fixers makes me worry, one of my guys was asked last week whether he was worried "which was round the 3 phase was". The answer "The correct way" seems to upset the meter man. The head was marked R,Y,B which I've always taken to be L1,L2,L3 !


Stu

As said before, you would be better off considering the possibility of splitting the house into 'wings' and do submains to feed them. Its often better to do 1 SWA for 40m than run another 10 circuits the 40m back to the intake.

If the heads that old, id consider getting the head replaced and possibly the cable as ukpn suggests.

The chance of 5 showers all being used at the same time is minimal unless they are running a youth hostel, so id start by distributing them 2 per phase with some common sense.
Use SWA for submains not crap T and E.........

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Lack of planning on your part doesn't make it an emergency on mine....

Every man has to know his limitations- Dirty Harry

i cant actually say it for fear of being thrown off this site... but it sounds as though i know exactly the kind of person who wants to squeeze 9 beds in. So with that in mind, your top priority is to make sure you get regular weekly or fortnightly payments.
For what its worth, pricewise, i'd be looking to bill £11500 for basics. near to 20k for alarm cctv network.

Unless clients wants multiple metering, i really dont follow the logic of so many electric showers.

Hi UK

Yes, I've checked with the DNO, it was one of the first things I did, they are fitting a PME this Monday, when that's installed I can connect a temporary supply. The cut out is 60A and the DNO has informed me they will be able to up that to 100A without a problem. I think I understand bonding requirements but will be happy to double check that, I also plan to supplementary bond the bathrooms/shower rooms in addition to RCD protection. The kitchen is surprisingly small, considering the size of house they want to turn it into, and, for the time being at least, they have no plans to extend it. I will certainly be seeking guidance as I go, as this is a really big project for me and is definitely taking me out of my comfort zone

Edited: 21 September 2012 at 09:25 AM by CoolHome

Hi Stu

The PME is being fitted this Monday, it was one of the first things I got organised.

Hi SKElectrical

If I get your meaning right, you're spot on mate, I've got 50% up front... Mind you, that was before I knew about 5 showers, so I've got to give him a revised price.... which I suspect he's not going to like.

Nobody I know understands his logic for wanting 5 showers (he's stark raving mad in my opinion) and believe me I've tried everything I can to convince him otherwise, but he plain won't have it, I don't know what else I can say to him to change his mind.

Hi Andy

The intake is in a tiny little cupboard, at ground level, in the lobby, which is why I'm having to put the CUs in the largest (and closest) cupboard available, which is under the stairs. I'm 6 foot tall and can stand up in the under stairs cupboard for a good metre or so before I have to start crouching down. I've absolutely no experience with TP&N boards so I'm probably going to stick with what I know and go for 3 CUs.

Hi Broadgage

That's what I'm going to plan to do. The 3 phase isn't connected yet, just an old 3 phase cut out utilising one of the phase connections, which is 60A. The DNO inform me there will be no problems upping them to 100A. He doesn't even have planning permission for the extension yet so I'm going to have to balance the circuits with this current re-wire job and then change the loads around when the extension gets built.... assuming it gets planning permission.

Hi Legh

1) Yes, purely residential

2) Agree, as no 3 phase equipment it going to be used.

3) I'm still working on him but, I don't get it either, he's adamant.

4) Main intake and isolation is in a little cupboard, at ground level, in the lobby, just the CUs and henley block will be in the under stairs cupboard, it's a straight run under the floor (for the tails) from the isolator.

5) The extension doesn't have planning permission yet, so all I have to work with is what's there now, I'll have to balance the load across the boards with the current circuits and then change them when the extension gets built.... If it gets built

Hi Zeeper

Yep, I've got that

One gas mains one boiler, could be he will charge tenants for communal heating and hot water, but his logic may be, he wants the tenants to pay addition bill for the electric showers, there again are you fitting a meter for each bed-sit.

Over crowed Britain, it's getting more difficult to provide the power.