HOW MANY MCB'S IN ONE BOARD


Postby BIG BILL » Sat Feb 11, 2012 10:54 am

Hi All.
new to forum new to sparking so please be gentle.
A friend of mine has offered me the job of a full rewire of his house to help me get an assesment to get on a competent person scheme. The only thing is i think its to much as he wants to split his house so his parents can move in with him.
The circuits are thus
Non rcd
1, 20 amp fridge freezer radial
RCD 1
1 50 amp for shower
2. 32 amp kitchen final ring circuit
3. 32 amp downstairs final ring circuit
4. 32 amp upstairs final ring circuit
5. 6 amp downstairs lightin
6. 6 amp upstairs lighting
7. 6 amp boiler radial
RCD 2
1 50 amp 2nd shower
2. 32 amp downstairs final ring circuit including 2nd kitchen
3. 32 amp 2nd upstairs final ring circuit
4. 6 amp 2nd downstairs lighting
5. 6 amp 2nd upstairs lighting

I know i can get a consumer unit to accomadate this amount of circuits and taking diverity into account is this amount and type of circuit layout possible in one consumer unit as there is no space for another. Would any of you sparkies take this on?
Any help and advise will be much appericiated Bill
BIG BILL
Posts: 3
Joined: Sat Feb 11, 2012 9:08 am

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Postby ericmark » Sat Feb 11, 2012 7:28 pm

I would be a little wary of so much on each RCD. I would be considering using RCBO's for at least some final rings I would normally consider only one final ring per RCD although adding lights which are unlikely to have items which have a leakage problem is not a problem as long as if it trips they are safe.

Also I have found to buy Ali-tube cable I can only buy a role. Can't get it by the meter. So although in theroy the non-RCD supply to fridge seems good in practice getting the cable which will allow it to be buried in the wall is a problem. So first see if you can get the cable at a reasonable cost.

What surprises me is no cooker supply? I have noted the increase in electric cooking I think as both safer than gas and it has more control (If using induction hob) so to re-wire without giving the option to use an electric cooker would seem short sighted. And now I would use 10mm for cooker supply as with induction hobs the power required has gone up.

I would also question two 50A supplies for a shower? Unless the occupant intends to use both together it seems odd to have two showers so we must I think assume they will use them together. With two items using the total supply to the board one has to ask is this wise.

My son wired a house with a large prospective demand and we talked over how to do it. We fitted a three phase fused isolator then three consumer units off this. The supply was single phase but since we had fused down to 100A in the isolator if required the DNO could have given more than 100A or three phase.
It proved not to be required the 100A DNO fuse never went but I would be interested on hearing what others say.

I am not saying what you intend is wrong. Just saying to think about the problems.
ericmark
Posts: 1191
Joined: Fri Oct 02, 2009 8:49 pm
Location: Mold, North Wales.


Postby BIG BILL » Sun Feb 12, 2012 2:39 pm

Hi ericmark.
thanks for your reply.As i mentioned at he begining of this post i'm new to sparking and i look to people like yourself for guideance,I see someone like you has a wealth of experiance,as you post a lot of replys to people's dilemas.
That said,would I make it better if i were to remove 1 of the rcd's and put in 2 50 amp rcbo's for the 2 shower circuits,and also some 32 amp rcbo's for some of the final ring circuits.. I'm not saying they intend to use both shower at the same time, its just the house is being split into 2 living areas, 1 for himself and 1 for his parents. The cooker circuits are not required as it wil lbe gas cooking.
My concern is the amount that is on the one consumer unit, before i came into this game i had seen so many houses that have had extensions built,a lot of which had an additional consumer units, however still supplied by one REC company fuse
Regards Bill
BIG BILL
Posts: 3
Joined: Sat Feb 11, 2012 9:08 am


Postby ericmark » Mon Feb 13, 2012 9:05 pm

For items where the user can't alter the amount of leakage and where it is unlikely to be high then combining them on to one RCD I don't see as a problem. This would likely include the shower.

For items where the user can alter the leakage or where one would expect it to be high then RBCO's likely the answer. That would include cooker and sockets. The cooker as they can absorb moisture. OK know your not wiring for one but giving an example.

Where the problem lies is with critical items which loss of power could cause problems. There are two factors to consider.
1) Will the separating onto an independent RCD (RCBO) significantly reduce the change of it tripping?
2) Will the separating result in the occupants being unaware it has tripped until too late?
Supplying for example the bungler alarm and freezer from same RCBO would mean in the case of failure the alarm would beep alerting the occupants to the fact the RCBO had tripped. But also of course it could also mean a fault with freezer will stop bungler alarm working.
As it is with my house when the power fails so do the lights with the exception of the emergency lights so very quickly one realises and resets the RCD.
To be able to buy Ali-tube by the meter would remove the problem. You may be able to buy it in your area but I did not find a supplier in may area selling it by the meter.

As to two showers one again has to consider what happens if it trips/blows DNO fuse. On a caravan site I worked on the main supply was feed direct from DNO fuse to a moulded breaker of 80A so should it be overloaded it would be a simple trip needing re-setting not having to call out the DNO.

There is no right or wrong method but since your being tested on this installation clearly you need to consider the method carefully. Running two showers from one 50A MCB would ensure both can't be run together but not really sure if one needs such a drastic method as likely as I said with previous post although possible very likely it will never happen.

I have said many times it comes down to a risk assessment and one must weight up what may happen and how likely and strike a balance.

If it was not going to be inspected as part of the Part P self certificate scheme I would likely do as you originally suggested. And allow in quote for a recall to change the odd item. However since you are being inspected then there is a problem as either method could be seen as both right and wrong and what you are trying to do is second guess on what the inspectors views will be.

However if you were already registered you would be free to ring your scheme provider and ask for advice on these points. To my mind to ring up and ask in it's self shows you are a good worker and does not really distract from what you are doing but adds to it. So I think the answer must be to ring up the prospective scheme provider and ask their advice.

So what I am saying is you want to be able if asked to be able to point the finger and say advice was sort on this point and Mr X. XYZ or ABC advised to do it this way. Not Joe Blogs or ericmark who you don't know from Adam on some internet forum.

There is no black and white answer it is a personal preference so you want it to be the person who is likely to be testing the works preference.
ericmark
Posts: 1191
Joined: Fri Oct 02, 2009 8:49 pm
Location: Mold, North Wales.


Postby BIG BILL » Tue Feb 14, 2012 7:10 pm

Hi ec
thankyou very much for the time and effort you have affroded myself with such a detaild answer.
I have spoken to the electrical company that work on behalf of the LABC,and aired my intensions. They are happy with the use of RCBO'S on certain circuits, this has put my mind at rest and i'm now happy to carry on with my rewire. As you quoted i can now point a finger,if there inspector is not happy.
Once again thankyou very much,and keep up the good work,
Bill
BIG BILL
Posts: 3
Joined: Sat Feb 11, 2012 9:08 am


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