Second distribution board

Postby digwie » Mon Jan 12, 2009 5:45 pm


I have a full distribution board but the last breaker (32a) has a single 4mm T&E currently going to a single twin socket about 15m from the DB. There is a small building extension planned for this area where the single socket is.
Do you think that a small 4 way sub board with a 32a MCB with a 20a breaker for a ring of 6 twin sockets and a 6a breaker for 9 low energy light fittings will be ok?

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Simply Build It

Postby ericmark » Tue Jan 13, 2009 12:37 pm

If feeding a board from a 32 amp supply the largest breaker one would normally fit in the board would be 25 amp more likely 20 amp and use radial rather than ring method. And 6 amp for lights would seem OK.
However do be aware of Part P. I would guess LABC will be involved anyway which means you will need to comply with all regulations and you may want to check if you need RCD etc adding at same time.

Postby digwie » Tue Jan 13, 2009 1:10 pm

Hi Eric & thanks for your reply,

Did you mean a 25a or a 20a for the Main breaker in the sub-board and then a 20a for the radial circuit for the sockets?

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Postby ericmark » Sat Jan 17, 2009 12:24 am

Sorry missed your reply.
Its all logic
If you supply a board with a 32 amp protection device then the board also supplies something with a 32 amp protection device plus other items as well then should there be an overload then the 32 amp supplying the board will always open first and the one on the new board is nothing more than a switch.
However even if you supply with a single 32 amp and then in the new board have two 25 amp devices although if both loaded it could take out the 32 amp main feed there is a chance if one output is overloaded and the other is light loaded the 25 amp device will open first.

An example would be on a caravan site where the caravan site owner supplies the pole with a 16 amp MCB you don't want to call out the owner if you slip up and put on too many items so either in the caravan you use a 10 and 6 amp MCB or you supply the 6 amp through the 16 amp MCB since the pole is likely colder than caravan the caravan trip will most likely trip first.

But this is not a hard and fast rule and if some item would normally draw 28 amp then supplying from a 25 amp MCB would not work and you may have to fit 32 amp MCB's in both supply consumer unit and new consumer unit.

Its all a case of using some common sense.

With only 6 sockets unless there is something special that uses a lot of power using a radial supplied from a 20 amp MCB would make sense.

It's only common sense no special rules.

Postby singer » Sat Jan 17, 2009 11:35 am

I notice at the start of this thread we dont have a warning from gardnico1307 to consult Part P requirements.
For a job like this where final circuits are being converted to distribution circuits, new circuits are being installed, protection and earth loop impedance values need to be within requirements of the circuit design and not to mention the bonding needs to be in place, it is imperative that all the requirements of Part P are adhered to.
This kind of work must be undertaken under the strict control of either an electrician who is a member of a competent persons scheme, or the building control body of the local authority who may choose to employ a third party(local electrician) to supervise installation, test and issue certificate.
It is never emphasised enough in these replies that this is a legal requirement and the consequences of many electrical DIY jobs could see you in court possibly loosing your house to pay the compensation, as no DIYer has public liability insurance!!
So to answer your question digwie you dont really need to ask it in the first place as its not a job you can do yourself unless you consult the local authority, in which case they will give you all the advise you need, or you can employ a Part P registered electrician who will take responsibility for all the requirements and will advise you accordingly.
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Postby TheDoctor4 » Sat Jan 17, 2009 12:01 pm


Apologies to Singer but he is absolutely correct. Part P should always be consulted before any DIY'er looks at any DIY electrical project!!

Click on the link to consult our Part P project:

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Postby ericmark » Sat Jan 17, 2009 1:45 pm

Because he said he had a small building extension planned I had assumed he would be going through building control. So referring to the stealth tax seems unnecessary.


Postby digwie » Mon Jan 19, 2009 9:20 am

Thanks for your replies gentlemen.

I have taken your advise and scrapped any idea of doing this my self and have enliseted the services of a professional.

Keep up thye good work of your excellent websirte and forum.


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