DIY Doctor

consumer units

Postby MALPASS » Sat Jun 12, 2010 8:21 pm

can anyone tell me the regs relating to consumer units, i have a couple of old rewirable ones and a rcd protected one , i want to get another rcd and do away with the rewirable , is their a maximum amount of fuse ratings or can you put how many you like in as long as the load does not exeed the box rating, in short can i put 200a worth of mcb in a 100a unit ?
any help would be apreciated
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Simply Build It

Postby ericmark » Sun Jun 13, 2010 1:11 pm

There is a limit to the diversity allowed but for example if you decided to use radial circuits one to each room for sockets then having 6 x 20A MCB's to supply what could be done with 2 x 32A MCB's with a ring main would not in real terms increase the total load. So some common sense is required.

When wiring a very large house I used a switched fused three phase isolator and feed the three 100A consumer units from the isolator each with a 80A fuse. The idea being either a single phase supply of over 100A or a three phase supply could be used in the future should the demand exceed supply.

However all type tested distribution units (Consumer units) seem to be 100A and to get any unit with more than 16 outlets is rare. Unless one was to use RCBO's in all supplies having more that 16 in a twin RCD unit would likely contravene regulation 314 where it is now made plain that the division of the installation into circuits must also take into account of danger that may arise from the failure of a single circuit such as a lighting circuit and reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation.

Specials where basically two consumer units are together one above the other may allow four RCD circuits but even with three phase distribution units which technically because they are not type tested you shouldn't use anyway for a house would need to be populated with RCBO's to comply.

At around £30 each even a 16 way consumer unit would cost around £500 and in the main this is the problem. Two split RCD ten way consumer units will cost around £160 and as a result it may not be economical sound to go for a single unit even when it may seem best option.

However in a domestic we also as well as wiring regulations have building regulations and Part P is a problem as the LABC charges are £100 plus so although you may be able to do the work it is likely uneconomical and the rules have changed and now they are allowed to pass on charges for any out sourced work so unless you can inspect and test your own work the council charges can exceed the labour charges of getting an electrician who is a member of a scheme to do the work for you.

So in real terms you will have to go with what the electrician doing the work is offering and all we say on here is just academic.

May be the new government will repeal the regulations but I would not hold my breath it seems we must now get the man in and DIY is a naughty word.
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Postby MALPASS » Mon Jun 14, 2010 9:40 pm

thank you ericmark most enlightning
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