Hi
My garage has an old CU to supply its lights and sockets; the garage CU is supplied from the main CU in the house.
I'd like to change the garage CU to a modern one, incorporating an RCD and a couple of MCBs.
I know it's OK to do a simple job like change a switch, but do the current regulations allow me to change a 'secondary' CU?
Steve
Assuming you do a competent job, who's going to know or care?
I am sorry Steve, I don't think nanny can let you do that...
It would be legal, however it would come under the scope of part pee.
Hence it would need inspecting by building control if you are not able to self certify the work, or treating with contempt and ignoring if you so desire!
Go ahead and change it, but ideally it should be on a separate isolate, not on the main CU.
What's the issue with having it wired from the house CU? (Mine was wired that way - by a pro, not me, I might add!)
David
But he's told us all now that he maybe going to do it and the Part P police could be listening
Dave
Don't know I'm afraid, but I guess the "regs" say so. I doubt that most electricians can understand them anyway.
I don't think so.
You have to have some form of circuit breaker or fuse before the run of cable to the garage CU to protect that cable.
Either this can be in the form of a separate MCB or fuse to the main CU, or it can be one in the main CU itself, rated appropriately.
The regs have nothing saying "you should not feed a submain from a CU"
In fact it seems quite a sensible idea if done properly.
Adam
Surely that's what the OP means - the two CU's must be in series, not in parallel otherwise how do you isolate the garage system on order to work on it's CU.
At least that's how I wired mine and that seemed rather obvious ! Based on the fact that the supply is just an ordinary 2.5 T &E, it's off a 15A fuse. I would rather you didn't ask any more about the system though - like how does it get to the garage.
Rob
The other option would be an MCB, RCBO or fuse in an enclosure fed from tails connected to a Henley block before the main CU.
Functionally, this is the same as going from a way on the main CU.
Certainly you would not do a long cable run to a garage CU directly from a terminal block (i.e. Henley block) before the main CU with no protection.
You can confide in us..... :-)
Either way is acceptable and the relative merits have often been discussed here.
If the garage is fed in parallel (from a service connector block in the meter tails) there has to be a separate switch-fuse at the house end, otherwise you'd be relying on the supplier's fuse to protect the submain cable to the garage, which would be completely unacceptable. Said switch-fuse provides the isolation (and strictly speaking should be capable of being locked In the off position to protect anybody working in the garage from any 'surprises').
Electricians should be able to understand the Regs, or at least implement them properly.
The only real "issue" I can see is if the garage supply was from the RCD'd side of a split load board, or a whole-house RCD. It would not then be possible to achieve proper discrimination between the house RCD and any RCD in the garage, and a fault on the garage installation would cause nuisance tripping of the house RCD.
Owain
Thanks for that, all.
Steve
I understood that for an outbuilding, they did.
But then the regs are bloody stupid anyway, why would it be sensible to float the earth to a swimming pool control panel, and have a separate earth spike?
Define "float the earth".
Adam
No, it is in fact quite common (and acceptable) to feed an outbuilding CU submain from an existing CU. The alternative would be to feed it from a service connector block in the meter tails via a switchfuse.
Why do you think that?
By float I take it you mean to leave the earth disconnected at the end the submain?
This is standard practice when making the outbuilding installation a TT earthed installation protected by its own RCD and a local earth rod.
It can be undesirable to export the house earth - especially over long cable runs, or in situations where it is difficult or impossible to ensure that all extraneous conductive parts can be bonded to the same earthing system.
But by doing so you get a much better earth plane.
On Mon, 23 Jan 2006 13:48:06 +0000 someone who may be Nigel Molesworth wrote this:-
Assuming this is true, this is important for what reason?
The ground potential varies, so spreading out the earth points and bonding these together give a more robust earth.
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