Should I fit a garage CU and separate the lights

[Markthisoneout]

I have a 1998 detached house with an integral single storey double garage built part in and part out of the side of the house. The house has a split CU with a 30mA RCD feeding 2 ring mains. One of the ring mains is looped into the garage where it feeds 2 double sockets and a single switched hanging light flex. I want to remove the light flex and fit 4 switched fluorescent battens.
Although I guess the wiring arrangement met the regs at the time the house was built, I am concerned that there is no separate mcb for the garage circuit on the main CU and even more concerned that the existing garage light is fed off a ring loop which has overcurrent protection of 32A!! It would be both very expensive and difficult to run a new circuit from the CU to the garage area which is diagonally opposite and at the other end of the house and it would also be very difficult to wire the garage light circuit into the main house lighting circuit. I am thinking therefore, of installing a Garage CU in the garage part of the house and connecting it to the ring loop that is there, effectively making that a fused spur. I would then be able to put the new lights on a more appropriate 6A mcb and the garage sockets would have their own overcurrent protection from a new 16A mcb. From my understanding of discrimination, I guess the Garage CU RCD would be largely redundant as the garage circuit as a whole emanates from the main CU 30A RCD. However, this must be better than the existing arrangement – mustn’t it? But is it legal and is this the only solution?
I would be very grateful for the views and advice of you experienced professionals out there because I really cannot see what else I can do, other than pay for an extremely expensive and disruptive rewire – just for better, safer lighting in the garage.

 

[Paul.M]

Ging by your detailed description (thanks btw) this job is'nt as big as you think for an electrician. All of it will b completed within a day and that includes testing and certification for your local building authority (which is needed if your in England or Wales).

This may seem like a nice little diy job for the weekend but theres more to it than running a few cables. I suggest getting a local sparky in to do this work and cert it, you could help him? Sounds like you have a good idea what is involved. post up yur area and see if we have someone close.

 

[malcolmsanford]

What you propose is not exactly illegal but bad practice. You seem to have a fair grasp of electrical theory but not sure if you know the regulations.

There is a reg that states that BS 1363 accessories can be fed via a ring final circuit. There is a reason for this, all these accessories contain fuse links the maximum of 13amp. Which means whatever you connect to these accessories can not exceed 13 amps and so not overload the ring.

Now if you fit a CU to a ring there is chances, be it ever so slim of in the future someone replacing that 16 amp MCB with a 40 say for a welding machine or some such, that means you are in danger of overloading the ring's supply cable. Be it ever so remote that will happen it could, which is why it is bad practice do do what you want

Surely a better, easier solution would be fitting a fused spur onto the ring and take your lighting circuit from that

 

[Rockingit]

Well, I don't think you need a new CU, no. There's nothing wrong with having the garage sockets as part of another ring if they are in effect 'the same building' - no different to having a bedroom on the same cct as the lounge, for example. As for the lighting, what I would do would be to fit a FUSED spur outlet at the point you need the spur to start and then take your lighting from off of it. A 5A fuse would be OK for 4 x fluorescents, for example.

There is actually an inherent danger of overload were you to fit a CU in place of a fused isolator, as someone in the future may not understand the limitations and just go banging MCB's into spare ways.

To quote a phrase "if it ain't bust, don't fix it".

 

[Rockingit]

******

There's an echo in here! Malc beat me to it.

 

[Markthisoneout]

I knew someone would give me a simple, easily understood answer on this site and so it has proved! Thanks very much malcolmsanford for such a speedy, professional response. Sorry I didn't respond earlier - working late again!

 

[Markthisoneout]

Like malcolmsanford above, you have given me a sensible answer with clear advice Rockingit so thanks for taking the time and trouble and for the speed of your response. What a great bunch of professionals.

 

[Rockingit]

thanks for taking the time and trouble and for the speed of your response. What a great bunch of professionals.

So where are we sending the invoice to? :biggrinjester: