Extending the ring main into the garage

[AnotherDesign]

Hi everyone

Working under a building notice I have recently built a garage joined onto the side of the house. The inspector is aware that power sockets are to be installed and I am aware that I will need to get a helpful electrician to check any installation after it is completed, and a certificate issued.

8 years ago when I completely rewired the house, I allocated one ring main out of 4, to two bedrooms and this garage. The loop required was taken though the wall and has been used up to now to power a socket in an old detached garage which has now been demolished for the new garage. The total area to be serviced by this ring is well under 70sq m. In the main CU the ring is protected by a Crabtree Type 1 32Amp MCB, which is protected by a 63A RCCB 30ma trip. On the ring so far are 5 double sockets and 6 single sockets. The length of cable used so far is approximately 30m. This is difficult to measure as it is now under floorboards and carpet The current max load at any one time on the existing sockets would be 2000 watts.

Can I: use this ring main to service a garage consumer unit having a 6A and 32A MCBs and 63A 30ms RCCB? The lighting would run off off the 6A and the power sockets off the 32A. If so, can I have another 50m of cable off this garage consumer unit or am I now restricted to 50 - 30(already used) = 20m?. How will the lighting cabling count within the allowable cable length?
Can I have any number of sockets as long as load used is not over 7200watts?
Does the 2.5 T&E have to be incased in plastic conduit or can it just be clipped to joists and walls?

I do realise I will have to get this work checked. However, I really would appreciate any advice. Many thanks. AnotherDesign

 

[BS3036]

What you are proposing would no longer be a ring circuit, so you can't just carry the ring circuit guidance across to this design.

It would be more like 1 spur plus one ring emanating from one point of the original ring, which is not permitted.

The new 32A MCB and the new RCCB would be contributing nothing as they do not offer any discrimination with the originals.

As to what you should do; I think you could actually use the ring as a continuous ring, with a fused spur for the lighting, or better still run a new submain from the CU. However I am not an electrician.

 

[oharaf]

You need to have this discussion with building control and / or your electrician.

If you require seperate power to the garage this will need to be fed from your an MCB in the CU to your garage CU, then all your circuits in the garage will be fed from there.

Personally I would be looking to give the garage its own supply

 

[AnotherDesign]

Thanks so far BS3036 and oharaf. I do have 20m cable allowance of the Ring Main that I can still use in the garage. Is there any reason why I should not just continue the ring main as I first intended with some additional sockets and put in some fused spurs for any remaining sockets that I cannot cover within this cable allowance. I am not intending a heavy load so much as a good variety of socket placement.

How do 'fused spurs' get included in a cable allowance of 50m? eg. If I have a ring main that is 50m in cable length, can I add a fused spur of 10 m to it, or will that be over the 50m allowance? Is there a limit to the number of fused spurs
or the length of cable that can be used on them?

Thanks again

 

[BS3036]

Subject to certain conditions, the On Site Guide indicates that a ring of up to 84m is allowed. Read table 7.1 on Conventional Circuits for details. I think the answer for extra length of spurs would be that you need to do some calculations if you have doubts that you are meeting the requirements for volt drop.

Personally I would have liked a separate circuit for the garage, but I can't think of anything in the rules stopping you.

I'm sure that if I'm wrong you will hear very soon from someone. As oharaf points out, you also need to have this agreed with an electrician before you start, as I'm sure you'll be aware that this method of certification is a very grey area, as he won't be able to issue an EIC for work he didn't do.