Consumer unit for garage conversion

[MGA]

Hi, I am converting part of my detached garage (a meter and half away from the house) to a utility room. Building control officer is involved and he's being very helpful. The roof is now finished and I am doing the walls. Next will be electrics, but I have a doubt: There is a cable (similar to the ones used for ring mains, i.e. 32AMP max?) going from the house to the garage, coming out of the consumer unit (it runs in a conduit). It is RCD protected. I am planning to install a mini consumer unit using this cable as the main input so it can be split into a ring main for sockets and then the lights. I have read a couple of books about it and it doesn't seem that dificult.

- Can I really use this cable as the input to the consumer unit?

- Will the earth on this cable be enough for the utility room?

- If all of this is possible, would I need to change the MCB on the consumer unit in the house for a special one? currently it's an RCD protected 32 AMPS.

Thanks for your help!

MGA

ps: don't worry, I will get it all checked by an electrician when they come to do the Kitchen in a few weeks and before I use the sockets!

[Christian McArdle]

Yes, probably.

No, probably not.

However, you really need to do some calculations to prove the circuit's suitability. For a start, the cable used for 32A ring mains is normally

2.5mm. However, this small cable can only be used because there are two of them in a ring. With a direct single cable, it needs to be larger. However, the fact that the circuit already has a single cable and is rated at 32A indicates that you might be lucky, provided you check the suitability first.

The exact size required depends on the length of the cable, the way the cable is installed and the rating of the circuit. We know the rating. We need to know the distance that the cable route takes and the way it is embedded into the building fabric. In particularly, we need to know if it is surface run, embedded in plaster, in conduit, or (worst of all) runs inside an insulated wall.

Once we've calculated the minimum size, we need to identify the size that is currently installed and see if it is good enough. If not, it either needs replacement, or we can calculate what rating can be used with what is already there.

Christian.

[MGA]

Thanks very much for your early response to my questions, Christian. I have had a closer look at the circuit to explore a bit more the things you mention in your message. The cable itself is not 2.5 mm, but 1.5 mm, and I made a mistake when looking at the AMPS of the MCB in the house consumer unit, it's actually 20 AMP. I guess this means I need to replace the cable and the MCB in the house for a 32 AMPS one?. The cable length is no more than 6 meters , 2 of them running between the house and the garage inside a metallic conduit.

So the plan is:

- To change the MCB in the house for a 32 AMPS one.

- To run an armoured cable from the house to the garage, reusing the conduit if possible.

- To install the consumer unit at the other end using the earth from this new cable.

Does this sound ok?

Thanks for your time!!!

MGA