Radial to Ring

BJC · Posted: Mon May 23, 2005 6:58 am Post Subject: Radial to Ring

I have a cooker circuit in 10mm T&E which is no longer required. Is it safe/lawful to create a ring at the end of this in 2.5 T&E and simply change the MCB in the consumer unit to 30A i.e. use the 10mm T&E as an extension of the CU. Otherwise I'll need to run two new 2.5 T&E cables.

Regards

Brian

pna · Posted: Mon May 23, 2005 3:18 pm Post Subject:

Yes - no problem in having part of the circuit in higher spec cable than is necessary.

ban-all-sheds · Posted: Mon May 23, 2005 4:09 pm Post Subject:

It's neither unsafe, nor unlawful.

BUT -

It is different, and electricians don't like different.

It makes testing more difficult, as the ring continuity etc tests can't be done at the CU.

There is always a chance that some future occupant will see a 10mm² cooker cable and but the MCB back up to 45A....

Try very hard not to do it like this.....

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I mustn't warn people that the "experts" on the plumbing forum can't be trusted to tell the truth.

BJC · Posted: Mon May 23, 2005 7:16 pm Post Subject:

Thanks chaps. Hadn't thought about future occupants misinterpreting my handywork. Will probably put in a new ring from the CU and retain the cooker circuit for a new fan oven I'm installing.

Big_Spark · Posted: Mon May 23, 2005 8:13 pm Post Subject:

As has been pointed out it is safe to do, and I have seen this done, but I would make a suggestion to anyone doing this.

Where the 10mm and the legs of the ring meet I would fit a small enclosure housing (as this is a kitchen) a 32A 30mA RCBO, so the 10mm is effectively a sub-main. I would do this because at a later date, someone may uprate the source breaker as it feeds 10mm, and this could result in someone overloading the 2.5mm (you have no idea what they may do in the future).

When dropping a cable size, you should always add overload protection for the smaller cable, to no do so is bad practice, bad design and potentially inviting dangers that can be forseen and thus avoided.

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BJC · Posted: Mon May 23, 2005 8:23 pm Post Subject:

Thanks for that FWL_E. That tip could save me quite a bit of work. I've never used an RCBO (don't even know what it is) but don't imagine it's much different from an MCB.

Big_Spark · Posted: Mon May 23, 2005 8:43 pm Post Subject:

BJ, an RCBO is simply a device that combines the functions of an RCD and an MCB in one neat package. They can be expensive compared to a simple MCB..£40 to £50 each, but they do provide total protection in the event of any type of fault occuring on the circuit.

A Two way enclosure, splah proof type, will be sufficient for your purposes.

Regardless fo the make of RCBO you Buy, I would suggest and MK enclosure, like this one (Part No: 5702s)

They look better than standard ones, but can withstand being in a kitchen without any undue influence from the environment they are in.

I would suggest a matched RCBO (Part No: 6932s)

I would further suggest that you do not buy these items from B&Q or any other shed, they will rob you blind, find your nearest Newey&Eyre or Edmundson Electrical, or similar dedicated wholesaler, you will save a significant amount on shed prices.

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I only do commercial and Industrial so Prescott can shove his Part P tax

BJC · Posted: Mon May 23, 2005 8:47 pm Post Subject:

I was just looking at that very housing and pondering whether it would do. Youv'e solved it all for me. Thanks. I've got an electric wholsesaler just around the corner so will purchase from them.

Thanks again.

Brian