New 4mm T&E feed to existing remote 2.5mm ring. OK or not?

[Mark42]

I'm building a small rental annexe at my own house, which will be supplied by a new local sub-main, with local metering. This includes one existing bedroom which will become part of the annexe.
This room has an existing ring final in 2.5mm, all in good condition. Rather than run two new 2.5s, a long way, in parallel to the new DB, may I run a single 4mm, then connect the existing 2.5mm 'tails' (now pulled back from the main house DB) to this locally? One less joint, and one less awkward run of cable. Protection would be a 25A RCBO.
I can think of no reason why this is not OK electrically, but I've never seen it done. It feels a bit 'bodgy' but so is running two great lengths of 2.5 in parallel for no particular reason.
Have I missed anything? Are there any regs about the CCA change within a circuit which might apply?

 

[telectrix]

sounds like a lollipop cct. no reason why not, but for the little extra, i'd use 6.0mm not 4.0mm. this will allow for any future expansion. and i'd firt a 2 way CU in the annex, fed by 6.0mm then separate MCBs for sockets and lights. RCD protection preferably in annex.

 

[Toneyz]

Why a 25A RCBO and could you get one for your C.U.
How long is the run back?

 

[Mark42]

sounds like a lollipop cct. no reason why not, but for the little extra, i'd use 6.0mm not 4.0mm. this will allow for any future expansion. and i'd firt a 2 way CU in the annex, fed by 6.0mm then separate MCBs for sockets and lights. RCD protection preferably in annex.

Sorry, I didn't explain it well!
The new DB in the annexe is a Hager three-phase, fed by 5-core 16mm NYY. It's being done properly . All local circuits will be from RCBOs because I like them.
The single 4mm I'm talking about will only ever supply the one existing RFC in a single bedroom (It was a small office previously).

 

[Pete999]

Sorry, I didn't explain it well!
The new DB in the annexe is a Hager three-phase, fed by 5-core 16mm NYY. It's being done properly . All local circuits will be from RCBOs because I like them.
The single 4mm I'm talking about will only ever supply the one existing RFC in a single bedroom (It was a small office previously).

Why three phase CU????

 

[Mark42]

Why a 25A RCBO and could you get one for your C.U.
How long is the run back?

I have a spare Hager 25A RCBO, and as the RFC will never possibly be loaded more than about 20A, then why not?
It's not that long a run, but it's in an awkward loft space. About 20m.

 

[Mark42]

Why three phase CU????

Why not????

I'm a great believer in good electrical design and load-balancing. Unlike the house, the annexe will be all electric. Electric space and stored water heating, and possibly some instant water heaters too. I don't want that lot on one phase!

 

[Pete999]

Why not????

I'm a great believer in good electrical design and load-balancing. Unlike the house, the annexe will be all electric. Electric space and stored water heating, and possibly some instant water heaters too. I don't want that lot on one phase!

Is there a 3 phase supply available?

 

[Mark42]

Is there a 3 phase supply available?

Eh? Of course. Would be a bit difficult otherwise

 

[Wilko]

Hi - I’d probably extend the ring. If it’s a long run it might need 4mm anyway.

 

[Toneyz]

How come you have a domestic 3 phase supply? Does you load warrant it?

 

[Pete999]

Eh? Of course. Would be a bit difficult otherwise

Eh? Of course. Would be a bit difficult otherwise

Forgive me if I'm misunderstanding this.
You say you have a 3 phase supply to the new annex, correct? Why are you considering running an extension from your house to feed the RFC? could you not just extend the RFC from the 3 phase board in the annex?
Again sorry if there has been any misunderstanding.
Edit Just read that you have taken the 17th "just for fun" that did it for me, I'm out.

 

[Mark42]

Forgive me if I'm misunderstanding this.
You say you have a 3 phase supply to the new annex, correct? Why are you considering running an extension from your house to feed the RFC? could you not just extend the RFC from the 3 phase board in the annex?
Again sorry if there has been any misunderstanding.

Yeah, I wrote the question in a hurry as I'm doing the work today!

1. New 3P supply taken to the annexe, with local meter.

2. Completely new install in annexe, all circuits with RCBOs.

3. But ... there's one existing RFC in an old office, which was part of the house, but will now become a new annexe bedroom. I'm disconnecting the old supply to this RFC from the house DB, then reconnecting to the new DB in the annexe, to bring the new bedroom within the metered zone.

4. None of the above really matters: all I asked is whether what I now know is called a 'lollipop' circuit (thanks Telectrix) is OK!

Cheers, Mark

 

[Mark42]

How come you have a domestic 3 phase supply? ...

I'm lucky: it was a farm before.

Does you load warrant it?

Not with the way I follow my family around the house, turning things off, no

 

[Pete999]

I KNOW I said " I 'm out" but you should have explained all the information from the outset, to avoid confusion and more importantly any bad feeling, just saying, Enterprise out.