Wiring - 2 radials into 1 MCB, amongst other things...

[RichardS]

Greetings all - returning to uk.d-i-y after a couple of years absence (mostly due to work. grrr.) Glad to see many familiar contributors still around.

Current project - kitchen extension and complete refit. Have a builder doing this (I'd love to do it but just no time) & electrician to do the electrics/certification (construction being done under Building Notice application, so no chance of any Pee avoidance. I'd want a regs compliant installation in any event).

One part of the work is to run SWA down the garden (about 15m) and connect up the shed. Shed has small garage CU in it, with 30mA RCD, 6A lighting ciruit MCB and 20A socket circuit MCB. 5 double sockets on the latter and about 8 metres of 2.5mm^2 t&e in total.

Now, I'm pretty sure that I have read here before from the 'lectrical cognoscenti that 2 radials into one MCB was unusual, but permissable. Of course I can't find the post now... Due to the layout of the shed workspaces & with thoughts of economy of installation, I'd prefer to install 2 radial circuits rather than one ring, so this is how I wired it.

Quick discussion with electrician today revealed he had a different take on it, and that he didn't think that such an arrangement could be compliant.

Does anyone have chapter & verse on such an arrangement?

Of course I could quickly adapt it to a ring, or even slot an adaptable box or similar so that the 2 x radials go into that, and single t&e to the shed CU, making it a single radial. But if I don't need to, I really would rather not do. A single ring would normally be on 32A MCB, of course, but is there anything fundamentally incorrect about putting the circuit on a 20A MCB? (the cable would if anything be over- protected in this arrangement, surely).

Also, I have an externally mounted socket, RCD protected. I'd rather not have to run two separate circuits to the kitchen (one RCD'd, the other not - for the fridge, etc), arguing that the external protected socket makes the use of internal sockets for external portable equipment very UNlikely. Are there other regs that come into play here?

On a final, OT, note - anyone working in The City know of any useful bookshops or other retailers where I might be able to get my hands on a

16th edition OSG? Decent bookshops in The City seem to be a bit of a rarity these days if my wanderings are anything to go by.

TIA

[dennis]

"RichardS" wrote in message news: [e-mail]...

Ask him how it would be different to running 20 cm of cable to a junction box and splitting it into two?

[Andy Hall]

Hello Richard, welcome back.

There's a commentary here on cable sizes and protection MCB ratings

Foyles would probably have it, otherwise go to any bookstore and ask them to order it in.

[RichardS]

:-) thankyou!

thanks Andy, I'd satisfied myself that 2.5mm^2 radials on a 20A MCB was fine for this (I haven't made a howler here, have I??)

It's more the point about that 2 radials terminating into the same 20A MCB that is the question - electician was exceptionally uneasy about this & until I get my mits on that OSG I couldn't ascertain whether it was within the letter of the regs other than a vague recollection of a post by a trusted regular that it was OK.

(Foyles is a little out of my lunchtime range unfortunately, even with the Brompton. Just wondered whether anyone knew of a square mile place, but I'll probably ring & order a copy from my home-local bookshop for collection on Saturday otherwise).

[Autolycus]

If you mean to buy your own copy, as against memorising odd pages that you browse,

shows several places selling it for around 16 quid with free delivery, though heaven knows when, with our present postal anarchy.

[John Rumm]

Long time no hear, welcome back!

Not even that unusual really. I think people get confused with radials thinking they need to be a linear chain of sockets connected from one to the next. The reality is with a radial any topology is acceptable. You can branch anywhere you like. So two legs connected to the CU is fine, as would be the second leg connected to the first socket. The design is such that the cable can take the maximum circuit design load, and the fault protection at the head end will protect the cable in any location.

It is compliant. You could even have a ring with a spur originating at the CU if you wanted. (this is explicitly mentioned in the OSG)

Ask your sparks to show where it is forbidden ;-)

No, that would also be fine...

There is even an obscure case where you could have two 2.5mm^2 radials each feeding a FCU or a socket outlet, and legitimately connect them both to a single 32A MCB, even though its trip current exceeds the rating of the cable. This would rely on splitting the responsibility for fault current protection and overcurrent protection. The MCB would provide adequate fault current protection for the single 2.5mm^2 cable (in much the same was as it protects a spur from a ring), while the local fusing in the FCU or plug is responsible from preventing overload of the single cable.

Not really (yet - 17th edition will change this).

[Owain]

Two radials into one MCB wouldn't be *two* radials - it would be one. Provided the MCB / consumer unit terminals are able to take the conductors, it is perfectly permissible to branch a radial, or even take an unfused spur, from the consumer unit in the same way as taking it from any other accessory, or a junction box.

Only thing is you might not want your shed lights on the same RCD as power tools, for safety if the RCD trips and you're left in darkness with power tools still spinning.

Fairy nuff. You could label the fridge socket 'not for external portable applicanes' if you want to make double sure.

Expect 16th Edition books to be cheap on Amazon soon, as new stuff is launched for the 17th Ed. Bookshops near Technical Colleges which teach electrical wiring are likely to have in stock.

Owain

[rrh]

IET bookshop, though not quite the City:

[David Hansen]

On Sun, 7 Oct 2007 19:50:35 +0100 someone who may be RichardS wrote this:-

To add to what the others have said, it would be good practice for this radial circuit should be labelled as a radial to avoid any future confusion.

[RichardS]

Just want to say thanks folks.

I think that Owain and John you put your finger on it in that even though it "looks" like two radials it is in fact one - I suspect the single daisy-chained idealised example of the radial is the one that sticks in peoples minds whereas it can be much more tree-like.

I did find the example of the spur off a ring connected directly to the circuit origin within the OSG - it's odd that the OSG doesn't also include definitions or examples for radial circuits. Perhaps it does and I just haven't found it yet.

I've also come to a decision on the kitchen circuits - am going to protect the general socket circuits with an RCD/RCBO and have a completely separate circuit for fixed appliances, including the fridge/freezer. The outside sockets will be protected by their own RCD/RCBO.

Incidentally - do external luminaires require RCD protection? Or can they be run directly off the lighting circuit? (they'll be low energy units so a small current draw)

thanks again

[John Rumm]

They don't require RCD protection (unless on a TT install). If they have it, then it ought to be a different RCD from that protecting the internal circuits.

[David Hansen]

On Thu, 11 Oct 2007 00:59:50 +0100 someone who may be RichardS wrote this:-

In addition to what John has said there are two situations to consider. One is lights on the outside of a house and one is lights in the garden/path/drive. The former can be fed from the "inside" circuit but it is better for the latter to be on their own circuits if only to avoid a fault on them preventing inside lights from being used.

[meow2222]

Outdoor electrics is generally of much lower reliability than indoor, so its not generally a great idea to put any outdoor stuff on indoor circuits.

NT

[David Hansen]

On Thu, 11 Oct 2007 05:24:38 -0700 someone who may be [e-mail] wrote this:-

Whether outdoor electrics is more reliable depends on how well it is done and how much weather it is exposed to. That is why I suggested a division between lights mounted on an outside wall of the house and ones further away. Millions of houses have the former on "indoor" circuits without great tales of woe. Indeed such lamps may experience less damp than lamps in a room containing a shower.