Maximum number of conductors in MCB for radial circuit?

[cornburn]

I can't find anything in the regs. On a ring it's fine to run a spur from the MCB (so 3x2.5mm). On a radial technically it doesn't matter where connections are made to the circuit as long as the cable at any given position can withstand the load. So how many connections from the MCB would be acceptable? Would you go by manufacturer's recommendation as to how many the terminal can hold?

I have to add 2 sockets into a radial run (currently a mix of 2.5mm & 4mm single cores and T&E on a 20A type C MCB). I could break into the main leg of the radial from a steel conduit inspection bend and leave a wago box hanging out of the bend (a bit messy IMO, even though hidden above a ceiling), but it's only about 6 feet further to the board so I'm thinking of just coming off the MCB instead. There are currently 2 connections in the MCB and it got me wondering how many you could technically fit.

And before anyone asks, this radial is the only RCBO on the board so I want to come off this circuit to save spending £100 on a Merlin Gerin RCBO just to put 2 sockets in.

Let the arguments commence..............................

 

[DurhamSparky]

hmm tricky one this, i have installed 3 radials from a MCB before for different lights, i assume as long as you have a decent connection and you do not stress the MCB retaining screw then as many as you deem practical. (obviously IAW Manufactures installation instructions) and don't over load the MCB!

 

[GT1]

yes im waiting with baited breath for the replies on this. Tried to find a similar answer some time ago and never reallly came up with an answer i was happy with..!

 

[Mr.Black]

Phone up your Part P scheme provider and let them earn their money.Then that way if it gets pulled for assessment..hey presto it was them that advised you .

Let us know what they said though.....

 

[Sintra]

have a read at reg 523.8 conductors in parallel. in my opinion 1 cable at mcb.

 

[acat]

I agree with above apart from anything else how much power would the client lose in the event of a trip of the MCB.


Chris

 

[ezzzekiel]

I would teat as a spur so never more than two at mcb although nothing wrong with radiating out from source like spokes of wheel except for space constraints

 

[bugsy]

I'd spend the extra £ 30.00 and install new RCBO = proper job

 

[Davey101]

Have been picked up on this by one of my fellow testers. Sintra is correct I believe, only one connection at an MCB except on a ring/spur arrangement (which seems backward to say the least).
On a practical level, I would have no problem with seeing two cables in an MCB , as long as ccc's are considered, and they are serving similar circuits i.e. two socket radials, two lighting circuits etc.

Would be intersted to know if you'd get pulled for it by your inspector?

 

[Des 56]

Other than saying, it should be only one wire at the mcb
I would be interested to hear the safety reasons why people think it cannot be more

If I was wiring a lighting radial and the lighting was in two opposite directions from the mains board,I would have no hesitation whatsoever taking 2 or more cables from the same mcb

In the above situation,I would say it is not only permissable,but a safer install because of the lower Zs that the branches would give, compared to a run which is doubling back on itself, just to please some obscure reason abot core numbers in an mcb

 

[Jason Smith]

A radial circuit, direct back to the board with a 20 amp MCB and 2.5, may serve an unlimited number of sockets. - Within a floor area of 50 sq. meters. A radial can be like a spider system as well. So without more info mate, do you need to consider bunching / you may be able to extend whats there .
However, the flavour of your post is about how many you can put in one mcb, very interesting. Its an important question to know the answer to as on a rewire of an old building, it may be necessary from time to time depending on whats in the way, like RSJ'S etc.

 

[Spudnik]

How does the RFC get connected if only 1 per MCB?

 

[cornburn]

Oooh it's all kicked off haha. Re the conductors in parallel comment, they wouldn't be in parallel as they would be feeding a different part of the circuit. From memory that reg covers multiple conductors feeding the same point (eg adding an identical cable for a distribution circuit instead of pulling the old out and installing a new one twice the size.

I could see no problem with a star arrangement if the building was an unusual layout and it shortened the total length of cable run, if the required points were in opposite directions then this does make perfect sense.

I suppose the main concern is whether or not the connection would be good enough with (example) 6 x 2.5mm terminated under it. The only way to really check this would be to screw down first, then take R1 readings with a long lead.

I've got the NIC assessor out in a couple of weeks so will see what he says. The board is a Merlin Gerin with a different RCBO design from most so last time I had to get them in they were £80+VAT, which is why I don't really want to buy another just for 2 sockets when there's a radial with 4 sockets already on the board.

Just to throw another spanner in the works, if wago connections are being classed as permanent commections I wonder if you could have an unlimited number of connections coming in from various legs of the circuit, connecting to a wago connector in the CU, then have one piece of 2.5 single core going into the MCB (or RCBO), thus ensuring a perfect connection for all parts of the circuit. In essence this would just be a junction box within the CU and might be a better option (we all know how unreliable screw connections can be sometimes with multiple cables under them)

Edit for Jason: The reason I don't want to come off the existing wiring is that it's all in steel conduit and to put a JB in would mean breaking into this and would look a bit messy (wago box hanging out of inspection bend). That's assuming I'd be able to pull back enough to do that at all. As the CU is the other side of the wall putting a new leg in direct from the MCB seems much easier and tidier.

 

[GrannySmiff]

So your asking if you can have two radials in one MCB/RCBO?.

 

[cornburn]

So your asking if you can have two radials in one MCB/RCBO?.

Well technically it's still just one radial circuit if it's all protected by the same breaker. If the MCB fed a JB, which then went out to the different legs of the radial then that would be perfectly textbook, the only different is this point on the circuit is under an MCB instead of a JB. I suppose in a JB each leg going out would have its own screw to ensure a good connection, and an MCB will only have the one screw.