Radial final circuits in garage

[sharpener]

Situtation is a new, small garage (16 sq m) fed from the house via a 16A mcb and 30mA main RCD. I am doing my own installation under a Building Notice to the local council building control dept.

Plans filed with the application are for a garage unit with 1 x 6A mcb for lighting, and 1 x 16A mcb for 2 double and 1 single sockets and a spur box for the garage door actuator, but I did not show any detail of the power circuit topology.

As the wiring comes at this CU from 3 different directions (left, right and below) I have brought 3 separate 2.5 mm^2 tails into it, one each from the two double sockets and one from the single socket via the spur box. The terminals of the breaker will accept 3 tails without a problem.

Since the cable (in trunking) is rated at over 20A and this circuit is protected by two 16A breakers in series I consider the requirements for protecting the cables against overload and short circuit are met. However the electrical contractors to whom the local authority have sub-contracted the inspection say this arrangement breaks some rule about radial circuits, though they cannot tell me which rule nor why the arrrangment is not safe.

Their recommendation is to daisy chain all four fittings, by using crimped connections in the CU to run additional wires out to each double socket and back. This will put 10m of cable and four additional intermediate connections between the breaker and the furthest outlet, which clearly makes no sense from the point of view of either reliability or fault clearance time or voltage drop.

Can anyone on this forum give me a definitive ruling either way as to whether what I originally proposed is allowed?

TIA

 

[Des 56]

I sometimes hear on this forum, that it is customary or required that only one conductor/one mcb
The regs require disconnection times to be met,they require overload and fault protection to be within limits,cable sizes to be adequate etc
They do not mention one mcb/one conductor

The inspectors may have preference for that set up, but that is personal preference
What you have done is neither dangerous or against any regs that I am aware of

 

[Engineer54]

Not the best practice, but perfectly acceptable, as Des 56 has stated above!! The three legs going to each of the socket outlets will be deemed as a single circuit as they are being supplied by a single MCB!!

 

[Brightspark2]

The main mcb needs uprating, you have no discrimnation between protective devices, you cant put a 16A MCB protecting a 16A MCB, uprate it, to say 20A depending on main cable used etc...


An OPD or MCB in this case can only protect one circuit, it cant protect more than one circuit, so if the inspecters are classing that as 2 or 3 circuits thats probably why there not accepting it im afraid which is techincally correct although may seem a little harsh in this instant...

 

[Darkwood]

The main mcb needs uprating, you have no discrimnation between protective devices, you cant put a 16A MCB protecting a 16A MCB, uprate it, to say 20A depending on main cable used etc...


An OPD or MCB in this case can only protect one circuit, it cant protect more than one circuit, so if the inspecters are classing that as 2 or 3 circuits thats probably why there not accepting it im afraid which is techincally correct although may seem a little harsh in this instant...

Disrimination regarding overload can be met as you state with a 20amp front end but discrimination cant be met regarding a fault current and partial discrimination in this situe is usually the best that can be achieved, this is acceptable and to highlight what i mean a 50amp mcb dosnt give discrimination to a 10amp mcb under short circuit conditions, its luck of the draw to which one if not both trip but agree totally with the direction of your post.

 

[Adam W]

You can have a maximum of one spurred socket per socket on a radial or ring. It is acceptable to spur straight out from the breaker, but what you've essentially done is spur out twice.
BS7671 refers to sockets being wired in a ring final, radial final, or spurred from either of those. What you've got is more of a 'spider' type arrangement which BS7671 makes no reference to.

 

[plugsandsparks]

Its a common situation wrt radials, i see it alot and sometimes have done it. I prefer the installation that way and i also mark the board as Sockets Radial. Having said that have never put three into one MCB but i guess its no different.
The reason i like it is its simplicity and cost effectiveness for cable (after all copper is a natural resource we dont really want to run out of). Think how many lighhting radials we see with 2 or 3 wires in when you included the em lights, no difference. It can make testing a little more complicated but in your siutation (in a garage) would have thought it was pretty obvious.

 

[Octopus]

However the electrical contractors to whom the local authority have sub-contracted the inspection say this arrangement breaks some rule about radial circuits, though they cannot tell me which rule nor why the arrrangment is not safe.

Their recommendation is to daisy chain all four fittings, by using crimped connections in the CU to run additional wires out to each double socket and back. This will put 10m of cable and four additional intermediate connections between the breaker and the furthest outlet, which clearly makes no sense from the point of view of either reliability or fault clearance time or voltage drop.

Can anyone on this forum give me a definitive ruling either way as to whether what I originally proposed is allowed?

TIA

1. IF you've paid LABC to inspect and their agents say its unsafe then ask them, in writiung to confirm what is unsafe and which regs apply.

2. Daisy chain using crimps in the CU - sounds like a bodge if I ever heard of one. IMHO a simple radial is the best solution.

3. This forum will NOT give you definitive answers. You are likely to get many conflicting opinions.

4. I recommend that step 1 above is the only way forward.

 

[JUD]

You can have a maximum of one spurred socket per socket on a radial or ring. It is acceptable to spur straight out from the breaker, but what you've essentially done is spur out twice.
BS7671 refers to sockets being wired in a ring final, radial final, or spurred from either of those. What you've got is more of a 'spider' type arrangement which BS7671 makes no reference to.

The requirement for only spurring one socket per socket only applies to ring circuits, NOT radials (Check App. 15). You can spur as many sockets off a socket as you like on a radial as long as all cables are sized correctly with regards to the OCPD i.e 2.5mm² on a 20A breaker.

As for the OP, apart from the discrimination issue between the two 16A breakers there's nothing electrically wrong with it -- it's just not the norm. A simple radial would be a better option.

 

[ukpita]

Come accross similar scenario in a property a few months back where customer CU had multiple conductors in every MCB, thought would run by Elecsa Tech as wasnt 100% if correct or not.

There advice was provide cable carrying capacities where greater than MCB then multiple conductors in MCB was whilst unorthadox was acceptable.... However they did advise that on the electrical cert needed to be mentioned and on the schedule of test results each individual conductor/leg needed to be individually listed and tested seperately... (IE MCB 1, leg 1, leg 2 etc.. etc..)

 

[sharpener]

You can have a maximum of one spurred socket per socket on a radial or ring. It is acceptable to spur straight out from the breaker, but what you've essentially done is spur out twice.
BS7671 refers to sockets being wired in a ring final, radial final, or spurred from either of those. What you've got is more of a 'spider' type arrangement which BS7671 makes no reference to.

To be exact, the configuration _as inspected_ had only one of the spurs actually installed at the time, even so they wouldn't accept it.

IIRC the On-Site Guide says that one spur per socket or item of fixed equipment (i.e. two, which can be from the breaker) is acceptable on a radial cct with 4 sq mm cable and a 30A breaker (A2 radial cct) but makes no mention of spurs for a final radial circuit with 2.5 sq mm cable and a 16 or 20A breaker (A3 radial cct). However it doesn't say (except by omission) that they are not allowed either.

Regarding the discrimination issue, the 16A breaker in the main CU protects the 2.5 sq mm cable out to the garage. The breaker in the garage unit is therefore not strictly necessary *at all*. but it would surely look a bit odd just to have the 6A mcb on the lighting circuit and nothing at all on the power circuit, also it is handy as an isolating switch. I agree that ideally in the event of an overload on the power cct it would be nice to be sure that the 16A mcb in the garage unit would be the one to trip (thus leaving the garage lights on), but I am not certain that making it a 20A breaker in the house would provide sufficient discrimination as I recall that the I squared t ratings of the two are both the same.

The sub-contract inspectors could not tell me the next step i.e. they do not know if they are going to be sent back to re-inspect or if I will be given written notice of their findings or what (some rigorous process this!), but as and when I hear anything further I will as Murdoch suggests ask what regs are being contravened.

Probably the easiest thing is for me to change my mind about having the two double sockets, as the only thing that is essential at right now is the spur box for the garage door actuator(!) They cannot prevent me from using a portable extension lead/4-way distribution board from the single socket outlet, even though common sense suggests that to have permanent wiring in trunking would be preferable.

Thanks for all the helpful comments.

 

[Guitarist]

You could have mounted a 4x4 adaptable box under the CU and brought your socket cables into this, all connected to wago connectors fed from the CU.
The other option, as your 2.5mm from the house CU is already essentially a radial, would be to dispense with the garage CU altogether and radial socket to socket then sfcu for lighting and sfcu for door circuit in 2.5mm.

 

[Adam W]

I'd go with post #8 - ask the person who has a problem with your work what their problem is with your work.