Best Method for Splicing into a Radial Power Circuit?

[Ultim8Powers]

Hi everyone, looking for a bit of advice on extending an existing radial circuit in my home (England, UK). I'm fairly experienced with basic electrical works, and regularly convert homes to 'smart homes', however the following has me stumped.

The house was rewired 5-6 years ago, prior to my purchase. Currently renovating so the ground floor ceilings have all been ripped down giving me easy access to the services.

Which of the following would you recommend as "best" (I suspect both are probably ok, with option B being the easiest):

A) Breaking into / splitting the radial (sorry about my terminology):

Or, B) Splicing into the radial:

A few other queries connected to the above:

1. From my research I’m getting conflicting information about the number of permitted outlets on a spur:
   • https://www.diydoctor.org.uk/projects/radialcircuit.htm - states that only a single outlet is allowed per spur
   • http://wiki.diyfaq.org.uk/index.php/House_Wiring_for_Beginners - shows multiple outlets per spur
2. Is the existing 16B MCB sufficient, from my research it should really be a 20A MCB? Some some sources state that 16amp is ok, others state that a radial must be on a 20amp MCB. Considering that I’m more than doubling the outlets, I suspect I need to upgrade to 20amp?
3. Is it permitted to place junction boxes in the ceiling void (they will be inaccessible once the new plasterboard goes up). I don’t really fancy the extra chasing required to spur from the existing sockets.
4. Are Wagos ok in the ceiling void, within Wago boxes of course?

 

[333rocky333]

1 spur from a spur applies to 32a rings not 16a or 20a radials, idea is you cannot overload cable, without the mcb tripping.
Rcd protection is also required

4 cables into a socket is poor design

There are far more superior MF junction boxes than using wagos and wagos boxes

 

[Ultim8Powers]

Thanks for your reply.

Rcd protection is also required

Absolutely, RCD is in the CU already, forgot to mention that.

4 cables into a socket is poor design

I'll look to see if I can change that, it's a tricky spot as the existing layout does not allow for a new vertical chase (there are water pipes in the wall above / I prefer to stick to the vertical wiring zones so no 'dog legs').

There are far more superior MF junction boxes than using wagos and wagos boxes

Sure, I'll just go for the regular round boxes.

Is it best to split of splice into the radial, i.e. A or B above?

Should I swap the MCB out for a 20A one?

 

[plugwash]

1. From my research I’m getting conflicting information about the number of permitted outlets on a spur:

https://www.diydoctor.org.uk/projects/radialcircuit.htm - states that only a single outlet is allowed per spur
http://wiki.diyfaq.org.uk/index.php/House_Wiring_for_Beginners - shows multiple outlets per spur

A branch in a radial circuit that is wired with cable rated to the full current rating of the circuit is not considered a spur and not subject to the guidelines on what can be on a spur.

2. Is the existing 16B MCB sufficient, from my research it should really be a 20A MCB? Some some sources state that 16amp is ok, others state that a radial must be on a 20amp MCB. Considering that I’m more than doubling the outlets, I suspect I need to upgrade to 20amp?

UK regs (unlike some countries) don't place an explicit limit the number of sockets on any particular rating of circuit. Presumably those writing the regs think (correctly IMO) that the number of sockets bears little relation to the amount of load. If there are less fixed sockets, people will just use more extension strips.

Still you shouldn't design circuits that you anticipate will be overloaded. You should think about what the sockets are likely to be used for and whether your design provides sufficient capacity.

Is it permitted to place junction boxes in the ceiling void (they will be inaccessible once the new plasterboard goes up). I don’t really fancy the extra chasing required to spur from the existing sockets.

If the junction won't be accessible then you need to use a maintenance free junction box (or use soldering or crimping, but those aren't options I'd generally recommend to the diyer).

It seems that the makers of maintenance free junction boxes have each taken a different approach, leading to different manufacturers products being better in different scenarios. IMO the best product for the particular application mentioned in this thread is the J803 from hager/ashley

I would try to keep the number of hidden joints to a minimum, even if it meant a bit more cable. The J803 has four cable entries, so you can get two branches from one box.

Honestly though, since you are clearly going to be getting a plasterer in anyway, i'd say your best option is to bite the bullet and avoid hidden joints completely. If you don't want to chip out the plaster by hand go pick up a cheap SDS+.

Are Wagos ok in the ceiling void, within Wago boxes of course?

The maker of the Wagoboxes (note that the Wagoboxes are not a wago product) say that some (not all) of their products are maintinance free junction boxes when used in accordance with the relevant instructions and the correct Wago terminals. Some combinations of box and terminal require adapter plates.

If you absolutely need a complicated junction with more than four terminals or more than four cables in an inaccessible location and/or you need to deal with 6mm² cables then the Wagobox products may be your best bet, but for simple scenarios there are IMO better options.

 

[JohnD]

Why are you opposed to having a Ring Final?

 

[Harry Bloomfield]

From my research I’m getting conflicting information about the number of permitted outlets on a spur:

• https://www.diydoctor.org.uk/projects/radialcircuit.htm - states that only a single outlet is allowed per spur

That rule is for a 32amp ring circuit.

• http://wiki.diyfaq.org.uk/index.php/House_Wiring_for_Beginners - shows multiple outlets per spur

That rule applies to the radial circuit you are proposing extending.

Is the existing 16B MCB sufficient, from my research it should really be a 20A MCB? Some some sources state that 16amp is ok, others state that a radial must be on a 20amp MCB. Considering that I’m more than doubling the outlets, I suspect I need to upgrade to 20amp?

So you could use method B, with suitable MF joints.

If it is wired in 2.5mm you could upgrade that to a 20amp MCB, if 16amp is not adequate. There was no rule that you had to have a 20amp MCB, just that 20amp was the maximum.

Alternatively, you might be better with method A, plus completing the circuit into a ring and changing the MCB for a 32amp one.

 

[bernardgreen]

Bite the cost bullet and use new cable to avoid the use ( and cost ) of any junction boxes.

The suggestion to convert to a ring final is also worth considering.

 

[ericmark]

The plan is to reduce the loop impedance to a minimum, in general this means using as little cable as possible, with a ring final with 2.5 mm² cable total should not exceed 106 meter we are told, with radials it is a lot less.

The way the volt drop which is normally the most limiting factor is calculated is not fixed in stone, with the ring we assume 20 amp at centre and 12 amp even spread, so with a radial it would be 20 amp at end and 12 amp even spread if using 4 mm², but with 2.5 mm² your maximum is likely 20 amp so it is simply 20 amp. This means 32 meters maximum.

When you re-wire some one signs for three parts of the re-wire, design, installation and the inspection & testing. So the person signing as designing the system needs to have worked out the volt drop, I will be frank unless working near the edge of what is permitted I don't work it out, but if you install a radial with incoming of 0.35 Ω and then test at end and find 1.13 Ω then you know you were on the edge with a radial, and I was worried about some one in the future testing my work and saying this is over the limit get him back to correct it. However in all my time only once have I had to reinstall a radial circuit using heavier cable, and I had told foreman before we started I didn't think the cable was heavy enough. It was for a shrink wrap machine and it did not work correctly with a volt drop.

I find radials are expensive to install, one ring can cover the area of 3 radials, OK the cost of RCBO's has dropped, around £15 for a fuse box type AC curve B, but if considering 3 rings that is £45 for RCBO's but with radials could cost £170 so it can work out expensive. Specially with a kitchen, we should I know have a dedicated supply to fixed appliance over 2 kW but with a ring we can normally get away not having a dedicated supply for washer/drier etc. But with radials of 20 amp one item drawing 10 amp and not much leeway for anything else.

It is down to design, and with a large fridge, (Size of about 3 houses) we wanted sockets for cleaning equipment, so ran 6 mm² in ceiling space with 2.5 mm² drops to each socket, the 6 mm² set out as a ring final that allowed 250 meters and still well within volt drop, with the drop down the wall being 4 meters in that case, a junction box in the ceiling/loft space was best option, but clearly not domestic, it was a warehouse for cheese, we don't run around with fork lifts in ones house, so in the house the drop is more like 1 meter.

I like the idea of with a house splitting ring side to side rather than up/down, it means if it fails no extension leads up/down stairs, it means more even load between the two, and less cable so better loop impedance. But the down side is if lights split up/down ensuring lights and ring final for any room not on same RCD, you don't what lights going off when some thing trips the RCD. This house is split front/back so used all RCBO's so no problem with sockets and lights on same RCD.

But down to planning, and that has to be done by you, your signing the installation certificate so you must design. Oh and for 2.5 mm² on 16 amp MCB around 42 meters max cable length.

 

[Ultim8Powers]

Thanks for all the prompt replies, super helpful.

I'm not opposed to making this circuit into a ring final. I have a twin blade chaser and a clear route back to the consumer unit, so a good suggestion. The only problem may be where I have 3 sockets together, would this require a fused spur, I think not as it would be two separate spurs off a single socket in the loop (4 wires into the 'master' socket is not ideal but unfortunately unavoidable in this position).

The sockets only serve a Living Room and Play room, thus only going to be running a TV, Laptop, table lamps, small gadgets and the occasional Vacuum Cleaner, however the total cable run including all branches and drops will be 46m, thus I think I will need to convert to a 32 amp ring final for that reason alone.

Cost is not the issue (just time), but I will pull new cables where possible to avoid hidden junctions, sticking with layout 'A' to maintain a compliant ring. Most of the existing drops are in oval trunking within the walls so hopefully not too hard to pull some new wires through.'

 

[bernardgreen]

 

[ericmark]

In the days when we did not consider the 1/3 rule when drilling beams, the cable to each drop was not that much, but with the 1/3 rule and taking cable along ceiling as well as wall, the cable length can become silly, so I think @bernardgreen idea is very good, down wall to first socket, along wall to other sockets, then up wall with last socket before corner. Not worried about cable length for cost, it is the volt drop that matters.

 

[JohnD]

View attachment 216515

The new layout can have numerous sockets and switches between the two drops, for example in a kitchen or workroom where you want a row of them about 150mm above the worktop.

If you already have drops to existing sockets, you do not need to add junction boxes.

You could achieve the socket layout in your "diagram B" much more simply.

 

[flameport]

I have a twin blade chaser

Then use it to chase the walls horizontally, rather than wasting vast lengths of cable and concealing unnecessary junction boxes in the ceiling

Is the existing 16B MCB sufficient, from my research it should really be a 20A MCB? Some some sources state that 16amp is ok, others state that a radial must be on a 20amp MCB.

Circuit ratings are dependent on the loads supplied, there is no 'correct' single answer.

The sockets only serve a Living Room and Play room, thus only going to be running a TV, Laptop, table lamps, small gadgets and the occasional Vacuum Cleaner

16A will be fine. 20A could be used if you want. Or even 25A.

however the total cable run including all branches and drops will be 46m, thus I think I will need to convert to a 32 amp ring final for that reason alone.

A ring is not a valid solution to a circuit being too long, and 46m is not a problem for a 16A radial with multiple socket outlets anyway.

If the chimney breast gets in the way, use one or more of the existing vertical drops and connect horizontally to the existing socket(s). There is no need for any junction boxes anywhere, or any new vertical chases.

 

[Ultim8Powers]

I am very limited as to where I can cut horizontal chases, the property was retrofitted with central heating a few years before I purchased the house (it's 120 years old btw). The main heating pipes are in the ceiling void, and micro pipes were used to drop down to the ground floor radiators, set within the plaster, thus horizontal chases would clash with the pipes in many places.

That said, horizontal chases work in two locations, I will do that for those.

The 3 sockets shown together are actually spread across both sides of a partition wall, the walls are only 50mm thick light concrete thus the sockets have to have shallow boxes and be staggered, plus there is a structural concrete beam above with only a thin skim of plaster, so no chasing possible. I will try to thread a wire straight through the middle box to spur off the existing spur, rather than have 4 wires going to the middle terminals.

I spoke to an electrician who did my mothers house and we was very against the idea of converting to a ring final. He said they are more risky as a fault can go undetected leading to a possible overload. He advised me that no branch can be longer than 42m, thus each branch can be measured separately back to the CU. I will keep the 16 amp MCU but upgrade to the maximum allowed 20 amp should I find it trips in the future.

I have redesigned my layout as follows... no more hidden junctions in the ceiling / less chasing.... yey!