Adding a new socket to a ring main

[Smil3r]

Hey,

 

I want to add additional socket on a ring main. Initial plan was to spur from existing socket C, however it appeared to be that C and all the other nearby sockets (B and D) already have spurs attached to them ( I am not exactly sure where these spurs from B, C and D are located, as all the wires are in the subfloor and I have not traced the cables nor did any continuity checks).

 

Could you please let me know if the proposed option on the right is feasible and within the regulations? Or do I need to find locations of either B1, C1 or D1 from B, C and D resprectively, do the break between them to place a 13amp FCU in between and wire these up in order as it should have been done the proper way?

 

Looking for the suggestions

 

 

 

[BotusBuild]

Get a qualified electrician and extend the ring main.

[Nickfromwales]

On 23/07/2025 at 08:30, Smil3r said:

Hey,

 

I want to add additional socket on a ring main. Initial plan was to spur from existing socket C, however it appeared to be that C and all the other nearby sockets (B and D) already have spurs attached to them ( I am not exactly sure where these spurs from B, C and D are located, as all the wires are in the subfloor and I have not traced the cables nor did any continuity checks).

 

Could you please let me know if the proposed option on the right is feasible and within the regulations? Or do I need to find locations of either B1, C1 or D1 from B, C and D resprectively, do the break between them to place a 13amp FCU in between and wire these up in order as it should have been done the proper way?

 

Looking for the suggestions

 

 

 

What you're proposing is permissible, just needs a good eye for making the connections in to the socket that will have 4x2.5mm2 cables in it. No need for the FCU as you can spur off a ring with 1x 1G 13a socket, or 1x2G 13a socket, or 1x 13a FCU feeding whatever, and the only time you need an FCU here is if you planned to take more than 1x the above from the ring.

 

Is it ideal, no, should you expand the ring main, yes, so the question would be; if you can get single cables to these extra outlets then why not run two and make it one big ring? You'd just Wago the cables in the back-boxes and open the ring up, quite simple to do and you can ask questions here if you need support.

 

If you're as much as 10% weary of what you're doing then give the job to a sparky. Just do the chases yourself in advance, install the back-boxes, and ask for them to just run cables and connect. A one day visit most prob so budget £250-300 for labour and consumables if you do the "donkey work"

[garrymartin]

9 hours ago, Nickfromwales said:

What you're proposing is permissible, just needs a good eye for making the connections in to the socket that will have 4x2.5mm2 cables in it. No need for the FCU as you can spur off a ring with 1x 1G 13a socket, or 1x2G 13a socket, or 1x 13a FCU feeding whatever, and the only time you need an FCU here is if you planned to take more than 1x the above from the ring.

 

But there is already a spur (C1) off C, so it isn't permissible to also add a FCU spur feeding an additional socket (C2).

 

Without seeing the cable routes, these appear to be the two easiest solutions:

 

1) disconnect C1 from C, add the FCU as a spur next to C, and then connect both C1 and C2 to the FCU

2) insert the FCU into the ring by disconnecting the cable between C and D, connecting that cable to the FCU along with a new cable between the FCU and C. You can then add as many extra sockets as you like off the FCU

 

B1 and D1 are fine - you can add a single spur off an existing socket on a ring.

 

Edited July 25 by garrymartin
Added note that B1 and D1 would be fine

[Nickfromwales]

1 minute ago, garrymartin said:

But there is already a spur (C1) off C, so it isn't permissible to also add a FCU spur feeding an additional socket (C2).

Not afaik. 
 

C is the ring, so C1 is a single radial spur, and C2 is a single radial spur, each from the ring. Technically this is fine and the terminals in the back of a socket are designed to take 3x 4mm2 or 4x 2.5mm2 also.

 

As an apprentice electrician the instructor took great pleasure in seeing if anyone had cut strands out of the 3x 4mm2’s when we had tasks to do 4mm radials with multiple spurs. Trick was not to twist the strands, which you learned quickly.

 

C1 and C2 off C is a go afaic, no problems there whatsoever, but obvs opening the ring for at least one of these would be the better job.

 

Good news these days is there are usually no 3kw electric fires with 13a plugs on them any more. 

[Nickfromwales]

Also, the FCU is entirely permissible, and then you could add as many sockets after it as you liked. Caveat is, after you’ve pulled more than 13a you’re on borrowed current and will eventually pop the fuse in the FCU. 
 

Ideal solution if you’re adding lots of sockets in a bedroom which will only have a hair drier, tv, lamps, pc etc, so run that radially off a FCU off the ring (which can still be a spur). 

[garrymartin]

1 hour ago, Nickfromwales said:

C is the ring, so C1 is a single radial spur, and C2 is a single radial spur, each from the ring. Technically this is fine and the terminals in the back of a socket are designed to take 3x 4mm2 or 4x 2.5mm2 also.

Oh, OK, didn't know that. That's really interesting. I understand that, as you describe, you essentially have two spurs, rather than a spur feeding two sockets, but I thought you were only allowed one spur from a single accessory;

 

"Only one unfused spur, supplying either a single 1-gang or 2-gang socket-outlet, should be connected to any accessory forming a part of the circuit (Appendix 15 of BS 7671 refers)." https://electricalapprentice.co.uk/connecting-equipment-to-a-socket-outlet-circuit/?utm_source=chatgpt.com#:~:text=Only one unfused spur%2C supplying either a single 1-gang or 2-gang socket-outlet%2C should be connected to any accessory forming a part of the circuit (Appendix 15 of BS 7671 refers).

 

But then Appendix 15 of BS 7671 only states that "an unfused spur should only feed one single or one twin socket-outlet only" - it doesn't explicitly say you can only have one unfused spur from a single accessory, even if that is commonly accepted...

 

[Nickfromwales]

2 minutes ago, garrymartin said:

Oh, OK, didn't know that. That's really interesting. I understand that, as you describe, you essentially have two spurs, rather than a spur feeding two sockets, but I thought you were only allowed one spur from a single accessory;

 

"Only one unfused spur, supplying either a single 1-gang or 2-gang socket-outlet, should be connected to any accessory forming a part of the circuit (Appendix 15 of BS 7671 refers)." https://electricalapprentice.co.uk/connecting-equipment-to-a-socket-outlet-circuit/?utm_source=chatgpt.com#:~:text=Only one unfused spur%2C supplying either a single 1-gang or 2-gang socket-outlet%2C should be connected to any accessory forming a part of the circuit (Appendix 15 of BS 7671 refers).

 

But then Appendix 15 of BS 7671 only states that "an unfused spur should only feed one single or one twin socket-outlet only" - it doesn't explicitly say you can only have one unfused spur from a single accessory, even if that is commonly accepted...

 

Indeed, often as clear as mud!

 

My very good sparky friend from Oxford (he’s wired a few BH members houses by coincidence) always took all 4 cables into 6-way push fit Wagos and then the 5th wire reconnected the socket that became the junction box.

 

This stopped the cluttered cabling at the socket terminal and made the process far more digestible. I’d recommend this approach for a competent DIY’r, but you MUST be sure to mark each core with a sharpie and use a combination pliers to push each core fully home into the Wago, or you end up with a crappy connection and likelihood of arc’ing or overheating.

 

Many ways to skin this cat, just needs competency and knowledge. 

[marshian]

2 hours ago, Nickfromwales said:

Good news these days is there are usually no 3kw electric fires with 13a plugs on them any more. 

you’ve just reminded me I need to wire in my 3kW immersion heater to a fused spur connection off the ring main (rather than the current 3 pin plug it’s currently wired into) before I turn off the boiler, drain the system for a couple of days to do a few circuit/rad valve changes prior to the heating season.

 

yes I know it shouldn’t be on a 3 pin but in 33 years it’s only been used twice (very much a energency use only)

[Nickfromwales]

1 minute ago, marshian said:

you’ve just reminded me I need to wire in my 3kW immersion heater to a fused spur connection off the ring main (rather than the current 3 pin plug it’s currently wired into) before I turn off the boiler, drain the system for a couple of days to do a few circuit/rad valve changes prior to the heating season.

 

yes I know it shouldn’t be on a 3 pin but in 33 years it’s only been used twice (very much a energency use only)

I met electricians on site last week who wired the immersion on to a 13a plug, and said it’s fine. 
 

Client wanted that on a ‘smart’ plug-in adaptor. I smiled and nodded……

 

I posted a pic a while back of a Willis heater is wired to a fused spur, and I got chased off into the hills……

 

Changed that to a 20a DP switch (16a RCBO) and slowly came out of hiding.

 

13a plug top is rated 10a continuous? @ProDave?

[marshian]

1 hour ago, Nickfromwales said:

I met electricians on site last week who wired the immersion on to a 13a plug, and said it’s fine. 
 

Client wanted that on a ‘smart’ plug-in adaptor. I smiled and nodded……

 

I posted a pic a while back of a Willis heater is wired to a fused spur, and I got chased off into the hills……

 

Changed that to a 20a DP switch (16a RCBO) and slowly came out of hiding.

 

13a plug top is rated 10a continuous? @ProDave?

I’d better state the case for defence (of the lazy)

 

1. it’s the shortest immersion I could find (seriously stubby)

 

2. it will heat approx 30 litres of a 115 litre tank before tripping out on thermal overload (temp of the water thermal overload - not melting the plug thermal overload)

 

3. it is absolutely a get out of jail solution for major gas boiler breakdown/failure 

 

4. I’m well aware it’s probably well past the ragged edge of acceptable

 

just an aside I once needed to do a bit of welding on a car - rather than turn the car round so the welder could be plugged into it’s normal 16 amp socket - I plugged the welder into an extension lead (un coiled just what was needed to reach the welder)

 

that ended well…….. Next time I used the extension lead it uncoiled to the same point - the rest had welded itself together / scrap basically

[Nickfromwales]

1 hour ago, marshian said:

I’d better state the case for defence (of the lazy)

 

1. it’s the shortest immersion I could find (seriously stubby)

 

2. it will heat approx 30 litres of a 115 litre tank before tripping out on thermal overload (temp of the water thermal overload - not melting the plug thermal overload)

 

3. it is absolutely a get out of jail solution for major gas boiler breakdown/failure 

 

4. I’m well aware it’s probably well past the ragged edge of acceptable

 

just an aside I once needed to do a bit of welding on a car - rather than turn the car round so the welder could be plugged into it’s normal 16 amp socket - I plugged the welder into an extension lead (un coiled just what was needed to reach the welder)

 

that ended well…….. Next time I used the extension lead it uncoiled to the same point - the rest had welded itself together / scrap basically

Did much the same thing testing an in-screed wire heater. 30m of a 50m extension lead rolled out from the site cabin and left to run for a couple of hours.

 

Centre of the plug top which I’d put onto the heater wire was just bubbling like a pot on a stove, had a molten middle with a small swirl of smoke rising from it.

 

Only 1800w combined, but constant. Cooked the lot. Heating worked well btw  

[marshian]

12 minutes ago, Nickfromwales said:

Did much the same thing testing an in-screed wire heater. 30m of a 50m extension lead rolled out from the site cabin and left to run for a couple of hours.

 

Centre of the plug top which I’d put onto the heater wire was just bubbling like a pot on a stove, had a molten middle with a small swirl of smoke rising from it.

 

Only 1800w combined, but constant. Cooked the lot. Heating worked well btw  

 

Home made induction heater was what I called it

 

Anyway apologies for the OT back to ring mains and fused spurs

[andyscotland]

5 hours ago, garrymartin said:

Oh, OK, didn't know that. That's really interesting. I understand that, as you describe, you essentially have two spurs, rather than a spur feeding two sockets, but I thought you were only allowed one spur from a single accessory;

 

"Only one unfused spur, supplying either a single 1-gang or 2-gang socket-outlet, should be connected to any accessory forming a part of the circuit (Appendix 15 of BS 7671 refers)." https://electricalapprentice.co.uk/connecting-equipment-to-a-socket-outlet-circuit/?utm_source=chatgpt.com#:~:text=Only one unfused spur%2C supplying either a single 1-gang or 2-gang socket-outlet%2C should be connected to any accessory forming a part of the circuit (Appendix 15 of BS 7671 refers).

 

But then Appendix 15 of BS 7671 only states that "an unfused spur should only feed one single or one twin socket-outlet only" - it doesn't explicitly say you can only have one unfused spur from a single accessory, even if that is commonly accepted...

 

Indeed. I think they've invented that requirement (or are repeating from a scheme provider who have invented it). Neither BS7671 or the On Site Guide specify any limit on the number of unfused spurs connected to a single outlet. The only stated limit is for outlets on a spur.

 

The only factor would be the requirement (only present in the On Site Guide) not to have more unfused spurs than outlets/appliances actually in the ring. And of course the capacity of the outlet terminals as @Nickfromwales said.

 

Spurs on ring finals are a bit of a historical anomaly. Despite being ubiquitous in the UK, until the 17th edition these circuits were barely covered in 7671 itself, and only detailed in the On Site Guide. The OSG is basically just there to save everyone having to work from first principles (and make mistakes).

 

The overall requirement is that the circuit is not likely to be overloaded for a significant period and that load is reasonably distributed around the ring. This is what allows using a cable that is technically undersized for the protective device (in fact a little overheating helps by increasing resistance and sending some current down the other leg of the ring).

 

However since there is no requirement for any particular spacing between outlets, you could without question have two double sockets or two junction boxes beside each other and a spur off each one.

 

Taking two spurs from one socket is essentially identical electrically - arguably better, since there are fewer sockets overall at that point in the ring.

 

But it does depend on a little common sense about expected use. If you want to plug in 6 phone chargers and laptops, no problem. If you're needing to plug in 6 kettles then that might not be the best circuit design.

[Nickfromwales]

8 hours ago, Nickfromwales said:

C1 and C2 off C is a go afaic, no problems there whatsoever, but obvs opening the ring for at least one of these would be the better job.

 

Just to clarify, the FCU is not needed, just go straight from C > C2. 

[marshian]

On 25/07/2025 at 18:10, andyscotland said:

 

Indeed. I think they've invented that requirement (or are repeating from a scheme provider who have invented it). Neither BS7671 or the On Site Guide specify any limit on the number of unfused spurs connected to a single outlet. The only stated limit is for outlets on a spur.

 

The only factor would be the requirement (only present in the On Site Guide) not to have more unfused spurs than outlets/appliances actually in the ring. And of course the capacity of the outlet terminals as @Nickfromwales said.

 

Spurs on ring finals are a bit of a historical anomaly. Despite being ubiquitous in the UK, until the 17th edition these circuits were barely covered in 7671 itself, and only detailed in the On Site Guide. The OSG is basically just there to save everyone having to work from first principles (and make mistakes).

 

The overall requirement is that the circuit is not likely to be overloaded for a significant period and that load is reasonably distributed around the ring. This is what allows using a cable that is technically undersized for the protective device (in fact a little overheating helps by increasing resistance and sending some current down the other leg of the ring).

 

However since there is no requirement for any particular spacing between outlets, you could without question have two double sockets or two junction boxes beside each other and a spur off each one.

 

Taking two spurs from one socket is essentially identical electrically - arguably better, since there are fewer sockets overall at that point in the ring.

 

But it does depend on a little common sense about expected use. If you want to plug in 6 phone chargers and laptops, no problem. If you're needing to plug in 6 kettles then that might not be the best circuit design.

 

I am right in thinking that ring circuits is very much a UK thing and stems from "economy" post war - less wire used and lower grade of cabling required?

 

In Europe they are radial??

[Mike]

2 hours ago, marshian said:

I am right in thinking that ring circuits is very much a UK thing and stems from "economy" post war - less wire used and lower grade of cabling required?

 

In Europe they are radial??

I believe that was the origin, and indeed they're not found in Europe. They no doubt regarded as a potentially dangerous British curiosity here in France. Potentially dangerous because, if the ring becomes broken, you could with 32A passing through cables that are too skinny.

 

[G and J]

1 hour ago, Mike said:

I believe that was the origin, and indeed they're not found in Europe. They do doubt regarded as a potentially dangerous British curiosity here in France. Potentially dangerous because, if the ring becomes broken, you could with 32A passing through cables that are too skinny.

 

So if they are radially wired presumably their CUs are very different to ours or they sit next to boxes that are the electrical equivalent of manifolds. 

[andyscotland]

1 hour ago, marshian said:

I am right in thinking that ring circuits is very much a UK thing and stems from "economy" post war - less wire used and lower grade of cabling required?

 

In Europe they are radial??

 

Indeed.

 

9 minutes ago, G and J said:

So if they are radially wired presumably their CUs are very different to ours or they sit next to boxes that are the electrical equivalent of manifolds. 

 

I don't know for sure, but from idle curiosity when on holiday they seem fairly similar, but usually with more ways (even small ski apartments tended to have a board more like the kind of thing you'd see on industrial/commercial installs here with sockets divided into multiple radials where UK practice would usually be 1 or perhaps 2 rings for a 3 room flat).

 

My hunch would be they essentially run the same cabling but instead of joining at the furthest point it becomes two radials at the CU.

 

1 hour ago, Mike said:

I believe that was the origin, and indeed they're not found in Europe. They do doubt regarded as a potentially dangerous British curiosity here in France. Potentially dangerous because, if the ring becomes broken, you could with 32A passing through cables that are too skinny.

 

Indeed - although as well as the ring itself the other factor is (in normal domestic with gas heating) it's not that common for a ring to be loaded up to the full 32A for any length of time, if it complies with the floor area & outlet type restrictions. Especially considering that even if the ring is broken the sockets are still split between two cables (assuming you're not unlucky enough to get a break between CU and first socket).

 

In fact that is the basis on which the regs allow the oversized protective device for the cable, rather than the ring per se.

 

One way to think of it is the places you're allowed to use a ring would also be suitable for 1 or 2 20A radials without any concerns that they'd be constantly tripping.

 

So although the ring is protected at 32A, most of the time the current will be below the 20A rating of the cable. The extra 12A just provides a bit of an extra buffer for short-lived concurrent loads.

 

That said, I prefer radials for new work.

[Nickfromwales]

58 minutes ago, G and J said:

So if they are radially wired presumably their CUs are very different to ours or they sit next to boxes that are the electrical equivalent of manifolds. 

Nope. You start at the CU (RCBO) and go to each socket one by one, a-la a daisy chain, and the last one on the line is where you terminate the run.

 

I do all current rewires (new electrical installations) with all radials where possible, so each bedroom on its own 20a radial, study same, hall/stairs/landing/attic same, living/dining same, and then put obligatory rings in for kitchen/utility. Outside lights and outside sockets/power always on dedicated circuits so that nothing getting wet can shut off something inside the home. 

 

All in the same CU, in the same location, just more ways and more runs of cable. Pretty basic tbf.

[Kelvin]

23 minutes ago, Nickfromwales said:

Nope. You start at the CU (RCBO) and go to each socket one by one, a-la a daisy chain, and the last one on the line is where you terminate the run.

 

I do all current rewires (new electrical installations) with all radials where possible, so each bedroom on its own 20a radial, study same, hall/stairs/landing/attic same, living/dining same, and then put obligatory rings in for kitchen/utility. Outside lights and outside sockets/power always on dedicated circuits so that nothing getting wet can shut off something inside the home. 

 

All in the same CU, in the same location, just more ways and more runs of cable. Pretty basic tbf.

That’s exactly how I did ours. A bit more cabling and a slightly bigger CU. 

[SteamyTea]

19 hours ago, Mike said:

They no doubt regarded as a potentially dangerous British curiosity here in France

I believe that EU wiring regs are based on our legislation, with a few minor changes for local markets.

The UK was the first European country to embrace electricity with major developments in the 1920s and 1940/50s. Why we have a very robust national grid.

[Mike]

On 28/07/2025 at 18:20, SteamyTea said:

I believe that EU wiring regs are based on our legislation, with a few minor changes for local markets.

There's certainly been international cooperation for decades, and current UK & European regs are based on CENELEC HD 60364 (itself a variation of IEC 60364) with local deviations. Ring mains, twin-and-earth cable, fused spurs, on-wall cooker switches and metal consumer units are probably the UK's main peculiarities.

 

On 27/07/2025 at 23:29, G and J said:

So if they are radially wired presumably their CUs are very different to ours or they sit next to boxes that are the electrical equivalent of manifolds. 

I've only studied the UK & French regs (informally, not as a qualification) but yes, modern French CUs are certainly much bigger. That's partly due to the use of radials everywhere, but also because the French rules just require everything to be split into more circuits. I'm not sure if that's a French peculiarity, or common across Europe.

 

For example, a UK 20A radial circuit can serve 50m²; in France it can serve just 12 sockets (doubles count as 2), or 6 sockets if serving a kitchen worktop. Plus separate circuits for each dishwasher, washing machine, freezer, etc. Consequently, excluding lighting, my living room has 3 radials and my kitchen has 8 (though I could have got away with 6). By the time I've added lighting, the other rooms, MVHR, water heating, a couple of Shellys, Willis heaters, a load-shedder, and a lightning protector, this is the CU that I'm installing (and that's just the first part):

 

 

In addition, I have several dimmers & LED drivers to accommodate, but I can't add them all to the CU without exceeding the mandatory 20% unused capacity, so they'll be going in a separate 2-row cabinet. Then, since it's an apartment, there's another 2-row cabinet for the smart meter and the main RCD / isolator.

 

And there's more - a separate communications cabinet for TV, telephone & internet cabling, with another for a broadband router / TV box.

 

My 'electrical technical space' will therefore extend from floor to ceiling with 5 separate cabinets plus an access hatch:

 

Edited July 29 by Mike

[Mike]

57 minutes ago, Mike said:

There's certainly been international cooperation for decades

Just looked this up out of curiosity. The first international conference was the 1881 Congress of Electrician's in Paris, which ran concurrently with the International Electricity Exposition, focused on defining and naming the basics - Volts, Amps, Coulombs, Watts and the like. That was followed by meetings in Chicago (1893) and 1900 (Paris), and discussions held at the latter and the subsequent 1904 meeting in St. Louis lead to the creation of the IEC (International Electrotechnical Commission), which first met in London in 1906.

 

The first UK wiring regs were published in 1882 by the Society of Telegraph Engineers and of Electricians, but I'm not sure what they covered. The first French law on electricity installations seems to be from 1895, concerning the safety of electrical installation in the streets, prompted by stories of horses 'dancing' in the streets of Paris after being shocked!

 

 

[G and J]

So with radial one could have an rcbo for each room’s sockets. (Maybe two rcbos for bigger rooms).  In terms of isolating faults and device failures that’s a fab idea.  Worth the cost of a bigger board and more rcbos/cable methinks.