Spurs

[ldoodle]

Hey,

I know you can only have 1 spur off a ring socket, but can you then spur off the spur for a light?

So ring socket > spur socket > lightswitch (by light switch I mean FCU with appropriate fuse).

I'm thinking specifically for a shed. I have an outside socket which is part of the ring, so wondering if I could spur off of this to a double socket in the shed, then spur off the socket for a light switch (FCU).

Don't really want 2 armoured cables (1 for socket, 1 for light) surface mounted to the outside of the house!

Cheers

 

[riveralt]

You cannot have a spur off a spur.
Replace the double socket with an 13A FCU and then run your sockets and lights from the from the fcu load side.
Any new sockets will need to be RCD protected so if your circuit is not already protected you could replace the original socket with a FCU/RCD.

 

[ldoodle]

You cannot have a spur off a spur.
Replace the double socket with an 13A FCU and then run your sockets and lights from the from the fcu load side.
Any new sockets will need to be RCD protected so if your circuit is not already protected you could replace the original socket with a FCU/RCD.

Ah gotcha, I think.

So;

from existing external socket (on house) > fcu (in shed) > socket (in shed) and light (in shed).

In other words, the FCU will have 1 cable coming in (supply) and 2 going out (load (2.5mm for socket and 1mm for light)).

Or do you mean external socket > fcu (13a) > socket > fcu (3a) > light

My CU is RCD protected.

 

[riveralt]

from existing external socket (on house) > fcu (in shed) > socket (in shed) and light (in shed).

In other words, the FCU will have 1 cable coming in (supply) and 2 going out (load (2.5mm for socket and 1mm for light)).

That is correct.
However, I would run 1.5 or 2.5mm ² T&E from the load side to another Switched FCU (3/5A) and then 1mm ² T&E onwards for the lights.
Sockets are as described.

 

[ldoodle]

Thanks.

So;

fcu (13a) --> double socket
..|
..|-> fcu (3/5a) > light


Not so bad, 'coz I can use the 2nd FCU as the light switch, rather than going fcu > light fitting > light switch.

 

[riveralt]

Not so bad, 'coz I can use the 2nd FCU as the light switch, rather than going fcu > light fitting > light switch.

That's the plan.

 

[PrenticeBoyofDerry]

I have an outside socket which is part of the ring

if so not really a good idea, I would much prefer this to be designed so it can be independantly isolated

so wondering if I could spur off of this to a double socket in the shed, then spur off the socket for a light switch (FCU)

You can fuse spur of a socket from the ring, then take further outlets from that point, but you are restricted to a maximum load of 13A after this spur.

The requirements for 30mA RCD protection on any additional sockets and new cable that is buried less than 50mm within wall, should also be complied to.

 

[ldoodle]

The outside socket was already there when we bought house. I plan, in the not too distant future, to get the CU changed, and as part of that get the outside socket and shed stuff put on it's own radial.

The outside socket actually 'backs' on to the CU so I (aka electrician) could take it out of the current ring easily, without using JBs to maintain the ring.

 

[PrenticeBoyofDerry]

The outside socket actually 'backs' on to the CU so I (aka electrician) could take it out of the current ring easily, without using JBs to maintain the ring.

So you could extend the ring at the CU and introduce DP isolation there for the external power.

 

[ericmark]

There are two types of spur fused and non fused. At one time we called a fused spur a radial but now it is called a spur which leads to confusion.

So the basic point is where the supply is fused to a greater value to what the cable is rated then you are allowed where there is very little chance of the cable being damaged to fit the overload device at the destination rather than the origin. 434.2.1 also limits the length to 3 meters although some argue spurs from a ring (twin 2.5mm) or radial (single 4mm) is a special case.

So you can connect one extra device to a ring which will have a fuse or pair of fuses in the case of a double socket only. However after it has gone through a fuse be it in a 13A plug or inside a fused connection unit you can then add as many devices as you like as they are protected by the 13A fuse.

The ring is a special case and there are some things not allowed even though at first it would seem there is no overload. For example your not allowed to take a spur with 4mm cable and feed daisy chained sockets even though the 4mm cable is rated high enough. This is because the ring may not load share across the two cables the same applied to figure of 8 wiring.

There are exceptions with a so called kitchen ring often it is not a true ring but two parallel conductors done because near impossible to get two 4mm into a grid switch.

As well as the rules on ring circuits there is now an added problem in that all new cable buried in a wall is either RCD protected or special cable and all sockets unless marked for one appliance only i.e. fridge only also needs RCD protection. Also before you can extend the circuit you need to work out if the increased loop impedance will be a problem either because of automatic disconnection or volt drop.

In real terms clearly many break the rules. This also means often sockets with two cables are not rings so testing must be done. Assuming you don't have the test equipment then using a fused connection unit (FCU) and if not already RCD protected the a RCD FCU is likely the best option for DIY.

There are plug in testers at around £50 which do a loop test but these will only show if the completed work complies they will not tell you if it will comply before you start.

 

[JohnW2]

The ring is a special case and there are some things not allowed even though at first it would seem there is no overload. For example your not allowed to take a spur with 4mm cable and feed daisy chained sockets even though the 4mm cable is rated high enough. This is because the ring may not load share across the two cables

AFAIAA, the regs don't actually forbid that, so one would have to be able to justify the design oneself. If, for example, the 4mm² (unfused) spur were attached at the midpoint of the ring, I would imagine that such a justification ought to be pretty straightforward, wouldn't you?

Kind Regards, John

 

[ericmark]

The ring is a special case and there are some things not allowed even though at first it would seem there is no overload. For example your not allowed to take a spur with 4mm cable and feed daisy chained sockets even though the 4mm cable is rated high enough. This is because the ring may not load share across the two cables

AFAIAA, the regs don't actually forbid that, so one would have to be able to justify the design oneself. If, for example, the 4mm² (unfused) spur were attached at the midpoint of the ring, I would imagine that such a justification ought to be pretty straightforward, wouldn't you?

Kind Regards, John

I would agree John one could design an installation which does not follow all the rules laid down in regulations and the accompanying IET books.

Many of the rules we follow are in the guides Book 3 for example does refer to figure of 8 and how to test to ensure it does not exist but seen nothing in BS7671:2008 about that or things like maximum number of cables in a conduit.

Appendix 15 (Informative) is seen by many to be part of the regulations and the circle with all the options does not include using 4mm² it is only with the radial final circuit arrangements that a 4mm² unfused spur is shown.

I have argued that the appendix is not really part of the regulations however since one is tested on items found in the appendix when taking the C&G2382 many consider it to be part of the regulations.

I am sure you remember the 14th Edition where what is now published in the guide was then part of the main book. However at that time it was not a British Standard it only became a British Standard with the 16th Edition. Up to the 16th Edition I do not remember any exam on being able to read a book.

433.4.2, 434.4, 523.8, all have to be considered and the latter is interesting as it states it does not stop the use of a ring final circuit. However 434.2.1 does not state any such thing so one has to assume the 3 meter rule applies to a ring final circuit!

This is a DIY forum and although interesting to talk about the finer points of the regulations I think it is unlikely any DIY guy will be able to follow what we are talking about. The ring final circuit is in its self an odd phrase as it is often not the final circuit where FCU are used. I think This is the link to the PDF showing the ring final circuit but not sure if non members can access it? So included picture for those who can't get it from website.

Ring Main

• ericmark
• 1 Jun 2009
• 6

IET Wiring regulation update

 

[JohnW2]

Appendix 15 (Informative) is seen by many to be part of the regulations and the circle with all the options does not include using 4mm² it is only with the radial final circuit arrangements that a 4mm² unfused spur is shown. ... I have argued that the appendix is not really part of the regulations however since one is tested on items found in the appendix when taking the C&G2382 many consider it to be part of the regulations.

I suppose it depends upon what you mean by 'part of the regulations'. Just in case we didn't understand, the note at the start of the appendices in BS7671 says "Appendix 1 is normative, and is therefore a requirement. All other appendices are informative, and are provided as guidance" - confirming that 'compliance' with most of the Appendices (including Appendix 15) is not a requirement for compliance with BS7671.

However, the fact that the Appendices are not compulsory requirements does not mean that they cannot be part of an examination syllabus. More important, whether informative or normative, the absence of something from Appendix 15 (or, indeed, from anywhere else in the regs) does not not necessarily mean that it is 'not allowed'.

This is a DIY forum and although interesting to talk about the finer points of the regulations I think it is unlikely any DIY guy will be able to follow what we are talking about.

That is very probably true of many DIYers (but clearly not all of them, since I am no electrician ). I only commented on the matter because you raised the issue of a 4mm² unfused spur from a ring final circuit (even if only to suggest that it was not allowed).

Kind Regards, John

 

[ldoodle]

Just another quick q...

Rather than having an FCU > socket > FCU > light setup, which would mean the shed light is on the socket circuit, can you get SWA with enough cores such that at the CU, 2 cores (L+N) go to the lighting circuit MCB and another 2 cores (L+N) go to the socket circuit MCB, maintaining the 2 separate circuits in one cable?

When we had outside electrics put in our old house, it was 2-core with mechanical earth.

Like ISDN installations . Is that do-able from the stance that you'd have 3 cores into one MCB rather than 2 cores in a traditional ring circuit.

At some stage I will get a new CU installed with more ways than I have now, so the shed (both sockets and lights) will be on their own circuits anyway.

Need shed power sooner rather than later though as dryer is going in there.

PS: When using the TLC VD calculator, do you use supply or load for power? I.e MCB is 32a so use 7.68kW, or load will be 13a (single double socket) so use 3.12kW?

4 core here http://www.tlc-direct.co.uk/Main_Index/Cable_Index/Armoured_SWA/index.html

?

 

[JohnW2]

Just another quick q... Rather than having an FCU > socket > FCU > light setup, which would mean the shed light is on the socket circuit, can you get SWA with enough cores such that at the CU, 2 cores (L+N) go to the lighting circuit MCB and another 2 cores (L+N) go to the socket circuit MCB, maintaining the 2 separate circuits in one cable? ... Is that do-able from the stance that you'd have 3 cores into one MCB rather than 2 cores in a traditional ring circuit.

At first sight, I can't think of any particularly good reason why you couldn't do that. Three conductors going into one MCB is far from unknown. Depending on the CU, terminating SWA directly into it may not be straightforward (or necessarily even possible).

However, would it be worth the effort? - you would have reduced the risk that a fault on the sockets circuit in the shed would take out the shed lighting, but would have replaced that with a risk that a fault on the lighting circuit in the house might take out the shed lighting.

Kind Regards, John