Add spur at Consumer Unit

[Jupiter01]

A spur from a ring at any point can only have one socket

What if you increase the cable size?

 

[Harry Bloomfield]

A spur from a ring at any point can only have one socket attached. If this is a double socket, the maximum load is 2x13amps.

Total 20amps on a twin socket.

 

[EFLImpudence]

Since the spur is a radial using 2.5mm cable, isn't its capacity less than a ring in 2.5mm as the load is distributed across the two legs of the ring?

Obviously the capacity of one 2.5mm² cable is less than two 2.5mm² cables.

Are you averse to the ring extension (when there is a need for multiple new sockets) because of the join it requires?

Not particularly because of the joint required; but because it is unnecessary.

Two cables running to socket(s) out and back by the same route is not a ring; it is two cables in parallel (although there are separate rules for cables in parallel).
They could both be connected to all the sockets supplied by this spur so you have a 5mm² spur - but all that is required is 4mm² cable so use that.

Trying to understand the reasoning.

I think it is called common sense - a rarity these days.

Can I also clarify, are you suggesting that a spur from any point on the ring (and in this case the MCB) could be for more than one socket if you increase the cable size?

Of course. Why do you even have to ask?

And it no longer requires an FCU at the start of the radial?

Think about the rule for an FCU.

According to the regulations; you may have a 2.5mm² spur to one socket (ignore double sockets for the moment); the cable has a capacity of 27A (yes, method C) but the plug fuse limits the current to 13A - all good.
You need another socket so you extend the spur; now you have a 27A cable, two sockets/plugs which limit the current to 26A - all good.
But now for some reason the regulations (sort of) say you must have at the source a 13A FCU. This is because that is all that is available and the lack of a suitable accessory; i.e. a 25A(27A) FCU.
You could install a 25A MCB at the source of this spur but that is not mentioned in the regulations - and would (in the case of a spur at the CU) be even more ridiculous as you may as well have it as a separate radial circuit.

Do you see where this is going?
We have silly rules because few people think about things. We have restrictive measures for the use of inadequately sized cable rather than just saying use 4mm² cable (method C) for spurs from a ring with more than one socket.

So, your asking "Shall I extend the ring?" is really asking "Shall I increase the current carrying capacity of the spur?".
(Edit - no, I meant your.)

The problem is that the regulation for ring final circuits is still written as it was for BS3036 30A fuses and no allowance has been made for the better characteristics of MCBs.

What if you increase the cable size?

As said, that is not mentioned in the regulations for some reason.

Although of course Appendix 15 is merely informative and not exhaustive.
You are free to design a circuit however you want as long as it is safe - but for some reason people and the regulations to an extent seem to think that an idiot in the future makes something unsafe now.

 

[Jupiter01]

Of course. Why do you even have to ask?

Because of this comment:

A spur from a ring at any point can only have one socket attached

 

[Jupiter01]

According to the regulations; you may have a 2.5mm² spur to one socket (ignore double sockets for the moment); the cable has a capacity of 27A (yes, method C) but the plug fuse limits the current to 13A - all good.

good!

You need another socket so you extend the spur; now you have a 27A cable, two sockets/plugs which limit the current to 26A - all good.

I know you suggested we ignore them but is the above true for two single sockets and a double socket? Do they both limit the current to 26A?

But now for some reason the regulations (sort of) say you must have at the source a 13A FCU. This is because that is all that is available and the lack of a suitable accessory; i.e. a 25A(27A) FCU.

But...the cable is rated at 27A and the two sockets will pull 26A and therefore not exceed the capacity of the cable. Isn't it the third socket that warrants the FCU as that potentially takes you above the capacity of the cable (if all sockets are maxed at 13amp each)?

 

[EFLImpudence]

Because of this comment:

Fair enough.

What TTC should have written is "A spur from a ring at any point can only have one socket attached - if it is 2.5mm² according to one way of reading of the regulations"


Of course, Appendix 15B does show an unfused spur(branch) to more than one socket with 4mm² cable from a 32A circuit.

What is the difference?

Also note that it does not call this an unfused spur.

 

[EFLImpudence]

I know you suggested we ignore them but is the above true for two single sockets and a double socket? Do they both limit the current to 26A?

Yes, they do but two single sockets is not allowed (according to some).
Obviously two double sockets would not be good.

But...the cable is rated at 27A and the two sockets will pull 26A and therefore not exceed the capacity of the cable.

True - so it should be allowed - or is - depending on how the regulations are interpreted.

Isn't it the third socket that warrants the FCU as that potentially takes you above the capacity of the cable (if all sockets are maxed at 13amp each)?

Yes, of course, but don't you agree that that is a daft way of making things safe? Use larger cable or larger fuse (in something other than an FCU).

OR

If, like some, one is worried about future idiots then why not specify that 4mm² must be used for all spurs. Simples.

 

[EFLImpudence]

If, like some, one is worried about future idiots then why not specify that 4mm² must be used for all spurs. Simples.

I would point out that such a rule would negate or contravene the regulations relating to the positioning (or indeed omission) of overcurrent protection devices which actually do allow two single sockets on a 2.5mm² spur from a 32A circuit.

So it seems that there are technical regulations and then there are "apparent cover-all" statements which contradict them.

 

[Taylortwocities]

What TTC should have written is "A spur from a ring at any point can only have one socket attached - if it is 2.5mm² according to one way of reading of the regulations"

Yes, if I could be bothered. But, quite honestly, @Jupiter01 has been playing at electrical stuff for many years.

He’s been asking questions on this forum for so long, and yet, every day, we get at least one very elementary question.
So I ran out of the ability to invest further time.

Sorry @Jupiter01. May be time to spend some time and think about what you are doing. You’ve got a lot of experience, already. You have(should have) an array of guides where you can find most of your answers. So, step back snd think, before you grab your DIYNOT life belt.

 

[EFLImpudence]

That is true. I am just strangely in a good mood today.

 

[JohnW2]

Total 20amps on a twin socket.

We've been through this umpteen times, and there is no clear generic answer.

BS1363 requires that double socket passes a temperature rise test at 20A total (14A + 6A), but that in no way precludes a manufacturer producing a socket which would pass that test (and any/all other tests) at a current of 26A (2 x 13A), and therefore coulod have a specified 'rating' of 26A total.

It therefore depends upon the specification of a particular socket. Taking MK as an example, their "Technical Data Sheet" (for all sockets) simply says:

When I asked, even the technical guys at MK were not able to tell me what that is meant to mean (because, I was told, "Technical Data Sheets" were written by the marketing dept.!) but it surely can only be interpreted (for a double socket) as either 13A (total) or 26A (total), not 20A?

Kind Regards, John

 

[Harry Bloomfield]

When I asked, even the technical guys at MK were not able to tell me what that is meant to mean (because, I was told, "Technical Data Sheets" were written by the marketing dept.!) but it surely can only be interpreted (for a double socket) as either 13A (total) or 26A (total), not 20A?

I thought I saw some marked in the moulding 'MAX 20amp), but then - why would anyone need two 3kw loads co-located?

 

[Taylortwocities]

I thought I saw some marked in the moulding 'MAX 20amp), but then - why would anyone need two 3kw loads co-located?

Indeed. But, should someone do this, then the load on the 2.5mm spur will still be inside the rated spec of 27amp*

* installation method permitting.

 

[JohnW2]

I thought I saw some marked in the moulding 'MAX 20amp) ....

I don't personally recall ever having seen that, but maybe you did - as I said, there is nothing stopping a manufacturer producing a double socket which only 'just' satisfies the minimum requirement of BS1263, hence presumably would have to 'rate' it at 20A.

, but then - why would anyone need two 3kw loads co-located?

Very unlikely, I agree, but not impossible.

Kind Regards, John

 

[JohnW2]

Indeed. But, should someone do this, then the load on the 2.5mm spur will still be inside the rated spec of 27amp*

Quite so, but ....

* installation method permitting.

Maybe the person(s) who invented Appendix 15 had it in their mind(s) that 433.1.204's minimum requirement of a CCC or 20A for the cable of a ring final circuit also applied to the cable of an unfused spur, such that it might not be adequate for 26A, but would be adequate for what they regarded (as seemingly other do) as being the (20A) 'rating' of a double socket - hence their guidance that an unfused 2.5mm spur could supply one double socket but not two single ones (which I don't think anyone disputes would each be 'rated' at 13A)?

Kind Regards, John