Unfused spurs

[JohnW2]

We are all familiar with the fact that, in the context of an RFC wired entirely with 2.5mm² cable, Appendix 15 ("Informative") of the regs indicates that an unfused spur should feed only one socket, whereas a spur connected via an FCU (fuse 13A) can feed an unlimited number of sockets.

I presume that this recommendation relates essentially to the current-carrying capacity of the cable - both within the spur circuit and also in the ring itself (since an extensive spur connected close to one end of the ring could result in most of the current going through one 'leg' of the ring - just as if many heavily-loaded sockets were installed close to one end of the ring).

If that is the case, then I think I could produce a fairly compelling argment for it to be acceptable to have an unfused spur with several sockets if it were wired in 4mm² cable, particularly if it were connected roughly at the mid-point of a ring. With such an arrangement, no cable would carry any more current than in the 'recommended' designs for RFCs or radial circuits. The wiring of the spur circuit itself would certainly be no problem since, with 4mm² cable, it would be identical to what is recommended for an unfused spur to a 4mm² radial final. As for the ring itself, the situation would be no worse than having several sockets close to each other on the ring at the point at which the spur was to be attached.

An advantage of such an arrangement would be that it could often result in an appreciable reduction in the total length of the ring (where most of the load would probably be), not to mention various possible practical installation advantages.

Any thoughts, or arguments against what I have suggested?

Kind Regards, John

 

[RossR]

Some arguments against:

1) It's not normal practice so will surprise anyone working on the installation in the future.

2) It isn't the same as having several sockets close together on the ring, because the sockets aren't close together.

3) You'll have to do more design work, as it isn't a standard design.

Personally I don't like rings. I'd just go the whole hog and install a 4mm radial, or a couple of 2.5mm radials.

 

[seasickstevie]

...since an extensive spur connected close to one end of the ring could result in most of the current going through one 'leg' of the ring - just as if many heavily-loaded sockets were installed close to one end of the ring).

You're basing that on the fact that the electric 'won't bother' to go the farthest way around because it's too far to go.

I don't think it works like that.

 

[RossR]

...since an extensive spur connected close to one end of the ring could result in most of the current going through one 'leg' of the ring - just as if many heavily-loaded sockets were installed close to one end of the ring).

You're basing that on the fact that the electric 'won't bother' to go the farthest way around because it's too far to go.

I don't think it works like that.

The shortest leg is likely to have a lower resistance. Both legs will have the same voltage drop (Kirchhoff's Voltage Law). Therefore the shorter leg is likely to have a larger current flowing through it (Ohm's Law).

 

[ban-all-sheds]

You're basing that on the fact that the electric 'won't bother' to go the farthest way around because it's too far to go.

Most of it won't - if the long route is, say, 20x longer than the short then less than 5% of the current will go that way and more than 95% will take the short route.

I don't think it works like that.

Maybe you don't, but you are wrong.

 

[JohnW2]

...since an extensive spur connected close to one end of the ring could result in most of the current going through one 'leg' of the ring - just as if many heavily-loaded sockets were installed close to one end of the ring).

You're basing that on the fact that the electric 'won't bother' to go the farthest way around because it's too far to go.
I don't think it works like that.

My Goodness! How do you think it 'works', then? It's not a question of where electricity 'bothers to go'. It's 'Electricitry 101' or GCSE Science. It's also a fundamental concept which anyone dealing with ring circuits really must understand.

More current will 'bother to go' along the lower impedance of the two paths back to the CU; if the load is very close to one end of the ring, then the majority of the current will take the shorter path.

Kind Regards, John.

 

[JohnW2]

Some arguments against:
1) It's not normal practice so will surprise anyone working on the installation in the future.

Agreed. That's the reason I asked the question - to see how people reacted. Mind you, even if 'surprised', one would not expect a competent person to have any problem with something that had been satisfactorily designed. Indeed, they might even be 'pleasantly surprised', since one doesn't have to look very far to find multiple socket unfused spurs wired in 2.5mm² !!

2) It isn't the same as having several sockets close together on the ring, because the sockets aren't close together.

In the context I was writing (the effect on currents in the main ring), it is essentially the same.

3) You'll have to do more design work, as it isn't a standard design.

Again agreed - but the amount of extra design work is trivial - I essentially did it (without the figures) in one short paragraph in my original post.

Personally I don't like rings. I'd just go the whole hog and install a 4mm radial, or a couple of 2.5mm radials.

I have some sympathy with that view. However, one issue here is that spurs are often added subsequent to the initial installation - and that is not the ideal time to be re-designing (or rewiring) the original circuit if that is not essential.

Kind Regards, John.

 

[echoes]

Would you classify the proposed topology as a 32A ring with a 32A-capable spur, or a 32A radial with a 'ring bulge'? Ouch.

 

[JohnW2]

Would you classify the proposed topology as a 32A ring with a 32A-capable spur, or a 32A radial with a 'ring bulge'? Ouch.

There are obviously several forms of words one could use to describe it, but that's just a matter of words.

What I'm talking about is a circuit in which the current carrying capacity of the path from every socket to the CU would be at least as high as it would be if the entire circuit was designed as a single ring - so I don't really see the need for an 'Ouch',

Kind Regards, John.

 

[echoes]

Don't disagree that such a design would be electrically sound.
The 'ouch' was refering only to the term 'ring bulge'.....

 

[ajrobb]

And the reason you don't want to split the ring and go for two 20 A radials (one with your new branch) is?

You'd have to ensure that you are between 40% and 60% of the way around the ring to ensure that a 32 A point load on the ring won't result in one leg being overloaded (20 A / 32 A = 62.5% and allow some variation in terminal resistances...). You then also have to allow for the ring to be extended, so this 'safer' area might move away from your 32 A spur.

It is very easy to overload one leg of a ring by adding a 20 A unfused spur to the first socket and this is allowed without any checks.

 

[ricicle]

Unfortunately in our trade there are those that cannot see past BS7671 and the fact that a circuit has been designed outside the scope of it's guidance will send some calling for the electrical police.
Granted it might throw some working in the domestic scenario, but I come across many commercial ring finals in 4mm² and would not have a problem with a multi socket radial spur from them. (Depending on all other circuit parameters)

 

[JohnW2]

Don't disagree that such a design would be electrically sound. The 'ouch' was refering only to the term 'ring bulge'.....

Ah, OK, I thought you were 'ouching' the concept of the design!

Kind Regards, John.

 

[JohnW2]

And the reason you don't want to split the ring and go for two 20 A radials (one with your new branch) is?

Whilst I am pretty sympathetic to the concept of radials, I am not very comfortable with 20A ones (since it's only too easy to exceed a 20A load on multiple sockets), and rewiring as a 32A radial would generally not be a very attractive option.

You'd have to ensure that you are between 40% and 60% of the way around the ring to ensure that a 32 A point load on the ring won't result in one leg being overloaded (20 A / 32 A = 62.5% and allow some variation in terminal resistances...). You then also have to allow for the ring to be extended, so this 'safer' area might move away from your 32 A spur.

Agreed. However, to keep this in perspective, one has to bear in mind that, despite good design intentions, one has no actual control of what will be plugged into which socket of the ring itself, so there's always the risk of one leg of a ring becoming overloaded, even if there are no spurs.

It is very easy to overload one leg of a ring by adding a 20 A unfused spur to the first socket and this is allowed without any checks.

Agreed - in fact, 'worse', since there is nothing in the regs preventing a 20A unfused spur actually being connected to the CU itself, and then loading it to a full 20A! Mind you, again one has to keep things in perspective. What you describe is no worse than plugging 20A worth of load into a double socket on the ring which is very close to the CU.

Kind Regards, John.

 

[RossR]

I come across many commercial ring finals in 4mm² and would not have a problem with a multi socket radial spur from them.

I don't think that's quite what the OP was suggesting. It was a 2.5mm² RFC with a 4mm² multi socket unfused spur (presumably on a 30A or 32A protective device).