Max number of fused spurs off a final ring

[Licklieder]

I'm sure I've seen somewhere in the 17th Edn. something that defines the maximum number of fused spurs allowable on a final ring circuit in terms of the number of sockets that are currently fitted.

Can anyone point me in the right direction ?

 

[deadshort]

Hi, my recollection is one for each socket on the ring if you wish.


Regards,

DS

 

[stem]

I always thought that a bit odd. If you want to add a spur because it's easier for routing to run a single cable, but you already have the same number of spurs as sockets on the ring, simply add an extra socket to the ring in some easily accessible location perhaps next to an existing socket, and then you can connect your extra spur wherever you like.

 

[Licklieder]

I always thought that a bit odd. If you want to add some spurs and you already have the same number of spurs as sockets on the ring, simply add more sockets to the ring and then you can add some more spurs.

Yes, that kinda defeats the point of having the regulation.....

 

[deadshort]

Hi, not really -that is a regulation !


Regards,

DS

 

[ekmdgrf]

I've heard the reference to one per socket and one from the MCB. So the general formula would be s = n + 1, where s is the number of spurs and n is the number of sockets.

That said, Appendix 15 give examples a ring with spurs taken from a junction box. I haven't found an authoritative reference to the actual number of spurs allowed (I didn't look too hard). It does mention consideration should be taken to not overloading the ring and mentions considering the floor area, with a historical figure of 100m2. Seems a bit of an odd metric really.

 

[EFLImpudence]

Isn't this is all 'guidance' or one of those things people just say because once they heard someone say it?


What about installations where ALL the sockets are spurs from junction boxes on the ring in the ceiling?

What has the floor area to do with anything?

 

[ekmdgrf]

Isn't this is all 'guidance' or one of those things people just say because once they heard someone say it?

Probably. As I said, I've failed to find an authority.

What about installations where ALL the sockets are spurs from junction boxes on the ring in the ceiling?

The owners had better hope that we don't find an authority

What has the floor area to do with anything?

It's mentioned as a guide by the BGB as being a metric for gauging the maximum size of the circuit. Curiously it's 100m2 for a 32A ring and 75m2 for a 32A radial. Work that one out. As I say, it's a curious metric.

 

[JohnW2]

Hi, my recollection is one for each socket on the ring if you wish.

Unless I'm mistaken, you won't find a regulation (in BS7671) which says that, or imposes any other sort of limit on the number of spurs (fused or unfused). As has been discussed, there is some guidance in Appendix 15 - but that is only guidance, and says nothing elplicit about the number of spurs.

The OSG says that the number of fused spurs is unlimited, and that the the number of unfused spurs should be limited to the number of sockets etc. on the ring - but, again, that's only guidance.

I personally can't see why there should be any limit of the number of spurs, fused or unfused. Almost every ring final will have more than enough socket outlets for the circuit to be seriously overloaded if large loads were plugged into several of them, and I don't really see why it matters whether the loads are connected to sockets 'on the ring' or which are spurs.

Kind Regards, John

 

[deadshort]

John,

If you take one from each socket on the ring- that would be the physical limit as you have no other sockets to spur an un fused spur from? That's a limit.

Regards,

DS

 

[JohnW2]

If you take one from each socket on the ring- that would be the physical limit as you have no other sockets to spur an un fused spur from? That's a limit.

As we often discuss, I don't think you'll find any regulation which actually says that you can't have more than one spur originating at one socket. In addition, as EFLI eluded to (and as recognised in App 15 of regs), spurs can originate from places other than sockets - e.g. JBs and FCUs 'on the ring' (the number of which is 'unlimited'), even the MCB.

The number of sockets therefore does not necessarily impose an absolute physical limit on the number of spurs.

In passing, if the OSG agreed with your view, it could simply say that the number of spurs (of any type) should be limited to the number of sockets etc. - rather than saying that only in relation to unfused spurs, but adding that the number of fused spurs is unlimited, couldn't it?

Kind Regards, John

 

[deadshort]

I made it clear I was referring to un-fused spurs. We know that the number of fused spurs is unlimited when protected by a FCU at its origin.

The use bold text to get your point across tells it's own story.


Kind Regards,

DS

 

[JohnW2]

I made it clear I was referring to un-fused spurs. We know that the number of fused spurs is unlimited when protected by a FCU at its origin.

Yes, you did refer to unfused spurs in that statement about the 'physical limit'. I didn't notice that (for which I apologise), probably because I did not expect that in a thread in which the OP asked about fused spurs.

However, I still don't really understand the point you were (and are) making. You are now acknowledging that (per OSG guidance) the number of fused spurs is 'unlimited' (which is the simple answer to the question in the OP), but if, as you said, you believe that only one spur can originate from one socket, the 'physical limit' to which you referred would also apply to fused spurs - unless, that is, you believe that one can only have one unfused spur originating from each socket, but as many unfused spurs originating from one socket as one can physically connect!

Kind Regards, John

 

[ban-all-sheds]

Where does it say that spurs (fused or unfused) can only come from sockets in the first place?

The former could originate from an FCU attached directly to the socket circuit cable, and the latter could come from a junction box. As could the supply cable for an FCU.

Even if you believe that you may have no more spurs than sockets that doesn't mean that any of them have to come from sockets.

 

[EFLImpudence]

What has the floor area to do with anything?

It's mentioned as a guide by the BGB as being a metric for gauging the maximum size of the circuit. Curiously it's 100m2 for a 32A ring and 75m2 for a 32A radial. Work that one out. As I say, it's a curious metric.

Yes, I wasn't questioning you but the point of the stupid 'rule'.

I have asked before and obviously no one knows why it is stated; it's pointless.