Quick queston on spurs

[BeeLZeebub]

I've been a viewer of this forum long enough that the mantra "you cant spur off a spur" is deeply etched in my brain. And I wouldnt ... oooh no, not me sir. However, there's a dull spot upstairs today which is preventing me recalling why you should not do the following:

From a spurred socket, wire-up a 3amp FCU in order to feed, say, a single (switched) light. That's spuring off a spur, but then, isnt that just like plugging a table lamp into a spurred socket? If you swap the spurred socket for a 3amp FCU, this would then limit the appliances you could use from sockets on the spur right? Is it simply to safeguard against future overload (however unlikely this could come about .... what, daisy-chaining a few dozen 100w lights?????)

Aaagh, please, someone, put me out of my misery ..... and if someone says that's just what the regs say, well .... I will find you .....

 

[riveralt]

From a spurred socket, wire-up a 3amp FCU in order to feed, say, a single (switched) light. That's spuring off a spur, but then, isnt that just like plugging a table lamp into a spurred socket?

No because the fuse in the plug limits the load on the cable supplying the spur (normally 2.5mm²) (max load of double gang socket circa 20A). Adding an FCU to the spur allows for potentially a greater load on the cable supplying the spur. Though probably still not enough to breach the current carrying capacity of the cable.

If you swap the spurred socket for a 3amp FCU, this would then limit the appliances you could use from sockets on the spur right?

Yes but if you were going to replace the spurred socket with an FCU then you would use a 13A fuse and the appropriately sized cable.

Is it simply to safeguard against future overload (however unlikely this could come about .... what, daisy-chaining a few dozen 100w lights?????)

The whole premise of BS7671 is safety and, in my view, standardisation. However even on a 3A fuse you would be limited to around 690w on a 13A fuse your limit would be closer to 3kw.

By the way the information contained in Appendix 15 of the regulations is informative only and not a requirement.

 

[ericmark]

I've been a viewer of this forum long enough that the mantra "you cant spur off a spur" is deeply etched in my brain.

Not strictly true what it is all about is when protecting a rine with a 30/32 amp protective device and then spur using cable unable to take that current you rely on the fuse in the FCU or plug. But spur from a 20A or 13A supply is OK.

And I wouldnt ... oooh no, not me sir. However, there's a dull spot upstairs today which is preventing me recalling why you should not do the following:

From a spurred socket, wire-up a 3amp FCU in order to feed, say, a single (switched) light. That's spuring off a spur, but then, isnt that just like plugging a table lamp into a spurred socket?

This is the point you have to ensure only one or two fuses are fed from the spur from a 32A supply and where many items come from this supply then it could exceed the rating of the cable. The ring is a type tested system and to use it you have to conform with the type testing limitations. If you don't follow the type testing then you have to test and design the whole system which is not easy. Since it was found there were radials being added with under size cable to ring finals to comply with type testing now you can only add one device.

If you swap the spurred socket for a 3amp FCU, this would then limit the appliances you could use from sockets on the spur right? Is it simply to safeguard against future overload (however unlikely this could come about .... what, daisy-chaining a few dozen 100w lights?????)

Once you fuse down to 13A you can add as much as you want as long as volt drop limits are not exceeded.

Aaagh, please, someone, put me out of my misery ..... and if someone says that's just what the regs say, well .... I will find you .....

So yes it is what regs say. They say if you want to use a type tested system to reduce the design work required you must follow the rules laid down.

However if for example to were to remove a single socket and back box on a ring and replace with a twin back box with a FCU in one side connected to ring and a socket other side connected to FCU you can then spur from that socket was as many sockets as you like.

 

[JohnW2]

However if for example to were to remove a single socket and back box on a ring and replace with a twin back box with a FCU in one side connected to ring and a socket other side connected to FCU you can then spur from that socket was as many sockets as you like.

Possible, yes, and compliant, but surely no-one in their right mind would actually do it like that - since it would involve 'unnecessarily' moving that socket onto the fused spur, thereby potentially reducing the amount of the 13A total available for other sockets on the fused spur. It would surely be better to connect them the other way around, leaving the existing socket on the ring and spurring off that to the adjacent FCU, which could then supply as many additional sockets as one wanted (with 13A max total load)? ... or am I missing something?

Kind Regards, John

 

[ericmark]

However if for example to were to remove a single socket and back box on a ring and replace with a twin back box with a FCU in one side connected to ring and a socket other side connected to FCU you can then spur from that socket was as many sockets as you like.

Possible, yes, and compliant, but surely no-one in their right mind would actually do it like that - since it would involve 'unnecessarily' moving that socket onto the fused spur, thereby potentially reducing the amount of the 13A total available for other sockets on the fused spur. It would surely be better to connect them the other way around, leaving the existing socket on the ring and spurring off that to the adjacent FCU, which could then supply as many additional sockets as one wanted (with 13A max total load)? ... or am I missing something?

Kind Regards, John

You are correct I was just pointing out there is a way within the final ring type tested system.

 

[JohnW2]

You are correct I was just pointing out there is a way within the final ring type tested system.

Indeed, but of the two ways of doing it within that system, one appears much more sensible than the other.

... or are you maybe just 'quibbling' about the fact that Appendix 15 of the regs shows an FCU connected to a ring from a JB on the ring, but not from a socket on the ring? If so, I find it very hard to believe that they intended to indicate that the former is acceptable, but that the latter is not - after all, terminals on the ring are terminals on the ring, whether in a JB or socket!

Kind Regards, John

 

[BeeLZeebub]

Thanks folks - sorry been waylaid ....

Just time for a quick question or two to you Riveralt ...

No because the fuse in the plug limits the load on the cable supplying the spur (normally 2.5mm²) (max load of double gang socket circa 20A). Adding an FCU to the spur allows for potentially a greater load on the cable supplying the spur. Though probably still not enough to breach the current carrying capacity of the cable.

OK - but does that not mean that my assumption is right? i.e. in the scenario I gave you - just one single light as the load on the circuit - the point of not allowing a spurred-spur would be to stop someone coming along and adding more 'load' ..?

Yes but if you were going to replace the spurred socket with an FCU then you would use a 13A fuse and the appropriately sized cable.

But again in my simple scenario, I wouldnt then want to connect a light to that 13am FCU without fusing down wouldnt I?

I'm asking all of this because I'd like to put a light in my understairs cupboard and it would make life easier taking power from a socket in the viscinity. That socket happens to be a spur from a ring. If I followed the regs as I understand them, I would replace the spurred socket with a 3amp FCU, which could then feed say another socket and a light .. but as I mentioned this would limit the use of appliances on that socket. I'm missing something ..... and for that I apologise

Thanks for the responses ...

 

[riveralt]

I'm asking all of this because I'd like to put a light in my understairs cupboard and it would make life easier taking power from a socket in the viscinity. That socket happens to be a spur from a ring. If I followed the regs as I understand them, I would replace the spurred socket with a 3amp FCU,

Not sure why you would do that, the purpose of the fuse is to protect the cable - so if I was replacing the spurred socket with an FCU I would use a 13A fuse which will give you greater flexibility later.

which could then feed say another socket and a light .. but as I mentioned this would limit the use of appliances on that socket. I'm missing something ..... and for that I apologise

Once you have replaced the spurred socket with an FCU everything after the FCU forms part of a radial circuit - so subject to cable sizes and the initial FCU fuse size you can run what you want.
So for example - replace your spurred socket with a 13A FCU - then run say 2.5mm ² T&E (you could use 1.5mm ²) from the load side of the FCU to another socket then run 1mm ² T&E from that socket to a switched FCU (3/5A) to a ceiling rose light.
There are other ways of doing this.

 

[BeeLZeebub]

Ok thanks Riveralt.

So for example - replace your spurred socket with a 13A FCU - then run say 2.5mm ² T&E (you could use 1.5mm ²) from the load side of the FCU to another socket then run 1mm ² T&E from that socket to a switched FCU (3/5A) to a ceiling rose light.

Ahem, for some inexplicable reason I had got it into my head that placing an additional FCU to feed the light after the spurred 13amp FCU was a no-no. Now that I think about, god knows why .... embarrassing, but there you go ....

Thanks for your time

 

[plugwash]

Strictly speaking I don't think you actually need the second FCU unless the light vendor explicitly specifies that their light requires such protectoin.