Spur off of ringmain - whys it allowed!?

[flyingsparks]

I have been wondering, why is not against the regulations to spur with 2.5mm cable (capable of taking 27A) off a ring main, which is protected at 32A. That length of 2.5 for the spur is not protected. How come you are allowed to do this??

 

[ban-all-sheds]

Because you can't overload the cable with one socket, and the protective device is OK to cope with faults on it.

 

[andy]

the max amount that can be taken from a spur is 13A

so because of this you cant spur from a spur

 

[Damocles]

The cable may have to be de-rated from 27A because of being buried in something. Otherwise we could have 45A rings using 2.5mm. (do not try that at home)

 

[ban-all-sheds]

No we couldn't - the rules for rings are that the cable has to be 2/3 of the rating of the protective device, not 1/2...

 

[has]

the max amount that can be taken from a spur is 13A

so because of this you cant spur from a spur

What if the spur is a twin socket outlet? Would this not allow more than 13A

 

[ban-all-sheds]

No - twin socket outlets are only rated at 13A

 

[mapj1]

This is one of those things that if it were dreamt up now would never be allowed, but in practice has worked so well for so long that it can't really be changed, without condemning satisfactory installations. (actually thats probably true of a ring main generally)
Ironically it means you can put a double socket on a 32A breaker on 2.5mm, but only if it shares that breaker with ring main.

In reality, there is always other stuff on the ring main, so you cant normally get the full breaker's worth down the spur, and as B-A-S says, officially double sockets only count as 13A, although in reality you could take more, at least until something gave way (but then you could use a pair of unfused 2 way adapters too).
Remember that the cable has to be overloaded for quite some time to overheat to a dangerous level, and most peak loads are not sustained for more than 10s of minutes.

 

[plugwash]

yes the fact that we ignore timespan of loads in our calculations means we end up with huge safety margins on the capacity

have you ever heared of a ring broken by a kitchen fitter causing any problems in practice?

 

[Damocles]

If we wanted to change the rules then we could have 45A rings run in 2.5mm. Technically this would be running things too close for comfort but in practice it would most probably hardly ever give rise to a problem. This is because ring safety relies upon quite a lot of factors all working together even though the design could theoretically be overloaded several different ways.

 

[mapj1]

Well, in extremis you could argue you don't need the final circuit MCB or fuse at all, except when the circuit is going to be overloaded, as short circuit protection could be provided further back (one big company fuse perhaps?).
Ask yourself honestly, when did you last change a final circuit fuse or reset the circuit breaker, except for a catastrophic fault?
It can happen, but it is mercifully rare. As evidenced by the safe operation of wiring in countries where the cable only needs to be rated for the fuse at the load end - a heinous crime here, but not often a problem in practice (mostly Asia and South America in case you are wondering).
However, like running without the earth, it is not dangerous in itself, but you have to understand when you are removing a safety net which you might need in case of fault.

There is a tendency here in the well fed first world to worry too much about non-real risks, and spurs are just such a non-risk.

 

[ban-all-sheds]

IIRC, until, or up to, the 15th, an unfused spur was allowed to have 2 single socket outlets...

 

[AdamW]

Some of you may remember FWL telling us about a house he visited last year which had all the rings wired in 4mm. Has anyone ever wired, or known of someone who wired, rings that were fused at more than 30/32A? Perhaps someone used 4mm and protected at 40/45A (or maybe even 50A at a stretch)?

I know, seldom required seeing as you could just wire 2 standard-spec rings instead of one monster ring.

 

[SUNRAY]

Sorry for the old bump but looking for something raise this thread.

Some of you may remember FWL telling us about a house he visited last year which had all the rings wired in 4mm. Has anyone ever wired, or known of someone who wired, rings that were fused at more than 30/32A? Perhaps someone used 4mm and protected at 40/45A (or maybe even 50A at a stretch)?

I know, seldom required seeing as you could just wire 2 standard-spec rings instead of one monster ring.

The insurance companies spec for a rewire in a thatched house was wire 2 sizes bigger ie 6mm² for a 32A ring and 2.5mm² for 6a lights. All had to be in original Copex (ali, paper & pvc stuff).
Custard of a job!!!

Anyone else had the same spec?

 

[JohnW2]

Sorry for the old bump but ....

11.5 years might actually be a record

Kind Regards, John