Clarification on additional sockets on a spur

[JohnS888]

Evening all

Got a query excuse me if not seeing the simple logic. On reading regs book it states can install one unfused spur off ring in 2.5 but if more sockets required need to first wire through a fused spur. My question is, is this all because of possibility of overloading one leg of the ring or is it to protect the first cable coming off the ring? If this is the case can any justification be made if ran a 4mm off the ring to first socket and then a 4mm to second. Thanks

 

[telectrix]

the reg.for fusing down is mainly to protect the spur cable (generally 2.5mm) from overload as the ring is protected @ 32A. if you were to install an unfused 4mm spur with several sockets, you run the risk of putting too muck load on 1 point of the ring, esp if it were close to one end of the ring.

 

[JohnS888]

the reg.for fusing down is mainly to protect the spur cable (generally 2.5mm) from overload as the ring is protected @ 32A. if you were to install an unfused 4mm spur with several sockets, you run the risk of putting too muck load on 1 point of the resp if it were close to one end of the ring.

Thanks Tel. So if a 4mm is used as initial cable of the ring this is then protected against overheating. If as you said though that socket coming off is at one end of ring is there now a over heating danger to one of these or is it just that because current isn't spread around ring adequately that it is just likely to trip out?

 

[telectrix]

if the spur is taken close to one end of the ring, then the short leg of the ring will carry most of the spur current.

 

[JohnS888]

if the spur is taken close to one end of the ring, then the short leg of the ring will carry most of the spur current.

Ok thanks so just no real possibility of getting round it then

 

[TJ Anderson]

Ok thanks so just no real possibility of getting round it then

What do you want to get around? If you ran 4mm, depending on correction factors you could most likely run it as a radial anyway with the 32A MCB.

 

[Dustydazzler]

If you require more than one additional socket then just split & continue the ring circuit

 

[radiohead]

In practice it does not matter at what point on a ring a spur comes from.
In the case of an unfused spur feeding a twin socket then potentially a load of 26a could be drawn at that point on the ring. With a fused spur the number of sockets on the load side is unlimited because overload protection @13a protects the spur cable both upstream and downstream from the FCU.
So in theory a fused spur is far less likely to result in excess load on one point of the circuit. But unfused spurs are far more common than fused spurs. In practice diversity and spread of load means 2.5mm ring circuits are rarely stressed unless poorly designed for known high load situations like commercial kitchens.

 

[JohnS888]

What do you want to get around? If you ran 4mm, depending on correction factors you could most likely run it as a radial anyway with the 32A MCB.

In practice it does not matter at what point on a ring a spur comes from.
In the case of an unfused spur feeding a twin socket then potentially a load of 26a could be drawn at that point on the ring. With a fused spur the number of sockets on the load side is unlimited because overload protection @13a protects the spur cable both upstream and downstream from the FCU.
So in theory a fused spur is far less likely to result in excess load on one point of the circuit. But unfused spurs are far more common than fused spurs. In practice diversity and spread of load means 2.5mm ring circuits are rarely stressed unless poorly designed for known high load situations like commercial kitchens.

Thanks radiohead is there any scenarios then you would be comfortable doing what I proposed....2 double sockets off 4mm from ring.
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If you require more than one additional socket then just split & continue the ring circuit

It's option dazzler but this job I'm already having to cut hole in plasterboard and another hole in skirting just to get a cable through trying to keep holes and trunking down to minimum
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What do you want to get around? If you ran 4mm, depending on correction factors you could most likely run it as a radial anyway with the 32A MCB.

No chance of running new circuit TJ have to come off existing ring

 

[7029 dave]

IMO, you dont have any choice other than extending the RFC, or spur off with 13amp switched spur then you can run as many as you like. (radial off the spur)

Personally I Dont like the latter