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

 

[radiohead]

Thanks radiohead is there any scenarios then you would be comfortable doing what I proposed....2 double sockets off 4mm from ring.

Absolutely not. Bs 7671 permits a maximum of 1 double outlet to be on an unfused spur from a ring.
The only theoretical reason I can see for wanting to put two double sockets on a 4.0mm spur from a 2.5mm ring would be because it was anticipated that these two sockets would be heavily loaded. That would create an unacceptable load potential on one point of the ring.
Where a number of lightly loaded outlets are to be installed then a 13a FCU spur for ease of installation would be sensible, but where more than one outlet is required and load is unknown always incorporate on the ring.

 

[Pretty Mouth]

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.

Potentially it could be 52A at that point on the ring, assuming the spur feeding the twin socket has been taken from the terminals of another twin socket.

 

[JohnS888]

Potentially it could be 52A at that point on the ring, assuming the spur feeding the twin socket has been taken from the terminals of another twin socket.

Can you clarify that for me. Seem to suggest even with one double socket being supplied from another double on a ring could be 52 amps on one leg of the ring?

 

[TJ Anderson]

Potentially it could be 52A at that point on the ring, assuming the spur feeding the twin socket has been taken from the terminals of another twin socket.

Anything over 32A for whatever reason will then be covered by the fuse or circuit breaker in place. The potential issue is long duration overload overheating and damaging cable insulation but It's not really such an issue anymore in my experience and certainly nothing like the level it is talked about. It was more of a problem with the 3036 fuses which would stay intact for long periods significantly over their rated current. The worst case of it where it was really an issue and had caused damage was in an office with 30A 3036 fuse in place. Panel heaters as well as the computers etc. It was pulling 47A when I clamped it at DB. Running for hours on end like this. 60898/61009 has far better characteristics.
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Can you clarify that for me. Seem to suggest even with one double socket being supplied from another double on a ring could be 52 amps on one leg of the ring?

You can easily overload any ring final circuit with the sockets available but it is designed to be limited by the overcurrent protective device and with realistic diversity it is not an issue to 99.9% of them that are in operation .

 

[radiohead]

Potentially it could be 52A at that point on the ring, assuming the spur feeding the twin socket has been taken from the terminals of another twin socket.

Yes of course, I stand corrected.

To the OP..... a ring with 10 double sockets could potentially be loaded at 260a. but as above, thats why Bs7671 stipulates a 32a OCPD to protect the conductors from prolonged overload.

 

[Pretty Mouth]

Can you clarify that for me. Seem to suggest even with one double socket being supplied from another double on a ring could be 52 amps on one leg of the ring?

At the terminals of the double socket spurred from, there could theoretically be 52A (26A from the spur, 26A from the spurred-from), but this would be divided up between the 2 legs of the ring, with more of it going down the shorter leg. While this overload is taking place, the circuit breaker is working toward disconnection, could take up to 10 mins for a 32A 60898. So if the spur was taken from a socket very close to the CU on a very long ring, then a cable rated to perhaps just 20A could indeed be getting the majority of that 52A for 10 minutes.

Highly unlikely though, and to design a circuit like this would be breaking 433.1.204