spur of a spur

[calum63]

Hi, I am struggling to understand why you cannot spur of a spur on a ring final? I know you are technically not allowed to do according to wiring regulations, but in reality how likely is there to be a consequence? I understand 2.5mm is rated at 26amps max if clipped direct,(it can probably take more in reality) but anyway, I propose to put a single socket up in the attic the most convenient way to do this would be to take the feed into an existing double socket which is spurred from an original socket as the wiring can be conceiled in the cupboard in a neat fashion.

I will do it by the book if need be but it will be an annoyance trying to get a cable from another socket up into the attic. I'm guessing the argument why it can't be done is overload? what if someone plugged in 2 fan heaters (who uses them these days anyway?) and then someone plugged in whatever else bla bla bla. In reality how likely is this scenario? The reason why I want a socket in the attic is for convenience if I need to do some work up there, it already has lighting but it would be convenient if there was at least a socket, for the most part it will be doing nothing, there may be a use for it in future, aerial equipment perhaps.

Another point I would like to make, I have worked along side dno's and I remember seeing a diagram of a street setup, there was a 4 core 185mm waveform cable leaving a substation which supplies approx 180 homes, so 60 houses on each phase( ,and a very long run at that! (bearing in mind this was just one of the distribution mains leaving the sub), IIRC there were 3 distribution mains in total leaving the sub, the sub itself serves around 350 houses (a very large housing estate) probably a very common setup considering the diversity is 1kw t 2kw per house, very low!, but even that is quite high for 185mm waveform, 60x8= 480 amps!, I thought 185mm was rated a 300 amps or something? How do dnos get away with these set ups and we don't?

Still the fact remains that distribution cable and substation transformer is in use for around 20 years now and neither has melted or had any problems?, almost certainly there has been some form of overload at times, either that or people arent using as much power as the silly regs claim. Doesn't that tell you something? so what difference is a piece of 2.5mm twin and earth running from an existing spur going to make?, how likely is anything going to happen to it?
Don't give me this unrealistic overload crap, in reality it won't happen certainly in my case and in most cases, most modern equipment is very low power anyway.
what is the problem? Or is this just another silly flawed reg.

sorry for a slight rant,

comments below, cheers

 

[ericmark]

I suppose technically when you add a fuse to a spur it is no longer a spur but a radial, however we refer to a fused spur and that is the way forward, I had a similar problem, and what I did was swap the original socket for a double and fitted a LAP grid socket, switch and fuse in the 4 way plate so I could fuse all the sockets to 13 amp and add another 4 sockets.

 

[John D v2.0]

Nice rant, anyway what you need is as above a fused spur serving both sockets.
Alternatively, you could plug in a fused plug into the socket and run that to the new socket. So a built in extension lead. Not as neat but can be easier.

 

[winston1]

what if someone plugged in 2 fan heaters (who uses them these days anyway?)

Lots of people or they wouldn't still sell so many of them.

I think you know the answer to your rant anyway. Your double socket could be loaded up to 20 amps (or even 26 amps), you go to the loft in winter and plug in a fan heater, another 10 amps say.

 

[JohnW2]

What eric and John have said - and, in any event, I think you already knew what the regulations require in this situation, and intend to 'do it by the book'.

As for all your observations, yes, the regulations are very conservative, and, despite the theoretical considerations, it is extremely unlikely that any harm would result from doing as you would like. However, the regs are the regs, and if one wishes to comply with them, one has no choice. They obviously have to draw a line somewhere - if multiple sockets on a 2.5mm² unfused spur from a 32A ring final were allowed, then it would be logical that they should also allow a 2.5mm² (clipped direct) radial (with multiple sockets) protected by a 32A OPD (which they don't). Again, it is very unlikely that such would ever result in significant problems, but the regs are 'being conservative'.

Kind Regards, John

 

[calum63]

thanks guys, I probably got a bit carried away, I will do it by the book, I'll use erics idea. Out of curiosity how has the substation and the distribution main I mentioned never ever caught fire or failed for 20 odd years without problems? surely 60 houses loaded on single 4 core 185mm waveform distribution main run a few hundred meters is pushing it a little? I thought 4 core waveform was rated for 300amps?, This has nothing to do with spurs I know but if there is anyone who has lots of experience in this field working with dno's, how do they make these decisions and know it works?

can 185mm take more than 300 amps? surely this must had been severely overloaded at times but somehow its still remained intact, like I say 60x8=480amps! and that's only the assumed 2kw per house.
cheers

 

[JohnW2]

Out of curiosity how has the substation and the distribution main I mentioned never ever caught fire or failed for 20 odd years without problems? surely 60 houses loaded on single 4 core 185mm waveform distribution main run a few hundred meters is pushing it a little?

I imagine a combination of diversity and the wide safety margins inherent in the 'current carrying capacities' we are used to working with. ... I would think, for example, that 2.5mm² T+E could probably carry 50A or more for significant periods without coming to any harm, or causing any harm. The melting point of PVC is around double the 'normal max operating temperature', and the ignition temperature much higher. With your underground (or even overhead) cables, there is usually no risk of a hot cable setting fire to anything.

Kind Regards, John

 

[Iggifer]

It’s actually got a sustained rating of between 360-390A and a cyclic rating of 398-441A (lower number is ali neutral, higher is extra copper neutral core).

Obviously only new installations will use wavecon, older PILC 185mm is good up to ~500A under the same situation. If it’s 20 odd years old it will almost certainly be PILC.

60x3 = 180
400x3 = 1200


1200/180 = 6.66A per house sustained.

That’s not even best case. Assuming PILC you’re looking more like 11A per house sustained. Which of course is unlikely.

I think my sustained load is somewhere around 2-3A with peaks of somewhere around 10-11A (Gas CH&HW, boiler from the Combi, gas hob and a small single oven)

If you then consider the fact that other than mealtimes, the peaks in load are often not the same for each house - I think the chances of even two people on the same phase firing their washing machine up at the same time is slim to none.

 

[calum63]

thanks iggifer that explains alot, so its basically all about diversity, getting over the fact that nothing will be switched on all at once. worst case if the sub and distribution cables were overloaded nothing bad happens straight away, I guess the weather helps the situation providing the majority of the uk is bloody freezing I guess that acts as a big cooling system for both the tx and cables.
cheers

 

[JohnW2]

...worst case if the sub and distribution cables were overloaded nothing bad happens straight away, I guess the weather helps the situation providing the majority of the uk is bloody freezing I guess that acts as a big cooling system for both the tx and cables.

The transformer, yes, but I doubt that the temperature in the vicinity of underground cables varies very much in the UK - unless, exceptionally, we have a very long period of very low temperatures.

As both Iggifer and I have said, it's really primarily down to diversity - plus the point I made that even substantial 'overloading' of cables (relative to their tabulated CCCs) does not necessarily do any harm.

Kind Regards, John

 

[Iggifer]

Western power work on the following ground temperatures. I personally think the winter one is a bit on the high side but these are all based on a lay depth of 500mm so it could well be

Autumn - 12°C
Winter - 10°C
Spring - 12°C
Summer - 15°C

 

[JohnW2]

Western power work on the following ground temperatures. I personally think the winter one is a bit on the high side but these are all based on a lay depth of 500mm so it could well be

Thanks - that's much as I would have guessed. A bit deeper, and there would presumably be even less seasonal variation.

In terms of the big picture (particularly the 'permitted' operating temperature of conductors), even at 500mm the difference between 10°C and 15°C is pretty trivial - just a handful of percent variation in the conductor-soil temp difference.

Kind Regards, John

 

[Iggifer]

Thanks - that's much as I would have guessed. A bit deeper, and there would presumably be even less seasonal variation.

In terms of the big picture (particularly the 'permitted' operating temperature of conductors), even at 500mm the difference between 10°C and 15°C is pretty trivial - just a handful of percent variation in the conductor-soil temp difference.

Kind Regards, John

Oh yea totally. For some reason WPD list the 500mm lay depth as the 'max' depth. Seems a bit odd but that's what their document says

 

[JohnW2]

Happy New Year!

Kind Regards, John

 

[calum63]

Hi, sorry to bring this back up, this time it is totally related to the addition spur, the substation and everything was just me trying to make a point and was rather annoyed at time., I realize the regs are the regs and if one wishes to comply then they must be followed whether we like them or not.
I have been rather busy so I have not actually got round to fitting the spur, As i mentioned before supplying the attic socket from the spur in question is by far the most convenient as there its opposite the airing cupboard and fishing a cable up the wall will be a doddle.
My current upstair circuit is on a ring, but I certainly do not need 32A upstairs far too overkill, Is it acceptable to downsize upstairs ring to 20a? (change the 32 amp breaker with a 20) at least this way all cables (2.5mm) for the circuit will be properly protected.

From what I have read If I had a 20 amp radial circuit then spuring from an existing spur (the way I want) won't be an issue.
However as its a ring its not acceptable to spur from a spur as there is a chance (unlikely) the cable can be overloaded.

So my question is, is there anything against the regs which prohibits me from downgrading upstairs ring to 20A?
Like I say I certainly do not need 32 amps upstairs. I won't drawing anywhere near that amount of current in my house, my overall current draw for the whole house is mainly 2 amps, 16 amps at the most when I am cooking or kettle on.
thanks