Spur from a ring main

[electronman]

Hi There,

I wondered whether someone could explain to me about taking spurs off a ring main.

Basically, I have noticed in the 16th Edition wiring regs that a double 13A socket can be fed off an exisiting socket that's on the ring main. Additionally, another single 13A socket can be connected to the double socket.

This means that there could be a current of up to 39 Amps flowing through one piece of 2.5mm T&E, assuming a worst case scenario of the maximum 13Amps being drawn from each of the three sockets. This doesn't sound very sensible to me, so wondered whether I was missing the point somewhere!!

Also, the current carrying capacity of the cable will be different depending on whether it's surface run or run in a cavity, or even chased into the wall, so what implications does this have with what I've stated above?

Thanks in advance for your help.

Electronman

 

[stevesey]

Diversity.


Try doing the same maths for the number of sockets on the ring main itself.

 

[JohnD]

... I have noticed in the 16th Edition wiring regs that a double 13A socket can be fed off an exisiting socket that's on the ring main. Additionally, another single 13A socket can be connected to the double socket...

No it can't.

 

[electronman]

So what you are trying to say is that the guide that I have, which is actually the 'electricians guide to the 16th edition wiring regs' is incorrect??

Now I am really confused, as it is clearly stated in the guide that it is acceptable practice to feed a single socket spur from an exisiting double socket spur, which prompted my question on here. I thought it was a bit strange to allow such a thing!

I kind of understand diversity, but still can't see how a ring main that is fused at the CU at 30A, can allow so many sockets that could potentially have 13Amps being drawn from each, as a worst case scenario. I just can't get my head round it and the maths just doesn't add up.

Could you please explain how the diversity calcs mean that this is practical, as I am really interested to know this.

Thanks,
electronman.

 

[FredFlintstone]

Re the spur, you're only allowed one spur per socket on the ring, UNLESS, you take a FCU spur which can then have any number of socket spurs after it because the fuse in the FCU protects the cable against over loading along that run of spurs.

With regards to the ring itself, I don't know the diversity calcs but don't forget that on a ring main, you've got BOTH ends of the ring connected to the supply so the electricity can come from either side of a socket.

I think the assumption is that on a typical ring there's going to be several low power items and a small number of high power items so the total draw won't be that high. If you had six electric heaters, a washing machine, a dryer, a dishwasher, kettle, toaster and microwave all on the one ring in use at the same time then you may be in trouble but from what I gather, diversity makes an assumption that you're not going to have all heaters on at the same time and you're not going to be boiling the kettle, using the toaster and using the microwave all at exactly the same time, while the washer, dryer and dishwasher are all simultaneously at the high draw stage of their cycles.

Perhaps a poor example but you get the idea.

Regards

Fred.

 

[davy_owen_88]

So what you are trying to say is that the guide that I have, which is actually the 'electricians guide to the 16th edition wiring regs' is incorrect??

Now I am really confused, as it is clearly stated in the guide that it is acceptable practice to feed a single socket spur from an exisiting double socket spur, which prompted my question on here. I thought it was a bit strange to allow such a thing!

I'm quite sure that you have misread the book but if it does indeed say what you insist then yes, it is incorrect. You can take ONE single OR ONE double socket from a ring main without fusing down.

I kind of understand diversity, but still can't see how a ring main that is fused at the CU at 30A, can allow so many sockets that could potentially have 13Amps being drawn from each, as a worst case scenario. I just can't get my head round it and the maths just doesn't add up.

Could you please explain how the diversity calcs mean that this is practical, as I am really interested to know this.

How many socket outlets do you have in your house? How many 13A appliances do you have in your house? How many of those 13A appliances are you likely to run at the same time? There is your answer.

A single length of 2.5mm² twin and earth can carry 27A when it is clipped directly to a wall. Remember that a ring circuit has 2 cables so current can come from either side reducing the load on any single length of cable (to 15A if perfectly balanced). The reason you can have so many socket outlets on a 30A circuit is because the fuse protects the cable from overload.

Davy

 

[robbierazz]

Hi Davy,

Thanks for explaining that, it makes sense now.

However, when you say about the ring being connected in such a way that the current draw could come from either side, I assume this wouldn't apply when using a spur as the spur has one length of cable feeding it that doesn't go back to the existing ring main. I understand the assumptions made when talking about diversity, and also that it's not a problem as the circuit is fused.

It is interesting that each socket is specified for a 13Amp rating, yet in reality in wouldn't be possible to connect many appliances to the ring that draw 13Amps, due to the total load being exceeded.

Thanks for explaining it to me anyway, even if it does seem a bit strange still.

Rob.

 

[Spark123]

So what you are trying to say is that the guide that I have, which is actually the 'electricians guide to the 16th edition wiring regs' is incorrect??

Is that the J Whitfield book? Where abouts are you looking in it? Admittedly my copy is the Sixth edition (2001) so could be slightly different.

 

[securespark]

This from the OSG:

Spurs.

The total number of fused spurs is unlimited but the number of non-fused spurs does not exceed the total number of socket outlets and items of stationary equipment connected directly into the circuit.

A non-fused spur feeds only one single or one twin socket outlet or one permanently connected equipment.

Such a spur is connected to a circuit at the terminals of socket outlets or at junction boxes or at the origin of the circuit in the dist. board.

A fused spur is connected to the circuit through a fused connection unit, the rating of the fuse not exceeding that of the cable forming the spur and, in any event, not exceeding 13A.

 

[dannyboi2003]

Hi Guys,

I remember back in my very early days that you could run an unfused spur from the ring main that could feed either one double socket or two single sockets. When did the regs change so that you could only have one single socket on an unfused spur and not two?

Reading this post got me curious to find out when and why the regs changed, any replies apreciated.

All the best
Dan

 

[securespark]

This from the 14th edition, 1 October 1966:

A.40 For ring final subcircuits complying with regulations A.30-33, non-fused spurs shall be connected to the ring at the terminals of socket outlets or at joint boxes, or at the origin of the ring in the distribution board. Non-fused spurs shall have a current rating not less than that of the conductors forming the ring. Not more than two socket outlets, or one twin socket outlet, or one stationary appliance, shall be fed from each non-fused spur.

This from the 15th edition, 30 March 1981:

Appendix 5. A non-fused spur feeds only one single or one twin socket outlet or one permanently connected equipment.

So, the answer to your question (When did the regulations change in respect to how many outlets can be wired on a non-fused spur?) is

30th March 1981.

 

[dannyboi2003]

Cheers secure

Dan

 

[securespark]

No worries!

Did your "early days" commence under the 14th?

I only ask because lots of folk here think you can run more than one outlet from a non-fused spur. Wondered why.

 

[empip]

This from the 14th edition, 1 October 1966:

A.40 For ring final subcircuits complying with regulations A.30-33, non-fused spurs shall be connected to the ring ....or at the origin of the ring in the distribution board. ..[/b]

There has been some huffing and puffing abaht this too, at times ...
I bet tis allowed in the latest regs too.

 

[dannyboi2003]

lol no they don't, i wasn't even born in 1981. My Dad had an old "home electrics" book which is where i got the two sockets on one spur thing from. I've never out more than one device on a spur (unless its fed to an FCU first) but all this talk of spurs got me curious to find out when the regs changed.

I just dug out that old book, its quite funny looking through it and comparing it to todays standards lol

All the best
Dan