Fused spur off a ring main cable size question

[Maddermanblue]

Hi,

The builders are extending the kitchen in my house at the moment which means putting in an extra 2 double sockets. I'm doing the electrics myself. I plan to to fix an unswitched 13A FCU to the spur off the ring main and put the 2 new double sockets on it.

I have been looking at the big red book; Appendix 15, the one with the ring diagram. I'm confused on the cable sizing. The diagram in the red book coming off the main ring is an FCU with a cable size of 2.5mm but coming off the FCU to a spurred socket is a 1.5mm cable, fair enough.

In another book, (Electrical Installations 2330 Level 2) it confirms the above but it says a MINIMUM size of the conductor is 1.5mm for a fused spur (I assume you can't go lower than 1.5mm but can go higher??).

Taking 'MINIMUM' into account does that mean I can use a 2.5mm cable instead of 1.5mm cable between the FCU and the new socket? Bearing in mind I need to run a washing machine and tumble drier and possibly a fridge off it...

I'm new in the world of electrics, passsed my C&G 2330 Level 2 and other bits not long ago and trying to gain experience and break into the field.
Another certified and registered sparky is going to come and inspect and sign off my work once its done - I'm just saving face asking him a question that may seem silly...

Any advice would be appreciated.

Maddermanblue

 

[truckster]

You can do it in 2.5mm always do. The trouble you may have is the load you are planing to put on it. With an fcu you are limited to 13A and a fridge. washng machine and tumble dryer are going to be more than that. Forget the fcu ad run the spur in 4mm t/e which is ok on the 32A ring mcb

 

[Deleted member 9648]

The minimum for a fused spur is 1.5mm as you rightly state.....this is because 1.5mm t/e is rated higher than 13a,the rating of the fuse protecting it.However,99% of sparks would use 2.5mm.......all that load you are proposing off a 13a spur is in the region of 20-25a all running.....Have you thought this one through?

 

[Deleted member 9648]

You can do it in 2.5mm always do. The trouble you may have is the load you are planing to put on it. With an fcu you are limited to 13A and a fridge. washng machine and tumble dryer are going to be more than that. Forget the fcu ad run the spur in 4mm t/e which is ok on the 32A ring mcb

Not acceptable as a spur from a ring, as it would result in uneven loadings on the ring cables. This would not comply with the regs.

 

[nickblake]

yes you can use higher the reason you can use a 1.5mm cable is the fact that its been fused down to a max of 13 amps , and yes you can go higher , the only problem i can see with what you are suggesting is the loading if you have a fridge washing machine and tumble dryer all off a fused spur there will be a good chance the FCU is over loaded what i would suggest is that you either cut into the ring to add these sockets or install a 4mm radial circuit from the CDU to the sockets and protect by a 32 amp MCB oh and dont forget this type of work is notifiable as it falls into the special locations IE the kitchen

Nick

 

[Maddermanblue]

yes you can use higher the reason you can use a 1.5mm cable is the fact that its been fused down to a max of 13 amps , and yes you can go higher , the only problem i can see with what you are suggesting is the loading if you have a fridge washing machine and tumble dryer all off a fused spur there will be a good chance the FCU is over loaded what i would suggest is that you either cut into the ring to add these sockets or install a 4mm radial circuit from the CDU to the sockets and protect by a 32 amp MCB oh and dont forget this type of work is notifiable as it falls into the special locations IE the kitchen

Nick

The minimum for a fused spur is 1.5mm as you rightly state.....this is because 1.5mm t/e is rated higher than 13a,the rating of the fuse protecting it.However,99% of sparks would use 2.5mm.......all that load you are proposing off a 13a spur is in the region of 20-25a all running.....Have you thought this one through?

I agree, I thought its too much a load for a 13A FCU. So cutting into the ring, making the new sockets part of the ring is ok and complies with the regs?
Origionally in the kitchen only has 2 singles and 2 doubles at the mo...

Regards

Maddermanblue

 

[Octopus]

I agree, I thought its too much a load for a 13A FCU. So cutting into the ring, making the new sockets part of the ring is ok and complies with the regs?
Origionally in the kitchen only has 2 singles and 2 doubles at the mo...

Regards

Maddermanblue

Splitting the ring and inserting your new sockets so they are now "in" the ring would be OK and a perfectly good way of getting the "load" to the new sockets.

 

[truckster]

Not acceptable as a spur from a ring, as it would result in uneven loadings on the ring cables. This would not comply with the regs.

Hi Wirepuller

Can you point to a specific reg? I don't think it's anymore onerous on the ring than the poular kitchen grid switch which may have every appliance on it all taken from one place on the ring, not arguing just discussing.

 

[Deleted member 9648]

No argument mate!.....appendix 15 covers what can and cant be done on a ring, 1(i) requires sharing evenly the load around the ring.

 

[truckster]

Which means that all these new builds with the lovely neat grid plate are probably non-compliant. I do remember reading something ages ago about this but can't think where it was, probably Professional Electrician or some such. Interesting thought.

 

[Hawk81]

Hi Wirepuller

Can you point to a specific reg? I don't think it's anymore onerous on the ring than the poular kitchen grid switch which may have every appliance on it all taken from one place on the ring, not arguing just discussing.

He is looking to spur 2 sockets. Which would make this a spur off a spur. Which is not allowed

 

[Des 56]

Hi Wirepuller

Can you point to a specific reg? I don't think it's anymore onerous on the ring than the poular kitchen grid switch which may have every appliance on it all taken from one place on the ring, not arguing just discussing.

Possibly
If it was near mid point of the ring it would be shared equally no doubt
It has been known to assess, say for a kitchen ring, where the appliances are going, so that the ring can be installed to take the position of the loads into account

 

[Gerry wall]

I would like to mention something here chaps but as far as I am aware you sohuld not be using 1.5 T/E cable in a ring main period. When you spur off you should use 2.5 T/E cable. If you were spuring off to a lighting circuit then from the spur you would use 1.5 T/E but beofre the spur 2.5 T/E.
As already mentioned the load would be way to much under these conditions. Just look at your appliances and add up the amps needed and you will find it is way more than 13A.
Once you have spured off you can add as many sockets as you like within under the regs but again you have to work out what you are going to plug into it.

Best bet would either be to extend the ring main or run a new cable from the consumer unit.

Hope it works out

Gerry

 

[truckster]

He is looking to spur 2 sockets. Which would make this a spur off a spur. Which is not allowed

Says who? This is the usual fall back position. Appendix 15 is guidance, not normative and Appendix 8 of the OSG gives advice on standard cct arrangements for households. It goes on to say "cct arrangements other than those detailed in this appendix are not precluded when specified by a competent person"

I am unconvinced by the "sharing the load" arguement in a domestic situation as the lengths of cable and power demand are insufficient to significantly affect current flow.

I go back to the gridswitch example, where you can connect 4,6,8 appliances to a single point in a ring, if load sharing capabilities were the big issue then I am sure these would have been banned.

Discuss.

 

[Hawk81]

Don't have my BRB with me but I do have my OSG and page 159 8.2.4 says in an A1 ring final circuit and an A2 radial circuit of table 8A a non-fused spur should feed only ONE single or ONE twin or multiiple socket outlet or ONE item of permanetly connected equipment. Or am I reading this wrong??

 

[truckster]

As I already said" Appendix 8 of the OSG gives advice on standard cct arrangements for households. It goes on to say "cct arrangements other than those detailed in this appendix are not precluded when specified by a competent person"

 

[Hawk81]

Yes but this is a standard A1 ring final as specified and therefore only one non fused spur should be taken from it.

 

[Gerry wall]

I think this is a FUSED spur and NOT a NON FUSED SPUR.

Gerry

 

[Hawk81]

I was refering to truckster's advice to take a 4mm non fused spur to do the 2 sockets

 

[Gerry wall]

Ok Sorry mate

 

[truckster]

Yes it is a standard cct now but by using a 4mm spur only makes it non-standard, not wrong.

 

[Deleted member 9648]

Yes it is a standard cct now but by using a 4mm spur only makes it non-standard, not wrong.

I would tend to agree....as long as it was properly designed and executed. The problem is what starts out as 2 twins on a 4.0mm spur might escalate into a whole radial on said spur.Thats why we have the one point per unfused spur guidance,makes sense in my opinion.

 

[Deleted member 9648]

I would like to mention something here chaps but as far as I am aware you sohuld not be using 1.5 T/E cable in a ring main period. When you spur off you should use 2.5 T/E cable. If you were spuring off to a lighting circuit then from the spur you would use 1.5 T/E but beofre the spur 2.5 T/E.
As already mentioned the load would be way to much under these conditions. Just look at your appliances and add up the amps needed and you will find it is way more than 13A.
Once you have spured off you can add as many sockets as you like within under the regs but again you have to work out what you are going to plug into it.

Best bet would either be to extend the ring main or run a new cable from the consumer unit.

Hope it works out

Gerry

The OP was asking if it is acceptable to use 1.5mm on the load side of a FUSED(13a) spur....which if you refer to the BRB app 15 you will find can run an unlimited number of sockets.
It has already been pointed out that in the circumstances a fused spur,either 2'5mm or 1'5mm will not supply the proposed load.

 

[mickys86]

I was refering to truckster's advice to take a 4mm non fused spur to do the 2 sockets

I definitely agree.
If you are taking a spur from a ring, you can only have one double socket at the most. But if you are coming of a FCU then it's unlimited amount bearing in mind that your limited to 13amps.

And someone mentioned about not being able to use 1.5mm? As has been said previously. You have fused the circuit down to 13a and (off the top of my head) 1.5 can take around 26a

 

[mickys86]

The best option is to open the ring.

 

[mickys86]

I definitely agree.
If you are taking a spur from a ring, you can only have one double socket at the most. But if you are coming of a FCU then it's unlimited amount bearing in mind that your limited to 13amps.

And someone mentioned about not being able to use 1.5mm? As has been said previously. You have fused the circuit down to 13a and (off the top of my head) 1.5 can take around 26a

Ok its 20a ref method c
Even ref method a is 14.5 and it's still higher than 13a

 

[Maddermanblue]

Thanks for all the advice, somewhat turned onto a healthy debate.
Am I right thinking that the BS7641

 

[Maddermanblue]

Ignore what I just posted, iPhone playing up and sent it prematurely...
As I was trying to say is it correct to assume the BS7671 book is open to interpretation?
Anyhoo, it's all change now, they can't make their minds up... It's been decided the washing machine and tumble drier is to go in the extension currently being built.
I am going to spur off the ring main for the 2 new sockets using a fcu, and use 2.5mm t/e no heavy loads are going to use it.
The extension is part of the kitchen, only a separate room, is ok to extend the kitchen ring main into the extension? It will house a tumble drier and washing machine and possibly a fridge.

 

[johnelectrician]

I really dont get why you dont just extend the ring main using a 30a junction box?

 

[Farmelectrics]

Why not extend the ring

 

[Octopus]

This thread is over 1 year old!!

 

[Farmelectrics]

And counting

 

[HandySparks]

I really dont get why you dont just extend the ring main using a 30a junction box?

Welcome to the forum johnelectrician.
I've not read the whole thread, but the OP wanted more than one socket on the spur, so it has to be fused to prevent overloading the spur. See appendix of BS7671.
By the way, the OP has probably finished the job by now, being a year on!

 

[johnelectrician]

Welcome to the forum johnelectrician.
I've not read the whole thread, but the OP wanted more than one socket on the spur, so it has to be fused to prevent overloading the spur. See appendix of BS7671.
By the way, the OP has probably finished the job by now, being a year on!

or he could of just extended the ring final circuit aslong as its within 100sqm, i thoought this would of been the preffered method that putting a 13a fused spur in place?

 

[SPARTYKUS]

Like using a non standard (but well thought out ) circuit, there are also no rules precluding one from contributing to a year old thread

 

[HandySparks]

or he could of just extended the ring final circuit aslong as its within 100sqm, i thoought this would of been the preffered method that putting a 13a fused spur in place?

Your phrase "extend the ring main using a 30a junction box" made me jump to the conclusion that you were suggesting an unfused spur. Apologies for the misunderstanding.