Circuit design for new extension

[Mechie]

Hi all

I've got a training assignment to do which is to design a circuit for a job I will be doing with my boss. He wants to see what I come up with having viewed the job and spoken to the client.

Basically the job is supplying lighting and sockets to a new extension comprising of three double socket outlet, two wall mounted up lighters controlled by one side of a two gang switch and one downlight controlled by the other side.

As far as I can see there is no easy way to extend any current lighting or ring final circuits without making a lot of holes and a lot of work which the client doesn't want, but there are two spare ways on the consumer unit which is right next to the new extension. So my suggestion would simply be run two new circuits from the consumer unit.

Only problem is that the client thinks he knows a bit about electrics "maybe he does?" and has started going on about only using one way from the consumer unit, he wants a radial supplying the sockets and a fused spur feeding the 3 lights.

seeing as the max current demand for the lights is about 1.3amps and I can't find anything in the regs which says I couldn't run the lights of a 5amp connection unit spurred off the socket circuit or spurred off an exsisting lighting circuit at the consumer unit. Am I missing something in the regs? And should I design my circuit as the client wants, or is there any reason I should suggest that the two circuits be run from their own circuit breakers at the consumer unit, eg, future proofing if he wants to extend the circuits, or a deveation from the regs if I've missed one

I noticed that the RCD takes up 3 ways in the CU, so I could suggest putting in a newer smaller RCD that would leave room for another way off the CU for future extensions.

Just wanted to hear your thoughts before I hand my design over to my boss.

 

[Badged01]

I take the view that the customer is king ... he is paying your bills and more importantly he can recommend you for further work. If what he wants doing meets the regs and in your opinion is safe to implement then do it. A satisfied and happy customer is always good for business!!.

I actually see no problem with a radial circuit for 3 sockets with a 3A fused spur off this feeding the 2 lights.

 

[RyanCheetham]

I did something similar 2 weeks ago. No easy access to CU from the extension so i broke into an existing socket on the backside of the wall and extended the RFC for my 3 sockets and i used an FCU to power my lights and 3 plated all at the switch. Obviously i was lucky enough to have a wiggle room on the circuit to add the extra sockets without overloading. So yeah there is no problem with what you propose

 

[stuartcourt]

If you can get to consumer can you not come off an existing lighting breaker

 

[Mechie]

If you can get to consumer can you not come off an existing lighting breaker

As it's feeding multiple lights would this not still need a fused connection unit, if so then it makes sense just to run the one piece of 2.5 twin and earth from the consumer unit to the extension, then FCU off that for the lights, would save on cable.

 

[Spoon]

As it's feeding multiple lights would this not still need a fused connection unit, if so then it makes sense just to run the one piece of 2.5 twin and earth from the consumer unit to the extension, then FCU off that for the lights, would save on cable.

Just my opinion, but seeing as the CU is "right next to the new extension" as you mentioned in post 1, then doing as stuartcoart suggested would be better, as long as the circuit can take the extra load for the lights.

 

[Mechie]

Just my opinion, but seeing as the CU is "right next to the new extension" as you mentioned in post 1, then doing as stuartcoart suggested would be better, as long as the circuit can take the extra load for the lights.

Yeah i agree, I just want to be sure so i can explain exactly why i've designed the circuit the way i have to the boss and also to explain to the client if it's different from the way he wants it. End of the day the boss and the client will make the finale call, I just want to investigate all the options and show that I've actually put some serious thought into it. I know it's not the most complicated circuit or installation but the boss is letting me take the lead on this job so want to show I'm taking it seriously and not just taking the easy route.

 

[Octopus]

Yeah i agree, I just want to be sure so i can explain exactly why i've designed the circuit the way i have to the boss and also to explain to the client if it's different from the way he wants it. End of the day the boss and the client will make the finale call, I just want to investigate all the options and show that I've actually put some serious thought into it. I know it's not the most complicated circuit or installation but the boss is letting me take the lead on this job so want to show I'm taking it seriously and not just taking the easy route.

Given that your outline of the job suggests a single up front RCD, you should comment on the increased risk of nuisance tripping.

 

[odawire]

Hi all


I noticed that the RCD takes up 3 ways in the CU, so I could suggest putting in a newer smaller RCD that would leave room for another way off the CU for future extensions.

What do you mean? are you saying its three pole?

 

[stuartcourt]

No he is saying it sounds like one RCD protecting all circuits so if it trips then you lose power to everything. Back to the new circuits I would have thought your boss would rather lights on lighting circuit, only really put lights off spur if lighting is inaccessible.

 

[stuartcourt]

What do you mean? are you saying its three pole?

sorry got confused some older RCD took up 3 ways (spaces) on consumer unit rather than 2

 

[Mechie]

sorry got confused some older RCD took up 3 ways (spaces) on consumer unit rather than 2

Yeah you got it, sorry probably my fault for not using the right language to explain myself.

So I think I'm going ahead with suggesting running the lights of the existing light circuit in the CU, Looking through BS7671 I can't see anything that say's I should treat a domestic light circuit any differently from a standard socket radial. So as I can't really extend the current light circuit without disconnecting the current light circuit, running my new piece of 1.5 into the extension to the lights, then back to the CU to connect back the existing circuit, I guess this will still be consider a spur of the existing lights connected at the CU. as it's powering more than one piece of fixed equipment I will need to install a 3amp fused connection unit?

If so does it matter where this is located, is does it need to be close to the lights as a fused unit for a cooker would be, or can I install it at the start of the spur next to the consumer unit.


If i'm missing something really obvious in the regs would someone mind pointing me to the right section

 

[Spoon]

I think you are getting a bit confused Mechie, or I am....
If you are going to wire the light in the extension into an existing lighting breaker then you do not need any additional fusing down to 3A. Just wire it up as you would a standard lighting circuit. Again, make sure the existing breaker can take the extra load for the lights.

 

[Mechie]

I think you are getting a bit confused Mechie, or I am....
If you are going to wire the light in the extension into an existing lighting breaker then you do not need any additional fusing down to 3A. Just wire it up as you would a standard lighting circuit. Again, make sure the existing breaker can take the extra load for the lights.

No sorry, I thinks it's me getting confused as I've never wired a lighting circuit like this. So I have a 6amp breaker currently supplying the exsiting lights. What I want to do is also run the 3 new lights off this breaker. Looking in the On Site Guide, The Electricians Guide and BS7671 I can not see anything covering this exact situation, the closest I can see if diagrames and info on wiring a radial for sockets.

Appendix 15 in BS7671 states "an unfused spur run in 2.5mm2 cable should feed one single or one twin socket-outlet only. An unfused spur may be connected to the origin of the circuit in the distribution board."

For a fused spur it says the number of sockets supplied is dependent on the load, so what I'm asking is whether I treat the lighting circuit in the same way, eg as there is more than one light should I fit a fused connection unit or can i connect straight into the breaker without one. I've been looking through all the books but can't see anything that answers my question.

 

[Spoon]

The breakers in the CU are to protect the cables in the circuit. What you intend to do is wire the lights in the extension in 1.5mm, as normal. Then wire this circuit back to a 6A lighting breaker... Making sure the extra load is suitable.
What is the existing current consumption when all the light are on?

 

[Mechie]

The breakers in the CU are to protect the cables in the circuit. What you intend to do is wire the lights in the extension in 1.5mm, as normal. Then wire this circuit back to a 6A lighting breaker... Making sure the extra load is suitable.
What is the existing current consumption when all the light are on?

Will total 5,7amp including the 3 new lights so the breaker can handle it

 

[Octopus]

Now you've got some pointers, is it time to run it past your boss?