Discuss 2 Lighting Circuits on one 6A CB in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

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NewbieRuffy

Hi All, I am newbie to this site and very new to Electrics also, so please excuse my ignorance. I have recently obtained Part P, 17 edition and 2392. As i am sure you are aware to gain access to NIC i need to carry out 2 jobs so they can be inspected by NIC (for a nice fee of course). I chose to change my very out dated CCU which went without too much hassle. I noticed when changing to the new board that the downstairs lighting circuit had 2 tails into the CB as it seemed to always had been like this i did a like for like change and thought would investigate after CCU change. I am now at the testing stage and after some investigation and elimination i have found that its 2 circuits on one 6A CB. 1 circuit serves the whole of the downstairs lights apart from the Porch light (which is where the CCU is) which is on the other circuit in the same CB. I am not sure why this has been done and when changing CCU noticed that all was original wiring and the porch is part of the original house.

Given there are 8 other lights down stairs, and working at 100w per light using O-law this equates to 3.4A and with the porch light even though on a different circuit its well within the limits of the 6A CB, question is, as this is going to be inspected by NIC, does the Porch light circuit need its own CB?

Sorry if i go on a bit and its a bit detailed, still new :)
 

sythai

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Dont worry it fine on the same mcb, maybe it was added at a later date and the CCU was the easiest place to pick up a feed from.

All the best with your NIC inspection

Sy
 

telectrix

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noy

thing wrong with that. even though there are 2 "circuits" coming from 1 MCB, it's classed as 1 circuit, so you will get your R1+R2 and Zs readings from wherever, and record the highest reading on your cert.
 

needasparks

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Hi fella, see your new to the forum. before you get bombarded with answers from all. set aside the calculations bit and give the overall view of this situation a think. What do YOU think you should do and why ?
It will save everybody a lot of explaining if we know how your thinking addresses this situation first.
 

telectrix

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NewbieRuffy

Thanks guys for all your feedback. I did do an inspection before changing CU and as i said in original post believed it wasn't going to cause an issue and would address later, however perhaps i should have addressed and investigated before hand.

As for my own thoughts, originally i thought was is the lighting on a ring circuit? of course after investigation i found 2 lighting circuits, you say put the calculation aside but i used this in my thought process and believed that 2 circuits on 1 CB shouldnt be an issue as the overall AMPS after calculations did not exceed the CB limit. I only asked my friend because being still new and not covered 2 circuits on 1 CB before and because i want this CCU change as one of my NIC jobs, i just wanted to make sure i was doing the right thing as at £300 per assessment I dont want to get it wrong the first time i do however appreciate your feedback.
 

telectrix

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if you have a spare way, then you can split them. if not, then they're fine as they are. as you say, the rating of the OCPD is not exceeded, so you're good to go.
 

needasparks

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As others have said its not a problem, some people don't like to see more than one cable in a breaker I don't see a problem with it myself as long as its not overloaded, common sence should provale over this. It could be confusing if the cables do different things especially if it covers more than one area eg: upstairs AND say kitchen lights. which tends to happen through lack of spare ways and should really be split up when at all possible, you can split the installation up as much as you like as long as its labelled correctly.
As long as you can jusitfy what you have done without being detrimental to the installation regarding identification, loadings and utilisation as tel said your good to go. Reason i said set aside the math initially was to ease more info out of you about your knowledge and how you are going to explain to your assessment.
Do expect some comments during your visit but its not all about what you have done but more about what you know.
 

Murdoch

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if you have a spare way, then you can split them. if not, then they're fine as they are. as you say, the rating of the OCPD is not exceeded, so you're good to go.
Surely there must be spare ways in a new CU??
 
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BMelectrics

i thought that each circuit needed its own form of protection??

sharing a breaker is bad practise.
 

Snapester

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I take that the porch light is on adequate sized cable for the breaker? I thought you need more than one for assessment? A few big jobs and one small or something, at least when i joined it was......... i think i might sell my test gear.....
 

Strima

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You can also run a spur from the origin of the circuit...
 
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Guest55

so what if the neutral was shared then......each on its own MCB then eh?...lol....
Well thats not the same as 2 cables sharing a mcb is it ? :)
which is the issue being discussed isnt it ?
 

Guitarist

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It is always a good idea to split the lighting if possible, 1 onto each RCD, so that you don't lose everything if 1 trips. The problem arises if the 2 circuits share a neutral (I think this is what sparks1973 was getting onto), as you will end up with an imbalance. Have you checked for shared neutrals? If you have, and the 2 circuits are completely separate, then I would split them across the RCD's. This is, after all, the reasoning behind dual RCD boards.
 
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NewbieRuffy

I did do a IR test between N/E and this what confirmed that they were separate. To be sure (as i am still a Newbie) i removed one of the circuits in the CU from CB, connector blocked it, and then energised (after replacing cover of course!) Then simply turned on the lights down stairs and just a case of elimination, i repeated for other circuit to confirm. I guess long way round but had time and didnt take too long.

I also do have a number of spare ways in the CU, I have just thought though i am installing a new kitchen (i also fit kitchens with my bro, so pretty handy) and think i am going to put in downlighters, so will more likely have to upgrade to a 10A CB so thinking leaving the one porch light circuit on 6A and moving the rest of the downstairs and new kitchen lights to 10A CB.
 

richy3333

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In the house we are in there are 5 lives stuffed in to a rewirable fuse, CU! Landlords agreed to let me sort the mess that are our electrics out though and pay for it :)
 
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NewbieRuffy

Totally agree Guitarist, the upstairs lights are already on the other RCD, I now plan to move the porch light circuit on to the upstairs RCD and put in a new 10A CB for the downstairs, as mentioned before, installing downlighters in kitchen very soon. I was taught that it was always best practice to have all circuits on separate CB where possible, hence i asked this original question.
 
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Deleted member 26818

At the moment, it is one circuit.
A circuit is defined as: "An assembly of electrical equipment supplied from the same origin and protected against overcurrent by the same protective device(s)."
The fact that there are two conductors in one MCB is immaterial, it is one circuit.
 
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Jonny66

I would leave it as is, cant see anything wrong with the porch light added via MCB, and agree with spinlondon that this is one circuit.
IMO, I would rather do it this way (at MCB) than hidden junction box under floor/ above ceiling.
 

ackbarthestar

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That's my thinking as well.
I would have wanted to know what each lighting leg fed and whether they could be separated or whether they shared a neutral. If the latter then they would have to be regarded as one circuit and kept together in the same breaker.

With these new 16 - 20 way CCUs you can have lots of separate circuits without the need for sharing.

One quick way to check besides IR testing across the neutrals would be to place one leg on one RCD and the other leg on another RCD. If both RCDs trip out when a light is switch on then you have a neutral across the two legs

- - - Updated - - -

That's my thinking as well.
I would have wanted to know what each lighting leg fed and whether they could be separated or whether they shared a neutral. If the latter then they would have to be regarded as one circuit and kept together in the same breaker.

With these new 16 - 20 way CCUs you can have lots of separate circuits without the need for sharing.

One quick way to check besides IR testing across the neutrals would be to place one leg on one RCD and the other leg on another RCD. If both RCDs trip out when a light is switch on then you have a neutral across the two legs

- - - Updated - - -

That's my thinking as well.
I would have wanted to know what each lighting leg fed and whether they could be separated or whether they shared a neutral. If the latter then they would have to be regarded as one circuit and kept together in the same breaker.

With these new 16 - 20 way CCUs you can have lots of separate circuits without the need for sharing.

One quick way to check besides IR testing across the neutrals would be to place one leg on one RCD and the other leg on another RCD. If both RCDs trip out when a light is switch on then you have a neutral across the two legs
 

Risteard

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As others have said we should be clear that there is ONE circuit supplied by the protective device. By definition you have one circuit (and indeed there can be good reasons for designing this way e.g. to limit Zs/volt drop).
 
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