Separate lighting circuits/borrowed neutrals

[JohnW2]

The OP's electrician has only highlighted a potential additional cost as any competent electrical contractor would do.

Given that the electrician's proposed solution is "... if this was the case then he would have to put the lights all on one circuit", I can't see what "additional cost" there should be - if anything, connecting two conductors to one MCB in the new CU should take a few seconds less than connecting them to two separate MCBs! ... if he proposes to charge extra for that, he would surely be some sort of cowboy?!

Kind Regards, John

 

[deadshort]

I cannot say, as i don't know the electrician and would not wish no make assumptions or judgements someone i have never met.

Regards,

DS

 

[JohnW2]

I cannot say, as i don't know the electrician and would not wish no make assumptions or judgements someone i have never met.

Fair enough, but what "additional charge" would you expect to add on for connecting two lighting circuits to one MCB (and neutral bar), rather than two?

Kind Regards, John

 

[Alfie25]

It only came up as we were discussing whether to go for a split board or for full RCBOs. I'm glad he mentioned it as it's simply got me learning/thinking more rather than just blindly nodding at what he's saying.

So if a 2 light circuit job is done properly, is the landing light usually on the upstairs circuit with the downstairs switch on the upstairs circuit too, or vice versa?

 

[deadshort]

The additional charge would be to remove the borrowed neutral and wire as 2 circuits.

DS

 

[EFLImpudence]

So if a 2 light circuit job is done properly, is the landing light usually on the upstairs circuit with the downstairs switch on the upstairs circuit too, or vice versa?

I would say 'normally' rather than 'properly'.

The line(supply) will go to the downstairs switch and the neutral back from the landing light.

 

[JohnW2]

The additional charge would be to remove the borrowed neutral and wire as 2 circuits.

As I said, that was not what the electrician was proposing. He said that if there were a shared/borrowed neutral "...then he would have to put the lights all on one circuit" - which is why I didn't understand your comment about him "highlighting a potential additional cost".

Kind Regards, John

 

[JohnW2]

So if a 2 light circuit job is done properly, is the landing light usually on the upstairs circuit with the downstairs switch on the upstairs circuit too, or vice versa?

It can be done either way around - although it would be logical for an ('upstairs') landing light to be on the upstairs lighting circuit.

In terms of the potential problem we're discussing, the issue is not about what light is on what circuit - the only important point (in that respect) is that, whichever circuit a light (and associated switch) is connected to, both the live and neutral for that light must come from the same circuit (which is not the case if there is a borrowed/shared neutral).

Kind Regards, John

 

[EFLImpudence]

it would be logical for an ('upstairs') landing light to be on the upstairs lighting circuit.

Well, the light is on both circuits; that's the problem.

To have two-way switching on a light upstairs using the conventional method of only two strappers the line has to be from downstairs otherwise it would be switching the neutral.

 

[ban-all-sheds]

No - I did not say I thought it was likely.

The electrician did, and what I said was that he could have good grounds for suspecting it. At the time we did not know how old the house(s) are, and now we do, what TTC said is true. But if the house(s) had been built in the 30's/40's/50's (as many were) then it's perfectly believable that they were wired as discussed, perfectly believable that he could know that, and perfectly believable that he could have encountered many where a subsequent split had created a borrowed neutral.

 

[JohnW2]

it would be logical for an ('upstairs') landing light to be on the upstairs lighting circuit.

Well, the light is on both circuits; that's the problem. ... To have two-way switching on a light upstairs using the conventional method of only two strappers the line has to be from downstairs otherwise it would be switching the neutral.

Needless to say, I understand all that - but was merely answering the OP's question. As I said to him, it doesn't really matter which circuit the landing light is on, provided that both L and N for it come from the same circuit. As you say, with 'conventional' (old) two-strapper wiring, that would not be the case.

Having said that, the situation I inherited in my house (probably late 70s wiring) was a little different. Although it basically used the 'old' method of 2-way switch wiring, it used 3C+E cable, with the L feed travelling in the third core, with the two strappers. ... so there are variations around.

Kind Regards, John

 

[EFLImpudence]

Although it basically used the 'old' method of 2-way switch wiring, it used 3C+E cable, with the L feed travelling in the third core, with the two strappers. ... so there are variations around.

That would be the modern (conversion) method and surely not involve a borrowed neutral, wouldn't it?


Edit - or rather a miswired conversion method.

 

[JohnW2]

No - I did not say I thought it was likely.

It's true you didn't use that word - so, if you believe that "likely" and "probably" have appreciably different meanings, then you're right:

But I stand by what I said - if there are 10's or 100's of houses just like the OP's in the area, all built when 1 lighting circuit and a twin & earth connection between the hall and landing switches was de rigeur, and the CU tells him that the split of the circuit was likely done some years ago, and he's seen this many times, then he is right to say that there is probably a borrowed neutral.

Kind Regards, John

 

[ban-all-sheds]

Dear God...

But I stand by what I said - if there are 10's or 100's of houses just like the OP's in the area, all built when 1 lighting circuit and a twin & earth connection between the hall and landing switches was de rigeur, and the CU tells him that the split of the circuit was likely done some years ago, and he's seen this many times, then he is right to say that there is probably a borrowed neutral.

 

[ericmark]

1) As far as I am aware it has never been permissible to borrow a neutral.
2) My house like many built in the late 70's and early 80's has one fuse for all lights.
3) Until people started to fit down lighters 5A was enough to run all house hold lights.
4) Most ceiling roses are rated either 5 or 6 amp.
5) It is permissible to go up to 16A for lighting as long as there are no items like ceiling roses which are only rated at 5 amp.
6) Where one is limited to 5A then splitting the lighting circuit was common.
7) Where three core and earth cable is run between the two way light switches there is not a problem.
8 ) Where the line is borrowed for another switch then one can produce mains hum on sensitive equipment.

Although the label is borrowed neutral what has actually happened is a line is borrowed so with a two way switching system you have 4 options two are off and two are one of the latter one will result in a borrowed neutral if only twin and earth is used between switches and a line is taken from another switch.

If that happens there are a number of options.
1) Do away with two way switching.
2) Re-combine the lighting circuits.
3) Replace the cable between the two switches.
4) Replace a cabled link between the switches for a wireless link.
5) Break the rules and ignore the borrowed neutral and feed both MCB's from the same RCD.

The easy option is to fit wireless light switches.

I would say you have selected a good electrician as he has highlighted the potential problem before it becomes a problem.

I have seen electricians even when the fault is highlighted due to a RCD tripping just put both MCB's on the same RCD which may stop it tripping but still leaves one with a borrowed neutral.

Every installation shall be divided into circuits, as necessary, to: (iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit.
(iv) reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation.

This is a bit of a problem. If you use a single RCD and it does not trip then it complies as would be the case with a caravan but if it trips then it does not comply, but the only real way to know if it will trip is to try it which is a bit late when you find it's tripping.

Taking either circuits likely to trip and putting them on their own RCBO or taking circuit which will cause problems if they trip and putting them on there own RCBO does remove some of the problems. Be it the ring on RCBO or fridge/freezer on RCBO it removes some of the problems of RCD tripping. Having two circuits in a kitchen off different RCD's means if there is a problem you can swap sockets to work out which item is faulty.

Also running lights from a RCBO will reduce the likely hood of them tripping but where there is no external light then an emergency light would be a better option.

It all boils down to a risk assessment my house with two rather old RCD's fitted in the early 1990's has a RCD trip on average 6 times a year my mothers house with all RCBO's in kitchen has never had one trip.