Stranded PVC cable, and junction boxes under floorboards.

Hi all

When I bought my house I was told it had been rewired in 1987. However there is quite a bit of stranded PVC/copper cable on the socket circuits [edit - just found some tinned copper / ally as well], and junction boxes under floorboards. When I have found a junction box under the floor, I have made an access panel in the floorboard fixed with screws, and relaid the carpet. There is no VIR cable.

I have also found that some parts of the lighting circuit have had no earth, these have mostly been replaced and I will do the last bit in the summer hopefully. I have checked for earth continuity back to the CU on parts I have replaced.

I have a TT system with whole house 30mA RCD and 7 Wylex NSB MCBs. All 16A radials apart from the lighting circuit which is 6A.

I can only conclude that the work in 1987 was not a full rewire but just a replacement CU and new RCD.

I would like to change my CU for one with 7 RCBO's in place of the RCD+MCBs.

Which leads on to the question: Is it certain that my house will require a full rewire if I wish to change my consumer unit?

Many thanks,

mogget.

Twin cable were about in the 60's but not the 80's. So you can assume it was an upgrade in the 80's.
Instead of fitting RCBO's could you not fit a 17th addition board board fitted with 2 RCD's & a main switch as it will be a lot cheaper than RCBO's. (Around £60 for the board with fuses)
The lighting defiantly needs rewiring with TWE cable & power might be ok, but would need looking at with a test.

No, there is nothing to say your old PVC cables are not serviceable, as long as they are correctly sized and installed. As long as they remain that way, they can outlast many CU changes.

You may however wish to break circuits for convenience (i.e. sockets up/down/kitchen) and lighting up/down. Additional wiring may also be required where borrowed neutrals are present.

That's comforting to know.

mfarrow said:

You may however wish to break circuits for convenience (i.e. sockets up/down/kitchen) and lighting up/down.

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I already have the circuits broken up and have done my best to trace every one. The single lighting circuit doesn't cause me any problems, and I'm not fussed about changing it unless it is required by regulations.

What about the junction boxes under the floor, do they count as accessible if you can easily lift the 'felt back' carpet and unscrew the panel?

I don't know how to identify a borrowed neutral, is this something that can be done DIY?

The only part of the system that's a mystery to me is the lighting circuit in the kitchen and workshop, this is also the part with no earth and I am planning to rewire this section, picking up the (earthed) supply under the bathroom floor.

I would consider accessing junction boxes under carpets as being accessible, however it's not widely considered best alignment with the regulations.

Borrowed neutrals will usually be found between lighting circuits, the most common one being where the landing light is supplied with line from the downstairs circuit, and neutral from the upstairs circuits. Your circuit is singular so you won't have that problem. As to splitting the lighting now it's really done for convenience, but if you and your CU are both downstairs when the lights go out then bugger all difference an upstairs circuit is going to make.

If you want to test for borrowed neutrals with what you've got, best way is to do some load switching whilst measuring line and neutral at the consumer unit. Each circuit should have the same current in line, as it does in neutral.

mfarrow said:

if you and your CU are both downstairs when the lights go out then b*****r all difference an upstairs circuit is going to make.

Click to expand...

My thoughts exactly.

If you want to test for borrowed neutrals with what you've got, best way is to do some load switching whilst measuring line and neutral at the consumer unit. Each circuit should have the same current in line, as it does in neutral.

Click to expand...

Am I correct in thinking that borrowed neutrals happen on 2-way lighting circuits? All mine are done the modern way with 3core and earth between the two switches, apart from the ones in the kitchen which are to be disconnected and re-done with 1-way switching. Will probably do most of that in surface mount due to the flat roof.

Thanks

mogget.

mogget said:

Am I correct in thinking that borrowed neutrals happen on 2-way lighting circuits?

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That's what the landing light is on, yes, and it's most likely here because of the upstairs/downstairs traditional circuit split.

Elsewhere on 2-ways, i.e. across the same floor, you will likely not likely need to borrow the neutral from another circuit, unless you're really lazy and pick it up from the nearest socket etc.

mfarrow said:

That's what the landing light is on, yes, and it's most likely here because of the upstairs/downstairs traditional circuit split.

Elsewhere on 2-ways, i.e. across the same floor, you will likely not likely need to borrow the neutral from another circuit, unless you're really lazy and pick it up from the nearest socket etc.

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Many thanks for taking the time to reply. I understand now. I know that the live and neutral both come from the same junction box upstairs as I put them in myself. Just a 3core+earth comes downstairs to the switch. So looks like it should be OK once I've re-done the kitchen lights.