Prepping for rewiring an old flat...

Discussion in 'Electricians' Talk' started by Qwerki, Nov 18, 2018.

  1. Qwerki

    Qwerki New Member

    I just thought I'd gather a few additional opinions on my current (if you'll excuse the pun) project. Please point out any issues you think I will need to rethink!

    I'm renovating my flat which is a 60's ex-council jobbie. Has a nice modern CU, but all wiring is original R&B singles in metal conduit embedded in the concrete, so they're not exactly what you would call replaceable. All sockets throughout the flat (one single per room!) are on a single ring circuit, and I'd like to ultimately put in new circuits for each room so I have a bit of granularity when I need to switch circuits off. Having more modern wiring won't hurt either. Initial plan is to extend the ring from the existing socket in each room to the new sockets, then ultimately get a qualified sparky in to run new circuits into each room and one by one remove each room from the ring by crimping the old wires until I can get the old ring disconnected entirely.

    Main problem is how to run the cabling so it's unobtrusive. Ceilings are low, and floor and ceiling are concrete, so whilst I initially considered putting in a new floating wooden floor throughout, getting the required depth for strength and running wires underneath is going to be difficult and will require a lot of nonsense adjusting doors and worktops etc. Boarding over the ceilings with wires behind is also not viable given that the walls are also concrete (so chasing every socket from the ceiling prescribed zone isn't practical) and I have some unfortunate structural lintels (preventing running the wiring behind coving or boards throughout).

    So I'm planning to run my wiring around inside hollow skirting with mechanical protection (given it's not a prescribed zone as behind coving would be) - I'm weighing up using either standard 20mm metal conduit, 3mm steel plate on the inside of the skirting, or 3mm thick 20mm steel angle. I'm not sure if those last two qualify as installation method B or not though.

    No problem within each room as there's plenty of room for a cable or two - no, the main problem comes in the hallway where all the circuits will need to run together down to the CU for about 2-3m. Four rooms with two 2.5mm T&Es results in 8 cables, which is:
    1. Going to mechanically be a tight squeeze into the space available (given you can only fit at best 3 T&Es into a 20mm conduit as they're designed for singles - I might be able to fit 8 into a square cross-section with the steel plate options though, but meeting the 45% space factor requirement for conduit may well be impossible),
    2. ...going to result in heavy deratings regardless due to grouping.
    So one option I've investigated was to run each circuit as derated 2.5mm radials instead to halve the number of cables. Whilst in theory I could be fine with 20A MCBs protecting the circuits, 16A seems reasonably standard enough and would go a long way to mitigating the grouping factors. It's a small flat so high regular loads are extremely unlikely. Worst case I suspect I could run 4mm from the CU to each room and use 2.5mm inside, if required.

    Does this sound reasonable? I'm obviously going to have to get a certified sparky involved for putting the new circuits in (who will hopefully point out any problems!) as I'm not allowed to touch the CU, but I need to plan in advance the runs into all the other bits I'm doing myself now with the walls etc....speaking of which, I have central heating pipes running near these wiring runs (potentially in the same skirting rebate). As temperature is the given reason for applying derating factors I want to make sure things will be safe, but I've not found proximity to hot pipes or heat sources mentioned in anything I've read. Is it sensible to just take the max external temp of the pipes as the ambient temp for the purposes of any calculations of what's viable?
     
  2. Comlec

    Comlec Screwfix Select

    You need to rethink the order of events.

    1. Contact sparky (Electrical Contractor)
    2. Agree design with with sparky
    3. Discuss with sparky a division of labour if they are agreeable.
    4. Sign the contract
    5. Start the work

    Not getting an electrical contractor on board before you start could cause you big problems further into the project.
     
  3. JP.

    JP. Screwfix Select

    The electrician who is going to carry out the job will advise accordingly Qwerki.
     
  4. They are very replaceable therefore wiring your lights will be very easy for an experienced Electrician.
     
  5. JP.

    JP. Screwfix Select

    I wonder how many people actually have used derating calcs within domestic rewire etc environs? Only interested.
     
  6. Comlec

    Comlec Screwfix Select

    Me. On a recent domestic rewire when reusing existing lighting conduit.
     
  7. baldelectrician.com

    baldelectrician.com Well-Known Member

  8. peter palmer

    peter palmer Screwfix Select

    If it was me I would get the flat back to just a shell, batten down the ceilings with 2x2 and board over.

    How are the walls solid concrete if the original sockets and wiring are below the surface, surely there is plaster on the walls to chop into? If not then dry line the walls with plasterboard.

    Projects like this really need to go back to basics, anything else ends up being a bodge. I would also renew all the plumbing whilst I'm at it. How is the place heated?
     
  9. Qwerki

    Qwerki New Member

    Thanks, all. Taking you each in turn:

    I've a few sparkys in my family who have advised it sounds reasonable, that's why I thought I'd fish around for a few more opinions so I'm not wasting whoever I get in's time with nonsense - I've not formally got anyone on board yet because I don't envision getting to the point I'll be able have the new circuits put in for quite some time, and adding new sockets to the existing ring in the meantime is fine for me to do.

    I'll take your word for that! There's an existing disused circuit that's been left in-situ, and when an earth was added for bonding the central heating it was done in mini-trunking on the surface. I assumed with all the hidden bends and the fact the existing wires don't feel like they have any give in them pulling through new ones was a no-no. If they can be pulled through though that perhaps opens up options for getting runs into the existing rooms. I'll discuss with my sparkys. :)

    I couldn't tell you the name of concrete construction. It's not Bison beams though - it's a high-rise so the flat's built up of large overall boxes, and the "lintels" I'm referring to are very clearly structural so I'm not going anywhere near them whatsoever. There's one type over the doorways in the hallway (I suspect bracing the box the bedrooms are in to the central building core) and the other is a strange section of ceiling the length of both bedrooms a couple of feet wide and three of so inches deeper than the main ceiling. Given it's sitting on top of a concrete support column between them I'm fairly confident it's supporting the floor of the concrete box of the bedrooms above mine. So also, I'm not touching that whatsoever because I'm not mad.

    I've got a EICR from when I purchased the flat, so I have a rough idea of what state things are in. It says the installation "appears" to be about 25 years old which would place it as mid-90s? So perhaps Deleted member 11267's post above is correct and the wiring has been replaced from the originals. I'm still suspicious though.

    I have indeed taken both rooms I'm working on back to a shell. It's complicated as I'm living in it, which limits things to a point.

    The ceilings are covered in old Artex, so I think drilling to fix battens is probably a no-no - plan is to get it and the walls all skimmed over.

    The rooms are dot and dabbed onto the concrete, and the existing electrical boxes are set into the concrete - I suspect they were all in place when it was poured given the little pile of lumpy concrete under the hole above the CU where the wires vanish into the ceiling concrete :)

    I am indeed looking at getting the plumbing altered as well - it's all centrally heated. Combi in the kitchen, 22mm feeding 3 rads, then 15mm feeding another 3 rads. One of the things I'm hoping to achieve in the project is hiding those pipes - the central heating was retrofitted over the skirting in the early 90s as I understand it - I suspect the disconnected wires I've found are from the long-gone original electrical storage heaters.
     
  10. Qwerki

    Qwerki New Member

    Inspired by this conversation I went back and had another look at the wires entering the enclosure where the CU is as I couldn't reconcile the narrow diameter I remembered the "conduit" having. ...and I now realise what they are - they're not conduit at all, they're mineral-insulated copper-clad cables, and presumably they must be embedded directly in the concrete. Was that a standard construction method in the 1960s for high rises?
     
  11. Bazza

    Bazza Screwfix Select

    Yes, MICC often used. But it does leave you without the option of drawing new singles into conduit that doesn't exist.
    Plasterboard/Dot and dab the walls would be best, but ceilings you'll need to batten out. That's not too bad for reasonably thin lekky cables.....but then you need to decide how you manage and conceal the plumbing services.
     
  12. Sometimes the lights were wired in Pyro and the power was in conduit.
     
  13. Mr Rusty

    Mr Rusty Screwfix Select

    If you use a decent outfit I wouldn't worry about the time. I had 18 months between first and second fix - no issues. Comlecs order of events is a good start. My experience with a tricky job is be upfront, trust your sparky, don't try and cut corners - be worthwhile offering to pay your sparky for a documented design if it helps you organise the rest of your project - nothing worse than installing a floor/ceiling and your sparks coming along after and needing to open it up!. On my job I set all the back boxes and cut many holes and chases ahead of first fix which my guy was more than happy to let me do.
     
  14. peter palmer

    peter palmer Screwfix Select

    I first fixed and extension across the road from me in May 2016, he's still not had it plastered.
     
  15. Sparkielev

    Sparkielev Screwfix Select

    I suppose you could put coving up and run your cables that way, as for the lighting I would batten the ceiling
     

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