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Like for Like Replacement of Steel Conduit

Discussion in 'Electrics UK' started by TedBk, 14 Feb 2020.

  1. TedBk

    TedBk

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    I intend to replace 20mm steel conduit that has rusted in existing floor screed. It probably has two 2.5mm singles red and black probably from the 1960s.

    The conduit is pretty straightforward it is just one 5m length from one side of the room to the other. But I will use galvanised this time.

    My question is should I just use two singles or use regular 2.5mm T&E, the new conduit MUST remain the primary CPC.

    And finally as it will be a tight fit without doing further damage to the installation. I intend extend the thread on one of length of conduit so that I use the coupler in a sliding fashion with a locknut so that I get the 5mm piece in place and then adjust the coupler.

    This will leave one thread exposed (ungalvanised) how should I protect the steel? Or is there a smarter way to this?
     
  2. securespark

    securespark

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    Have you got any pictures of the situation?
    Must the conduit remain the CPC?

    If, for example, the conduit is linking one socket to another, could you use PVC, but run a cpc connected to the boxes at either end?
     
  3. flameport

    flameport

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  4. Adam_151

    Adam_151

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    Bear in mind, that if it was installed in the 60s then it won't be 20mm. It'll be 3/4" which looks almost the same until you look closely, You can get coverters both ways around, put remember if you are wanting to cut and joint onto an existing length, then you'll need a 3/4" die to thread it, then you put the conversion on and then a 20mm coupler or besa box. I'd avoid using the conduit as CPC if you can(as others have said), suppliment the new length with an internal CPC. Use of the conduit as the CPC requires all joints to be tight, sound and have paint removed from under them, and the proper equipment to test it was sound is obselete now. And even from back in the day, I still see instances where it never had continuity from when it was installed 40 odd years ago
     
  5. Risteard

    Risteard

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    Don't use twin and skin. Singles are the only correct cable to use (with an additional cpc single cable if desired).
     
  6. SUNRAY

    SUNRAY

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    Twin and earth is fine but not necessary.
     
  7. Risteard

    Risteard

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    It's not fine. It's completely the wrong cable type for containment. I would repudiate its use in conduit in the strongest possible way.
     
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  9. SUNRAY

    SUNRAY

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    Why?

    It's only a cable, it won't be unsafe, it won't work any differently to singles.

    What particularly makes it wrong?
     
  10. JohnD

    JohnD

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    in the same way that there is a "correct" way to hold a teacup, or to take the top off an egg.

    What colour waistcoat do you wear in the enclosure on race days?
     
  11. TedBk

    TedBk

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    Thanks for all the replies. I will use 2.5mm stranded cable singles. (for conduit)

    I have found some elbows, which will save me having to borrowing a bending machine. If I succeed in getting it all fit without having to leave any exposed threads. Is it necessary to still use something to cover the threads that I cut myself with http://www.zinga-uk.com/product-range/zinga to maintain the corrosion protection of the galvanisation. Or is Zinc on the coupler sufficient?
     
  12. Simon35

    Simon35

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    Without pictures it's not that clear what you are doing, but it all seems a lot of effort to replace a bit of conduit. Can you not just thread a bit of T and E through, in a plastic conduit if you prefer?
     
  13. sparkwright

    sparkwright

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    Too much effort using metal conduit.

    Putting the elbows on is a bad idea anyway.

    It means drawing new cables in could be difficult, as you don't have a nicely formed bend to draw cables in without damaging them.

    PVC conduit is the way to do this, with live neutral and earth singles, and get the earth from the back boxes. A bending spring will help you to form your own bends.

    Was the old conduit rusted to the point that there were holes in it??

    Since the whole installation is likely to be rusty conduit, why not just replace the wiring only to include a separate earth wire.

    That may solve everything.
     
  14. TedBk

    TedBk

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    This piece of conduit is only in the floor screed i.e. less than 50mm from surface. The rest are burred in concrete in the slab floor, ceilings and walls etc etc. I would say it ended up like this because they made a mistake in the construction and need to get cable from one side to other.

    Anyway it is totally rusted. i.e. to the point that in the middle there was almost nothing left. This was caused by leaky balcony window in the building.

    The rest of conduit seems fine, I am fixing the floor anyway and the leaks. I just want to put it pack to the same standard it was 50 years ago.
     
    Last edited: 18 Feb 2020
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