Metal CUs and TT

Discussion in 'Electricians' Talk' started by Garry Ingham, Sep 16, 2018.

  1. Garry Ingham

    Garry Ingham Member

    Installing my first TT domestic. I love hagar consumer units but with this stupid amendment three on metal consumer units.( not brilliant with TT) too messy drilling for the 32mm entry gland. Does any one know what other c CU s there is out there.that has the 32mm cut out for the tail entry gland
     
  2. seneca

    seneca Screwfix Select

    Contactum, I've been using them since AMD3 started.
     
  3. peter palmer

    peter palmer Well-Known Member

    Whats wrong with drilling a hole in them, I do it all the while. Denmans curve range have a 32mm knockout but only on the sides, the ones top and bottom are square knockouts, I wonder what genius thought of that idea. I like the boards other than that so I tend to drill my own hole for the gland.

    Personally I use these, cheap as chips and will drill an RSJ if needed.

    https://www.amazon.co.uk/gp/product/B075ZQPSV7/ref=oh_aui_detailpage_o06_s00?ie=UTF8&psc=1

    Only downside is the depth isn't very much on them and the pilot drill will break when the cutter bites as the pilot puches through, I tend to remove the cutter, drill with pilot only and then put the cutter back.
     
  4. Bob Rathbone

    Bob Rathbone Well-Known Member

    Does amendment 3 actually state that the CCU must be metal, or does it state 'Non combustible' or a similar term.
     
  5. unphased

    unphased Screwfix Select

    I use the WISKA TKEP40 version and its a 40mm hole for the cable entry gland. This is the size you need for 2 x 25mm2 tails and a 16mm2 earth through the same hole.
     
    Last edited: Sep 16, 2018
  6. spinlondon

    spinlondon Well-Known Member

    It’s TT, no need for a 16mm2 earth.
    Come to think on it, I doubt any normal domestic installation would ever require a 16mm2 earth.

    Yes Bob, it does say non-combustible material.
    It also says they must comply with BS EN 61439-3.
    If you can find a BS EN 61439-3 compliant CU which is not constructed of metal, then you can use it.
     
  7. unphased

    unphased Screwfix Select

    They don't make a gland specifically for a TT tails arrangement. You are of course correct, you only need a 2.5mm2 main earth on to a rod and that's all you used to find. Its surprising how many 16mm2 are attached to rods though. 16mm2 earth is required for PME earths only. Its not needed for TN-S or TT.
     
  8. seneca

    seneca Screwfix Select

    I use 10mm from c/unit to the earth rod on TT, I know even that's a bit ott but I like the mechanical strength of 10mm compared to smaller sizes.
     
    retiredsparks likes this.
  9. spinlondon

    spinlondon Well-Known Member

    Not aware of any requirement in BS7671 for 16mm2 earths.
    Minimum CSA where PME conditions apply, is 10mm2.
     
  10. spinlondon

    spinlondon Well-Known Member

    Minimum CSA is 4mm2 where there is no mechanical protection.
     
    retiredsparks likes this.
  11. Sparkielev

    Sparkielev Well-Known Member

    2.5mm only if mechanically protected, I generally use 10mm
     
    retiredsparks likes this.
  12. peter palmer

    peter palmer Well-Known Member

    I used a 10mm once on a job that was TN-S as its all I had. Some months later we used the job for an assessment and I'd completely forgotten about the earth. Assesor made us replace it there and then with 16mm siting no iffs, no butts it had to be at least half the tails CSA. Pointless arguing with him so we just changed it.
     
  13. spinlondon

    spinlondon Well-Known Member

    Yep 2.5mm2, a sheathed single.
     
  14. seneca

    seneca Screwfix Select

    That's how I understand it Peter, I only use 10mm on TT systems.
     
  15. peter palmer

    peter palmer Well-Known Member

    You can use the adiabatic equation to find the minimum earth size, in my case a bit of 2.5 earth would have done the job safely despite what any petty rule might say to the contrary.
     
  16. unphased

    unphased Screwfix Select

    Main earth conductor on a PME is 16mm2 if used with 100A cut-out and 25mm2 tails. I can't quote the Reg but I am confident that's the required size on a PME. It could even be a DNO requirement. The minimum CSA on a TT would be 2.5mm2 subject to the 'other' requirements being met. It always brings out the pedants when you start mentioning this that and the other. :rolleyes:
     
  17. retiredsparks

    retiredsparks Well-Known Member


    Never heard of stepped cutters ?
    Much neater and faster.
    auto hole sizes...us "professionals " have been using them for years :cool:
    RS 71ACCZXmaFL._SL1027_.jpg
     
  18. peter palmer

    peter palmer Well-Known Member

  19. spinlondon

    spinlondon Well-Known Member

    There is no Regulation stating 16mm2 in BS7671.
    The minimum CSA quoted in BS7671 for where PME conditions apply is 10mm2.
    10mm2 will satisfy the adiabatic equation for almost all protective devices on 100A supplies.
    Might be one or perhaps two devices which would require a larger CSA.
     
  20. unphased

    unphased Screwfix Select

    Unless there are calculations available to show how the csa has been derived, table 54.7 is the default method. That is where the 16mm2 comes from. 543.1.4 Where it is desired not to calculate the minimum cross-sectional area of a protective conductor...
     
    Last edited: Sep 17, 2018

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