who downrates 32a mcb to 20a for ring mains when cables under more than 100mm of loft insulation ?

Discussion in 'Electricians' Talk' started by ram, Dec 4, 2017.

  1. unphased

    unphased Screwfix Select

    This thread now clearly demonstrates exactly where I stand with Regs. We have two very good sparks, one with a practical and sensible application, one with a pedantic unrealistic application. I adopt the practical stance, every single time. I have never understood the need for such drastic measures advocated by the regulations where insulation covers cables. It is far too onerous and should be massively relaxed. Bazza explains this admirably.
     
    Dr Bodgit likes this.
  2. leesparkykent

    leesparkykent Well-Known Member

    A lot more than you by the looks of things ;)
     
  3. unphased

    unphased Screwfix Select

    This is how I see it. Cables must first be loaded heavily for them to get warm. Insulation retains warmth, it doesn't in itself make them get warm. The current passing along the conductor causes the heat, not the insulation. A house circuit will never get so hot as to be a problem. FACT. The IET have totally over estimated the effect of insulation on a circuit that is relatively lightly loaded. AFAIK no circuit is ever so loaded up it will overheat. This is in a domestic situation. In industrial applications it will reach high temperatures, not in a house it won't.
     
  4. leesparkykent

    leesparkykent Well-Known Member

    I personally don't see how you can deem it unrealistic. The Current carrying capacity of the cable is either adequate or not in relation to the tables in BS7671. BS7671 says the cable must have a current carrying capacity of not less than 20A when it comes to the application of a RFC on a 30 or 32A OCPD. If you reference the tables in BS7671 you are given the current carrying capacity of the cable with installation method taken in to account.
     
  5. Dr Bodgit

    Dr Bodgit Well-Known Member

    I reiterate my point about applying common sense...
     
  6. unphased

    unphased Screwfix Select

    I am not disputing the need for derating a cable, Lee, I challenge the overly zealous application of the derating. In a laboratory it would be proved, I am convinced of that. The derating of a cable if it passes through 400mm of its length in insulation is patently ridiculous. I have no confidence in what the IET publish because they spout though shalt without any reference to how they derive these onerous requirements. They should be revised to align with reality. Diversity of insulation if you like. They are always taking worst case never actual. Theory not practice. If someone can prove to me that a cable passing through just 400mm of insulation loaded to 20A overheats then I am more than happy to resind my view. I just cannot see it needs it, I really can't.
     
  7. unphased

    unphased Screwfix Select

    I challenge your statement that standard circuits require derating. I am sure I read in one of the guides that diversity of load is taken in to account.
     
  8. leesparkykent

    leesparkykent Well-Known Member

    I believe ERA technology carried out the testing which the IET base their CCC tables on when it comes to flat twin and earth installed with thermal insulation.
     
  9. Bazza-spark

    Bazza-spark Well-Known Member

    So you are now saying that for the first 16th editions of the regs they got it wrong and only the 17th is crrect? God help us with the 18th edition!

    Are you an electrical engineer, an electrician or a regs follower?
     
  10. leesparkykent

    leesparkykent Well-Known Member

    No you are trying to put words in my mouth to suit your argument/opinion.
    existing installations that have been constructed in accordance with earlier editions of the Standard may not comply with the current edition in every respect, but this does not necessarily mean that they are unsafe for continued use or require upgrading. How ever when carrying out periodic inspection and testing for the purpose of producing a detailed electrical installation condition report, you are providing an assessment of the in-service condition of the electrical installation against the requirements of the edition of BS 7671 current at the time of the inspection, irrespective of the age of the installation.
     
    Risteard likes this.
  11. spinlondon

    spinlondon Well-Known Member

    Wouldn’t you just install a larger CSA cable?
     
  12. Coloumb

    Coloumb Well-Known Member

    Um, so you go into a loft, see the cables covered in insulation. It's been like it for years. What code do you give it?
     
  13. Dr Bodgit

    Dr Bodgit Well-Known Member

    As you guys know I'm not a qualified spark..but in this situation I'd only "code" it if I knew the ring final was heavily loaded for significant periods of time and I could justify the "warning". 2.5mm T&E is good for 20A radials, right? So on a ring final that's 40A as there's 2 cables, and its already down rated to 32A via the MCB. 2.5mm is probably rated nearer 25A max, so 50A down to 32A, that's a significant down rating right there for starters.
     
  14. Coloumb

    Coloumb Well-Known Member

    The way I've understood things is that 7671 treats all cables as already running at their maximum capacity in that they are constantly running at 70 degrees. Obvs. is this was the case then yea, your gonna have a major problem if you start sticking them in loft insulation, but in a domestic, when are cables ever going to get that hot? To me it sames the same as de-rating cables that are "bunched". I've often seen posts where members ask should cables be de-rated when running several through a hole in a joist. A literal interpretation of the regs would suggest that de-rating is required, though in the real world that's nonsense.
     
  15. JP.

    JP. Screwfix Select

    At the end of the day you must adhere to the regulations surely unless you can inject a valid departure?- so if derating is indicated then this must be carried out within regulation parameters. Myself I see no get out clause with ref to said tbqh, although of course know that some departures can be written in the box on the cert..but not anything to do with derating which cannot be departed from.
     
  16. JP.

    JP. Screwfix Select

    In the real world who has actually used derating calcs in a dom situation?

    I rest my case.
     
  17. Dr Bodgit

    Dr Bodgit Well-Known Member

    I'd go along with that - comply or explain. Cables are already derated for example, so really the question requires some expert judgement: are the cables, given the overall circuit design, use, installation etc. safe enough? Are there mandatory regs that are breached and if so, can this breach be explained away? Just because a reg is breached doesn't mean its unsafe, however you'd better be bluddy sure its still OK.
     
  18. JP.

    JP. Screwfix Select

    Its a minefield Doc - but in reality I wonder how many sparks have used domestic derating calcs generally? I think it all boils down to common sense really.
     
    Last edited: Dec 5, 2017
    Dr Bodgit likes this.
  19. retiredsparks

    retiredsparks Well-Known Member


    common_sense_isnt_t_shirt-rb1a25615aa674b7d913f84da27598e29_k2gl9_324.jpg
    Rs
     
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  20. JP.

    JP. Screwfix Select

    Lol - Merry Christmas RS
     
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