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. retiredsparks

    retiredsparks Well-Known Member

    I suspect that during 1942–1947 when rings were introduced into UK and other countries.......insulation was not taken into account.
    The average standard home (house) had ring in the floor void and fed up and down sockets.(no insulation)
    The attic had lighting ccts...no insulation.
    Bungalows might have rings ccts in attic...no insulation.
    Now insulation thrown in everywhere.
    If today we were designing a new cct...using the calcs would we take into account the derating factor of 100mm to 200mm full insulation ?
    I appreciate the difference between theory and practice.
  2. JP.

    JP. Screwfix Select

    I mean normally the Ring/Radial final is not run in the loft and dropped to sockets etc, maybe the derating is more applicable to lighting circuits which are broke@6amp normally and probably are derated enough at 1mm, however maybe 1.5 might be indicated in some insulation scenarios
  3. leesparkykent

    leesparkykent Well-Known Member

    One that I deep appropriate to the situation/installation.
  4. leesparkykent

    leesparkykent Well-Known Member

    I design installations, whether that be domestic, commercial, industrial or agricultural taking correction factors into account.
  5. retiredsparks

    retiredsparks Well-Known Member

  6. Comlec

    Comlec Well-Known Member

    In the UK the fixed wiring has been over-engineered for years. Probably why there are so few fires and electrocutions caused by fixed wiring. Most of the problems tend to originate from poor workmanship rather than poor design. I would be more worried about mechanical damage and the mice in the loft. But if the regs say stand on your head and the NICEIC add you must do so whilst wearing a tutu, then that's what must be done :):).

    *** If cutout seals are always missing then surely, during inspection, lofts are inaccessible (often due to the presence of asbestos, or padlocks)
  7. Bazza-spark

    Bazza-spark Well-Known Member

    If you go back to the 1940's and 50's when there was very little in the way of heating and people were using electric fires with lots of bars running in different rooms then there was a possibility of overloading. Over the years insulation in walls etc has improved, central heating in some form or other, double glazing and the like have all helped reduce demand. No more 100 watt tungsten lamps, replaced with energy efficient LED's. Loading has reduced dramatically.

    As with all things, safety margins are applied to cables too. If a cable is rated at 27 amps, it is probabaly capable of carrying 40 amps indefinitely. Derating is applied by the manufacrturers and others to give a sensible safety margin.

    If we have to say that in a new installation a cable has to be derated because it is going under 200mm of loft insulation so be it, however when carrying out an EICR on a property that has had insulation thrown over it after being in service fo a number of years do we have to tell the owner of the property it needs to be rewired with bigger cables because they have added insulation? Don't think so!

    Common sense must prevail. If you can see the installation is suffering because of deterioration then maybe the advice should be to look to a rewire. If there is no sign of damage or deterioration and the installation complied with the regs at the tie of installation as far as can be determined what is the problem?
  8. spinlondon

    spinlondon Well-Known Member

    I think the question should be:
    What numpty would cover wiring with more than 100mm of loft insulation?
  9. retiredsparks

    retiredsparks Well-Known Member

    Just about everyone.....lol.....
    270mm is now recommended by some councils in lofts.
    Dr Bodgit likes this.
  10. Dr Bodgit

    Dr Bodgit Well-Known Member

    270mm is the required amount to come "up to standard" these days, so its more than just some councils.
  11. Mr Rusty

    Mr Rusty Well-Known Member

    I am in the process of restoring an old house. So far electrics are up to first fix. My sparks (bona-fide multiple-employee contractor - not a one man band) has installed wiring in the loft, knowing exactly what I will be doing. I've now installed 100mm between the joists and cross rolled 200mm over them, so my wiring is under plenty of insulation, and the ring is on a 32MCB. Admittedly not much up there - lighting circuits and a short section for a socket outlet in the attic. I am not going to start questioning a professional sparks.............I have to presume it is all OK :)
  12. Dr Bodgit

    Dr Bodgit Well-Known Member

    Noooo! Its a fire hazard, your house will burn down, rip the lot out!!!
  13. retiredsparks

    retiredsparks Well-Known Member

    I was being "diplomatic"....before some n'erdowell numpty started jumping up and down screaming that there is a council somewhere that is happy with 150mm...
    Dr Bodgit likes this.
  14. Coloumb

    Coloumb Well-Known Member

    Lighting ctts on a 6 or 10a breaker generally don't need de-rating as the cable/breaker ratio is already pretty much half the ccc of 1mm/1.5mm
  15. Dr Bodgit

    Dr Bodgit Well-Known Member

    I know ;):p
  16. JP.

    JP. Screwfix Select

    Nice one
  17. JP.

    JP. Screwfix Select

    4.3.6 - Thermal insulation correction factors

    The use of thermal insulation in buildings, in the forms of cavity wall filling, roof space blanketing, and so on. is now standard. Since the purpose of such materials is to limit the transfer of heat, they will clearly affect the ability of a cable to dissipate the heat build up within it when in contact with them,

    The cable rating tables of [Appendix 4] allow for the reduced heat loss for a cable which is enclosed in an insulating wall and is assumed to be in contact with the insulation on one side. In all other cases, the cable should be fixed in a position where it is unlikely to be completely covered by the insulation. Where this is not possible and a cable is buried in thermal insulation for 0.5 m (500 mm) or more, a rating factor (the symbol for the thermal insulation factor is Ci) of 0.5 is applied, which means that the current rating is halved.

    If a cable is totally surrounded by thermal insulation for only a short length (for example, where a cable passes through an insulated wall), the heating effect on the insulation will not be so great because heat will be conducted from the short high-temperature length through the cable conductor. Clearly, the longer the length of cable enclosed in the insulation the greater will be the derating effect. {Table 4.5} shows the derating factors for lengths in insulation of up to 400 mm and applies to cables having cross-sectional area up to 10 mm².

    Commonly-used cavity wall fillings, such as polystyrene sheets or granules, will have an adverse effect on p.v.c. sheathing, leeching out some of the plasticiser so that the p.v.c. becomes brittle. In such cases, an inert barrier must be provided to separate the cable from the thermal insulation. PVC cable in contact with bitumen may have some of its plasticiser removed: whilst this is unlikely to damage the cable, the bitumen will become fluid and may run.
  18. Bob Rathbone

    Bob Rathbone Well-Known Member

    Who would put the cables under the insulation? I would do all I can to avoid this situation, run them on top, clip to the truss stabilizer woodwork, and when they come down through the insulation, run them through a 100mm short bit of pipe. All of this would only be necessary with power circuits, not lights with 1mm 2 and a 6A breaker as even a 50% derating would leave the cable in range of protection by the CB.
  19. retiredsparks

    retiredsparks Well-Known Member

    Bob, most insulation is in a retrofix situation...and the cable slack is not usually enough to do this.
    But on new builds it's possible.
  20. Pollowick

    Pollowick Well-Known Member

    Would that be a short length of something like 20mm overflow? or real conduit? :p

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