Rockwool Against Cable - Verification Needed Guys

Discussion in 'Electricians' Talk' started by JP., Oct 13, 2017.

  1. JP.

    JP. Screwfix Select

    Morning lads. Just a very quick question. In my loft I have single cables going to lights (single t+e from loop in box) max current no more then what 10 watts (led lamps) Cable is clipped to joist.

    Ok to bring rockwool onto the cable?

    Thanks lads.
     
  2. CGN

    CGN Screwfix Select

    No probs with your proposals JP...stack it up :)
     
  3. Dr Bodgit

    Dr Bodgit Super Member

    I would guess the majority of homes have that in their loft, including mine.
     
  4. Bob Rathbone

    Bob Rathbone Screwfix Select

    The cable will probably be 1mm with a current carrying capacity of around 12A, now if you assume it is completely surrounded by insulation and apply a 50% de-rating, that makes it 6A, the breaker is probably 6A so it will be OK. It is not the load that is important here, it is the rating of the protective device that must not exceed the cable rating after correction factors have been applied.
     
  5. Dr Bodgit

    Dr Bodgit Super Member

    In my case the cable is 1.5mm :)
     
  6. JP.

    JP. Screwfix Select

    Thanks for the clarification guys - now to get laying the rockwool, not my favourite job by any means
     
    Dr Bodgit likes this.
  7. Dr Bodgit

    Dr Bodgit Super Member

    I used Knauf Earthwool JP, no need for a mask even.
     
  8. JP.

    JP. Screwfix Select

    Thanks Doc - hopefully we will be moving in the near future so will remember that one.
     
  9. Risteard

    Risteard Well-Known Member

    No, surely it is the design current. Having a 63A protective device does not make a circuit draw 63A if it has a 100W load connected.
     
    leesparkykent likes this.
  10. Dr Bodgit

    Dr Bodgit Super Member

    Having a 63A protective device means that a maximum of 63amps can potentially be drawn, so the cable with whatever derating is applied, has to be able to handle that.
     
  11. Dr Bodgit

    Dr Bodgit Super Member

    You can then hopefully do up your new loft like mine!
     
  12. retiredsparks

    retiredsparks Super Member

    No Mr Bodge...JP likes to do his jobs properly !.....:p
    Rs
     
  13. Risteard

    Risteard Well-Known Member

    No. The cable needs to be protected against overload (where applicable) and short circuit.

    The derating need only be applied to the load the cable will carry - the protective device is not relevant. It is simply there to ensure that ADS occurs. Otherwise we wouldn't be allowed to omit overload protection under any circumstances (meaning grossly oversized cables), however we are.
     
    leesparkykent likes this.
  14. Bob Rathbone

    Bob Rathbone Screwfix Select

    It is the reduced current carrying capacity of the cable that is of paramount importance, this will limit the maximum design current for the circuit and ultimately, the rating of the protective device which must be large enough to carry the intended load, yet small enough to protect the smallest conductor in the circuit (before any secondary fuses). The fuse or CB must protect the conductors, after de rating has been applied. Remember that the issue is heat build up due to insulation, if not by selecting the protective device correctly, how else would one limit the design current to within the safe limits for the conductor.
     
  15. Dr Bodgit

    Dr Bodgit Super Member

    That's more along the lines of what I was thinking.

    Say you typically have a radial of 2.5mm T&E and a 20A MCB (lets assume the max current such a cable can take is 20amp). If the cable is covered in insulation so is de-rated by 50% and can only now carry 10Amps surely you'd need to fit a 10Amp MCB?
     
  16. Bob Rathbone

    Bob Rathbone Screwfix Select

    Yes, or more likely you will increase the cable size and keep the 20A MCB.
     
    Dr Bodgit likes this.
  17. Risteard

    Risteard Well-Known Member

    Yes your Cc; Ci; Cg; Ca; Cr factors are applied for the cable sizing. But the rating of the protective device is not necessarily the design rating of the circuit. (Except perhaps with a circuit serving socket outlets as the loads are generally unknown.) But these factors are applied to the design current as this is what the cable will be carrying.
     

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