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.
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.
Thanks for the clarification guys - now to get laying the rockwool, not my favourite job by any means
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.
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.
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.
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.
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?
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.