Wiring safety & acoustic insulation

[PrenticeBoyofDerry]

If I was to consider up-grading the 4mm cable what do you think about jointing it in only at the studs where I want the insulation to go?

would that be permissible? I understood the whole circuit would need to be upgraded if the derating factor was applied?
can we not split the ring in to two separate radial circuits, this then allowing for 20Amp devices?

 

[lou1996]

So that appears to leave me up the creek as they say, I need to derate the fuse so that the derated current can never be exceeded, but the law will not allow lower value fuses? That sounds a little silly surely if the fuse is smaller it's safe?

It's the Wiring Regulations rather than an actual law, but there is an actual law requiring you to make reasonable provision in the design and installation of electrical installations in order to protect persons operating, maintaining or altering the installations from fire or injury, and the accepted way of doing that is to comply with the Wiring Regs.

Ring finals are a special case of cables rated at less than the protective device and supplying accessories and appliances in parallel but not complying with any of the requirements that parallel cables otherwise must, and that special case only admits 30/32A devices.

If I was to consider up-grading the 4mm cable what do you think about jointing it in only at the studs where I want the insulation to go?

You could do that, but wouldn't it be less aggro to replace the cable entirely? Or replace it between the first/last sockets before/after it enters the wall?

Or would hidden cable joints be illegal?

Not if crimped.

So to clarify as far as I understand:
I cannot fit a 20A fuse on a final ring?
I need to derate my 2.5mm cable if it is run inside conduit next to sound insulation inside a stud wall?
4mm is the only compliant/safe way to run a final ring if the cable is run inside conduit next to sound insulation batts inside a stud wall?

No, yes and yes respectively.

But - how much of the circuit runs in the walls? All the way through horizontally, or up and down from the floor? How serious do you need the sound insulation to be? Obviously not very or you'd have to be doing a lot more than stick in some rockwool, so could you create insulation free channels for the cables without the extra sound leakage being a problem?

If I was to consider insulation free channels for the ring cables what size should they be?

If I was going to split the ring into two radials what kind of derating should I use? Previous advise has suggested that running cables inside insulation in the studs I may need to de-rate by up to 55%, and based on what you have said about clipped 2.5mm being rated at 27A, that would mean I'd need something like 10A for each radial cct.
Is the choice of fuses for radial circuits unrestricted, or are there other rules that I should to be aware of?

On a general point, it is puzzling that the regs do not allow a fuse of less than 30A on a ring, when in this situation current will actually be shared between the two arms of the ring so it would appear even safer. Yet the regs allow a derated fuse to be fitted on a radial circuit, when all the current flows in the one cable? I can't see the logic in that. Is it because the regs assume a fault on the ring e.g. so that it behaves like a radial because a joint has failed, even then what's the difference from a radial??

 

[PrenticeBoyofDerry]

Ideally, if you can split the ring final in to two equal radials with divided load, providing the area covered by each radial circuits does not exceed 50m2, a 20Amp breaker can be used, as the derating factor for the cable run in conduit through insulation will allow 2.5mm2 T&E to carry 20Amps.

 

[lou1996]

Ideally, if you can split the ring final in to two equal radials with divided load, providing the area covered by each radial circuits does not exceed 50m2, a 20Amp breaker can be used, as the derating factor for the cable run in conduit through insulation will allow 2.5mm2 T&E to carry 20Amps.

Your suggestion sounds the best way forward as it is fairly easy and doesn't involve changing cables. Is the type of conduit regulated or can I use corrugated plastic conduit over these cables? I've already got some of this stuff and it's 20mm internal diameter, I brought it for sleeving copper pipe as it goes through walls etc

I understand it's the regulations, but I still can't understand why a 20A is allowed for a 2.5mm radial circuit but not for a 2.5mm ring main, is there an explainable reason for this?