Wiring safety & acoustic insulation

lou1996 · 18 Sep 2010

I want to fit sound insulation into the cavity in some studwalls in my property, and was planning to use mineral wool bats (e.g. from wickes).
My query is that since these bats also have thermal insulation properties, what should I do to ensure that the cables remain safe and do not gradually overheat when carrying load currents below the circuit fuse rating?
The studs carry both lighting cables (to/from wall switches) and mains ring cables (to the 13A sockets).

I had thought that perhaps conduit would be adequate since it would ensure an air path around the cables, but I'm not a Part P qualified sparky so need a little advise.

MalcofArabia · 18 Sep 2010

Yes your right to check. Though insulation can de-rate a cable upto as much as 55% it is normally thought that where small cable are used, as in lighting and power this should not affect them to much.

ie Even 1.00mm^ cable is rated at 11amps and derate it down to half and you still have 5.5amps and most breakers are 5 or 6amp

The same with 2.5mm^ really. It is only when you start to fit larger loads such as showers and cookers that you really do need to take care.

To be completely safe you can do checks by finding out what breakers the circuits are protected by and then amp read the circuits at various times and loads to see what the usage is.

This is really sometimes beyond the DIY'er and you may need to call an electrician in to be completely reassured.

PrenticeBoyofDerry · 18 Sep 2010

There is a derating factor that is applied to your cable when totally surround by insulation, this can lower your cables current capacity to half it's normal rating.
What size cables and devices do you have for your circuits? ie 1.00mm T&E for lights protected by 6amps and 2.5mm T&E socket (radial)protected by 20Amps.
Sometimes you can get away with not having to upgrade your cable, especially with the light circuit, some other power circuits are effected by it though.
If you can install the cables so they are not totally surrounded this will help. ie. one side of cable is touching plasterboard wall the derating will be reduced.
A little late with my post as "MalcofArabia" has covered most of it, but we both seem to be of the same train of thought

If the cable was run in conduit in an insulated wall, a 1.00mm T&E cables current capacity would be reduced from 16 to 11.5 amps and 2.5mm from 27 to 20 amps, 4.00mm from 37 to 26 amps.
This is assuming that the rest of the circuit has no derating other derating factors or other factors such as fuse types and grouping of cables need to be added.

ban-all-sheds · 19 Sep 2010

MalcofArabia said:
The same with 2.5mm^ really. It is only when you start to fit larger loads such as showers and cookers that you really do need to take care.

Rubbish.

You may not use a cable rated at less than 20A in a ring final.

2.5mm² clipped direct is 27A, so if it gets de-rated to 50% of that you're in trouble.

To be completely safe you can do checks by finding out what breakers the circuits are protected by and then amp read the circuits at various times and loads to see what the usage is.

What good would that do?

Ib ≤ In ≤ Iz

lou1996 · 22 Sep 2010

PrenticeBoyofDerry said:
There is a derating factor that is applied to your cable when totally surround by insulation, this can lower your cables current capacity to half it's normal rating.
What size cables and devices do you have for your circuits? ie 1.00mm T&E for lights protected by 6amps and 2.5mm T&E socket (radial)protected by 20Amps.
Sometimes you can get away with not having to upgrade your cable, especially with the light circuit, some other power circuits are effected by it though.
If you can install the cables so they are not totally surrounded this will help. ie. one side of cable is touching plasterboard wall the derating will be reduced.
A little late with my post as "MalcofArabia" has covered most of it, but we both seem to be of the same train of thought
If the cable was run in conduit in an insulated wall, a 1.00mm T&E cables current capacity would be reduced from 16 to 11.5 amps and 2.5mm from 27 to 20 amps, 4.00mm from 37 to 26 amps.
This is assuming that the rest of the circuit has no derating other derating factors or other factors such as fuse types and grouping of cables need to be added.

Thanks for your reply, my lighting circuits are 1.5mm & protected with a 6A MCB, the ring main is 2.5mm & has a 32A MCB.

The appliances that are fed from the ring main are: a washing m/c, Tumble dryer, a fridge/freezer and a chest freezer.

If I understand your comments correctly using a conduit in my situation is probably the best as it will derate the cable the least.

I've estimated the peak current draw from the ring at about 18A (2kW each for W m/c and Dryer + 150W for the two freezers), so using that logic would I be OK if:
I fit conduit around the 2.5mm cable, and change the MCB to 20A.
And I'm thinking that the fuse rating on the 1.5mm cable is already de-rated enough even without any conduit and the cable is buried in the insulation in places (although I'll try to get it next to the plaster board wherever I can).

How does that sound?

lou1996 · 22 Sep 2010

ban-all-sheds said:
MalcofArabia said:

The same with 2.5mm^ really. It is only when you start to fit larger loads such as showers and cookers that you really do need to take care.

Click to expand...

Rubbish.

You may not use a cable rated at less than 20A in a ring final.

2.5mm² clipped direct is 27A, so if it gets de-rated to 50% of that you're in trouble.

To be completely safe you can do checks by finding out what breakers the circuits are protected by and then amp read the circuits at various times and loads to see what the usage is.

What good would that do?

Ib ≤ In ≤ Iz

Thanks for your response,
I've considered what you have said and think I understand what you are getting at. My ring main cable is 2.5mm and so is rated OK.
Hence I think that I need to determine the de-rated current and check that this is greater than the max current that my appliances might draw, and if this is the case then I don't need to change cable. In that case then I can simply fit a fuse with a rating just enough to avoid tripping out under "normal" peak currents which should operate trouble free whilst protecting the circuit from currents being drawn which might otherwise cause the cable to overheat.
Have i understood the important factors, if you need more details of the actual situation, have a read of my reply to the other helpful guy.

Chri5 · 22 Sep 2010

lou1996 said:
How does that sound?

Fine, where possible containment in conduit would be better.

It's a curve ball, but have you thought about updating to 4mm TE as either 2 x 20 amp radials or 1 x 32 amp ring ?

If you have access due to lagging work why not, and lets be fair the majority would expect better than 20 amp service in an area which could warrant more.

PrenticeBoyofDerry · 22 Sep 2010

I to, would prefer the upgrade rather than dropping the fuse rating.

lou1996 · 23 Sep 2010

Chri5 said:
lou1996 said:

How does that sound?

Click to expand...

Fine, where possible containment in conduit would be better.

It's a curve ball, but have you thought about updating to 4mm TE as either 2 x 20 amp radials or 1 x 32 amp ring ?

If you have access due to lagging work why not, and lets be fair the majority would expect better than 20 amp service in an area which could warrant more.

Click to expand...

I feel that to try and up-grade cable is probably impractical as I'd likely have to pull the ceilings down (the studs are creating 4 small rooms in a side extension). Unless you mean that I should make cuts and joints so that 4mm is inside the studs only? But then isn't that inserting lots of potentially unreliable joints into the ring? I'd welcome your thoughts.

Also could I use the corregated plastic conduit, the type that can be easily retro-fitted? It's got 20mm inside diameter and I've got plenty of that so it would be convenient for me.

lou1996 · 23 Sep 2010

PrenticeBoyofDerry said:
I to, would prefer the upgrade rather than dropping the fuse rating.

For practical reasons (see other reply to Chri5), I feel that to up-grade might be just too much hassle. The rooms on the ring are really utility rooms, so I am thinking that 20A is probably never going to be a problem, there is heating in the rooms so I shouldn't ever need any electric heaters.
But as previous I'm open to your views as to the pro's and con's, thanks.

PrenticeBoyofDerry · 24 Sep 2010

If you install a 20Amp fuse the cable will be safe.

ban-all-sheds · 24 Sep 2010

Ring finals are not allowed to be on 20A devices.

30A fuses or 32A MCBs are all that are permitted.

PrenticeBoyofDerry · 24 Sep 2010

Lacking information, thought I had read someone ask about splitting the ring to a radial then a 20A can be used and still with 2.5mm.
apologys

lou1996 · 24 Sep 2010

ban-all-sheds said:
Ring finals are not allowed to be on 20A devices.

30A fuses or 32A MCBs are all that are permitted.

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?

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? Or would hidden cable joints be illegal? I feel a little uncomfortable about cable joints, but I'm probably being a bit paranoid?

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?

Can you confirm, please?

ban-all-sheds · 24 Sep 2010

lou1996 said:
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?

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Wiring safety & acoustic insulation

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