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Topic Title: "Top-up loft insulation"
Topic Summary: Unsatisfactory inspection report outcome
Created On: 31 October 2009 10:19 AM
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 31 October 2009 10:19 AM
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deadtester

Posts: 32
Joined: 21 March 2008

I'd welcome information from anyone who's been involved in this issue where local authority or housing association properties have been concerned.

A single storey housing association house recently inspected is wired with all circuit cables (T&E) running through the roof space, properly installed, clipped to joists etc.

When the installation was originally done the design cable sizing for the required circuit capacities was ok for the installation method, which was equivalent to method 100 now, at worst.

However, since then 150mm of additional thermal insulation has been rolled out on top of the lot - equivalent to method 101 at best, or 50% derating required at worst ......

This is clearly an unsatisfactory situation which invalidates the original design cable size and protection provision regarding load current capacity and/or overload protection.

I'm considering advising the client to remove the additional insulation to render the electrical wiring satisfactory again. Not all circuits are affected, but otherwise rewiring of the ring circuits, cooker circuit and shower circuit could be necessary in this example, if the insulation is to stay !

I foresee any such recommendation causing trouble, and this has to be a common situation where housing bodies have ordered top-up insulation to be installed in single storey properties.

Any experiences or advice please?
 31 October 2009 10:33 AM
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Testit

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Wouldnt worry too much...

Ring circuits, well I recently donwgrading one due to thermal insulation on installing a new board, coupled with poor installation methods I felt it necessary...

Given the diversity of domestic houses its unlikely to overheat.. showers only run for short duration so wont get anywhere near warm... though loft spaces can get hot in the summer seasons mind, the size of cables you would require if you halve all the cable capacities myay make it impractical...

Worst case scenario...
6amp breakers 1.5mm OK
32amp rings... maybe knock them down to 20amp...
20amp radials... stick on a 16
Shower cable.... wouldnt worry not on long enough..

Otherwise just recommend periodics to ensure no issues....

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Online Services - http://propertydevelopment.org.uk

Experience can sometimes show that cost prevails over quality and safety, such little self-value that people hold.
 31 October 2009 10:35 AM
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sparkiemike

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1mm2 T&E is rated at 14A, so 50% of that is 7A, so your 6A MCB or 5A fuse shouldbe OK

I presume this problem is mainly on the socket outlets, depending on the loads, could you not de-rate the cable by changing the CPD to one with a lower current rating e.g. from 32A to 20A?.
 31 October 2009 10:37 AM
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lamps

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could you not just box in the cables
 31 October 2009 11:51 AM
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lamps

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hi can someone where its says to derate cable by 50%
as i can see method 101 says 1.5mm can be 13a
 31 October 2009 11:51 AM
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normcall

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I think you will find the contract for the loft insulation installed under the government scheme says that cables must be moved above the insulation.
My heart always sinks when I open a loft hatch and see a sea of yellow insulation!

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Norman
 31 October 2009 12:09 PM
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briggsy6

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Originally posted by: lamps

hi can someone where its says to derate cable by 50%


Page 104 big red book - two paragraphs above table 52.2 (Sorry, don't have regs book in front of me right now, or I'd quote the exact reg number.)

 31 October 2009 01:14 PM
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lamps

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cheers briggsy
so is the relationship here the higher the op temp the higher the resistance of the conductor
 31 October 2009 02:45 PM
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Testit

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lamps
You shoul dbe able to find in the regs that anything over 0.5m of insulation should halve the cable capacity.. the reason is that the cable will be unable to dissipate its heat under loading, so its rated value is subject to deration so that it doesnt heat up so much to maintain the safety of the cable... If you have osg its table 6b page125..

For domestic it doesnt make much difference if you derate by putting a 20amp for rings, 16 for 2.5radials etc, as unlikely they will ever trip.. loaded kitchens will still likely hold on a 20amp.. even for heavier circuits remember to derate the cable capacity not the breaker capacity installed then you usually dont have to derate by much....

Edit: failing that you can install thicker cable where it runs through the insulation only..

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Experience can sometimes show that cost prevails over quality and safety, such little self-value that people hold.
 31 October 2009 02:57 PM
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lamps

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yes i read that bit. just was a bit unsure why.so its because it will stay hotter for longer reducing its life span
thanks testit
 31 October 2009 03:46 PM
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Testit

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or it will overheat if run at full loadings as it cant dissipate its heat normally,,

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Experience can sometimes show that cost prevails over quality and safety, such little self-value that people hold.
 31 October 2009 04:04 PM
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rocknroll

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Okay I watched this thread and now from a Building Services perspective, generally the type of insulation used in the loft and the walls of the interior of the house is not necessarily thermal insulation, it tends to be low density insulation with some thermal qualities, if you were to use high density thermal insulation then you would have problems like SBS, your ceilings would start to drip, thermal insulation would normally be located in the roof space, outer walls and under the floor. The feelings generally is that domestic installations are robust enough without any need to derate cables anywhere near 50%, the air to material ratio in loft and partition insulation is high so dissipation of heat which is low is not a problem also cables tend to clipped to beams or laying on the plasterboard so heat is dispersed this way as well, loft insulation is fine for eliminating a certain amount of heat loss from the space below but in the insulation and above the insulation it is not so good. There have been odd occasions where cable has suffered some effects in insulation but this is rare, the general feeling is the cable was overheating from a fault, overload or poor connection anyway and not a result of normal operation.

Aside from this, I was with a surveyor quite a while back on a development and this subject came up, he said, "I wouldnt cover the cable with insulation", and when the electrician asked why, he said, "well, you wont be able to find them afterwards".

I personally feel that the lighting and ring main will suffer little effect and perhaps if you doing a rewire maybe a cooker or shower cable might be best left exposed as they tend to get a bit loaded sometimes, but on a PIR just mention this fact rather than suggesting everything has to be derated to 50% and get laughed at by others.

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------

Edited: 31 October 2009 at 04:24 PM by rocknroll
 31 October 2009 04:28 PM
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lamps

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but if regs say it we have to adhere to it
 31 October 2009 04:56 PM
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rocknroll

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Originally posted by: lamps

but if regs say it we have to adhere to it


The regs also give you a value to work to 0.1W/m²K and most surveyors will tell you most of the rubbish out there will not come anywhere near this value when installed.

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------
 31 October 2009 05:23 PM
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Testit

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Wont disagree with you there R&R, though it could say something about initial design in that 20amp rings in domestics would normally suffice, and if for some reason there was some serious insulation installed no problems...

Out of curiosity.. you say it comes nowhere near 0.1W/m²K .. would you be able to state the actual value for wall and loft insulation methods?

Edit: Time to get me thermocouple out on a hot summers day..

-------------------------
Online Services - http://propertydevelopment.org.uk

Experience can sometimes show that cost prevails over quality and safety, such little self-value that people hold.

Edited: 31 October 2009 at 05:30 PM by Testit
 31 October 2009 05:26 PM
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mitten

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Rearrange these:
Worry too much you!

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Jason
 31 October 2009 05:40 PM
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rocknroll

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You will find this in Part L1B where there is threshold and minimum values, in existing buildings it is often difficult to achieve anywhere where they recommend.

The trouble with loft insulation it is very poor and water vapour and air move through it fairly easily, but if you started using rockwool, sheepswool etc; in lofts and inner walls then there can be associated problems where ventilation would be required, the main areas where thermal insulation is used is roof space, outer walls and floors but obviously you still need some form of minor insulation in dividing walls and ceilings for a measure of heat loss and acoustic reasons.

Its take some doing to achieve anywhere near 0.1W/m²K.

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------

Edited: 31 October 2009 at 05:54 PM by rocknroll
 31 October 2009 05:49 PM
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Testit

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That's a no then..

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Experience can sometimes show that cost prevails over quality and safety, such little self-value that people hold.
 31 October 2009 09:42 PM
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frspikeyhead

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Remember that the de-rating values applied to current carrying capacities presume full load conditions under a length of time. Find me a domestic where you can say for example that the upstairs lighting circuit is anywhere near full load or ring circuits other than a kitchen. The only circuits concerned are generally instantaneous water heaters, showers, but even then they are only used for a short duration of time unless you live in my house with 3 women.
 01 November 2009 01:53 PM
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deadtester

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Thanks for all the interesting comments.

That there's probably no actual problem of cable deterioration in a domestic situation, is my feeling too,
especially if loft insulation generally doesn't cause cable heating to the extent that the regs say. (ref. R&R above).

Also, the responsibility for the situation lies with the installer of the insulation and/or the client for not properly considering possible adverse effects. (ref. normcall's comment above).

However, in the context of a PIR, all non-compliances with current wiring regs should be identified first, and then advice should be given on what (if anything) needs to be done about them. i.e. coding 1 to 4.

The circuit schedule with the PIR is going to state installation method 101 for all circuits, and cable and circuit breaker sizes will look unsuitable (to an informed person).

{Just to put in a note here - I don't think that replacing a 32A breaker with a 20A on an existing ring circuit is really compliant with regs .
A 20A ring circuit isn't an Appx. 15 standard circuit. Such a circuit would only be a suitable design where the known or estimated load isn't expected to exceed 20A, not where 20A may be exceeded a bit now and again, but not for long so it doesn't really matter ..... (even though that's probably true.) Incidentally the convectors of two small and one larger storage heaters are connected to the ring circuits here.}

Can I give this code 4 and overall satisfactory ? I'd probably have to confirm that cables aren't showing signs of deterioration to do that, and also check whether grouping (where clipped to joists) wasn't further reducing current capacity (especially re. storage heater and immersion heater circuits.) Guess what - the insulation would have to come out to do that (and I'm not removing it !).

"Back covering" (horrible phrase) seems to be code 2 and overall "unsatisfactory", remedy client removes insulation and thinks again.

But this has to have been an issue elsewhere ......

I know this is all very pedantic and by the book, but this is an "institutional" client. And the wiring regulations are pedantic - and theywrote the book !
IET » Wiring and the regulations » "Top-up loft insulation"

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