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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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 01 November 2009 02:42 PM
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Avatar for Testit.
Testit

Posts: 2962
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[Q{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.}.


Unusual view.. 20amp on a ring would still be compliant with the regulations IMO. Because it doesnt have a picture with a 20amp breaker on it for a ring doesnt make it non compliant... In relation to deration if you allow 27amps per conductor assuming 2.5mm you can allow approx 13.5amps per cable or 27amps on the ring.... now u may argue that's not 1.45 but then the red book allows 20amp on 2.5mm cable radials... generally speaking you are feeding any point with 5mm cable but with the current split by the ratios of the conductor resistances... this is no more than in any standard ring and assumes a 13amp loading on any accessory as maximum, though 2 X 13amp appliances on one socket is another argument... Personally I would say derating to 20amps would sufficiently satisfy 50% deration on a ring circuit assuming that the regulations condone the extra 2amps by virtue of stating a 20amp on a radial is OK which is essentially no different.... No problems from me sticking 20amps on a ring circuit, I'll sign that off as compliant... remembering that circuits designed with OCP are done so for protection of the cable and 20amp rings are therefore compliant...

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 01 November 2009 04:23 PM
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rocknroll

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I suppose you have to look at what you are trying to achieve and because you see a bit of loft insulation do you, double the size of the cable or reduce the MCB by a half.

Thermal insulation is something that is high density and in containment.

Loft insulation is something that is low density and has containment one side and the other side radiation to space.

A PIR is not legal document but an inspection report that has this clause; "Cables concealed with trunking and conduits, or cables and conduits concealed under floors, in roof spaces and generally within the fabric of the building or underground have not been inspected", to inspect the cables would require the use of PPE because of the removal of the insulation and obviously eat into the short time you need to do a basic periodic inspection.

They did not write the regulations, BS7671 is mainly a copy of IEC 60364 and many other BS and EN documents in a concise fun book for all to read.

There is no evidence that loft insulation affects cables to the degree of derating by 50%, even thirty years ago in rural areas where the preferred method was hay which is far superior to the loft insulation used today there was no evidence of cable thermal damage, the only damage was as a result of mice, rats, squirrels and gliss gliss.

As far as the paranoia about myths and folklore relating to standing in the dock because you put down a code 4 instead of a code 2 you have to remember 20 times more people win the lottery than tradespersons prosecuted for a clerical error.

If you think you need to apply a factor to this you perhaps need to determine from the book what factor you need to use, I think it was deemed to be about 0.9 where cables were clipped to the bottom of the beam or laid on the plasterboard and 0.87 for cables clipped on the top of the beam, but the average cable loading is around 67% so the loss of 3 or 4 amps wasnt really significant for domestic circuits, also evidence has shown that increasing the thickness of the insulation to 270mm or 440mm has very little effect on the cable, as far as grouping in domestic circuits is concerned this is not relevant as grouping applies to cables of the same size and carrying the same current.

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
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"Oh! The drama of it all."
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"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
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Edited: 01 November 2009 at 06:51 PM by rocknroll
 01 November 2009 05:53 PM
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Testit

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I would tend to agree for domestics, until of course you get the cannabis growers and immigrant collonies using multiple heaters absolutely everywhere.... perhaps then there's an ambient temperature deration.. perhaps more so in the summer if running through the attick....

I have no qualms still about sticking 20amp breaker on a ring...

I recently derated a ring due to poor installation with tight cable bends, mildly compressed cables by back boxes and running through insulation.. in such circumstances its thermal value didnt matter to me.. though doesnt it say that if thermal value isnt known you should adopt the given values? Cant quite recall mind, but I may be wrong...

No qualms about it tripping.. Kitchen ring as it goes, and the wall tends to get rather warm for a few hours where steamers and whatever else steam happily against the splash backs heating up the walls nicely.. nowhere near their operating temperatures of course...

Why did grouping come into this?

Off to watch the box anyway.. enough nonsense for one day.

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 02 November 2009 11:14 AM
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OMS

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This may be worth a read.

It's highly unliley that additional loft insulation will have anything other than a monor effect - cables are rarely loaded as much as you think and insulation rarely meets the thermal transmittance criteria of 0.1W/m²K used in the regs.


OMS

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 02 November 2009 11:39 AM
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OMS

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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?


A compliant UK roof structure is about 0.16W/m2k.

Using insulation with a condctivity of about 0.044W/mk then that's about 270mm thick. Typical mineral wool is about 0.04W/mk (say about 300 thick).

It will then depend on where in the insulation layer the cables are (ie fully embedded or clipped and covered on one side only.

The BS 7671 value of 0.1W/m2k for a ceiling only is hugely conservative

Regards

OMS

Regards

OMS

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 02 November 2009 12:39 PM
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lamps

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saying that a typical lighting ctt run in the loft in a domestic property
will have 6 lamps 4 downlighters off a 6amp breaker
all clipped direct all run without bunching no tight bends we need not worry
how would you work out w/m on this ctt

thanks in advance
 02 November 2009 01:11 PM
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OMS

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how would you work out w/m on this ctt


Add up all the watts and divide by the area served ?

OMS

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 02 November 2009 01:19 PM
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lamps

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so say 10x 100 watt lamps over a 15m ctt =1000/15=66.66/1000=0.06w/m2 is this correct
 02 November 2009 01:32 PM
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OMS

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Not really no - you (I guess) are trying to work out power density.

You need the floor area served not the circuit length. So lets say the 10 lamps serve 50m2.

The power density would be (10 x 100)/50 = 20w/m2

Or have I misunderstood what you were asking

OMS

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 02 November 2009 01:42 PM
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lamps

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no i was working out wrong lol
so say 4 rooms with rooms 3x2=24m2
10x100/24=42w/m2 is this correct
 02 November 2009 01:50 PM
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OMS

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It is yes

OMS

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 02 November 2009 02:00 PM
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lamps

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10x100/24=42w/m2
thanks oms so now i know how many watts a m2 how do i fully determine
how to derate ?if i even need to say ctt clipped direct to joists in 100mm of insulation
 02 November 2009 02:05 PM
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OMS

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Woah - determing of installed power density has very little to do with derating of cales due to thermal insulation - very different animals used for different purposes I'm afraid.

What is it exactly that you are trying to achieve

Regards

OMS

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 02 November 2009 02:22 PM
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lamps

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well i recently saw a job that has recently had the loft insulated and i am really trying to learn how i can determine if anything needs derating
 02 November 2009 02:55 PM
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OMS

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Try the link I posted above as a starting point

OMS

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 02 November 2009 02:57 PM
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lamps

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will do thanks
 02 November 2009 09:05 PM
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deadtester

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Finally I'll tell you what I've done with this PIR ......

All circuits are described as installation method 100 on the circuit schedule, i.e I'm saying that this (17th ed.) method best describes the original installation design, before extra insulation was rolled out over the cables at some later date.

Then I've added a code 3 recommendation for consideration to be given to further investigation of the effects (if any) that this detrimental influence may have had on the electrical cables.

Overall satisfactory outcome (because mainly everyone's saying don't worry about it !). Cheers!
 18 February 2011 12:48 PM
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pomistea

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Originally posted by: Testit

I have no qualms still about sticking 20amp breaker on a ring...



I recently had an argument about this where the other party claimed that in accordance with 7671 you MUST protect a ring circuit with a 32A MCB.

I can't find any reason not to protect a ring circuit with a 20A if you so desire.
It seems that 32A is what is generally used so people take this as a rule when it's not.
 18 February 2011 01:41 PM
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AJJewsbury

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the other party claimed that in accordance with 7671 you MUST protect a ring circuit with a 32A MCB.

They probably didn't spot the subtle "may" in 433.1.5
- Andy.
 18 February 2011 07:02 PM
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JonSteward

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Appendix 15 states

A ring final circuit starts and finishes at the distributionn board where it is connected to a 30A or 32A opd.

To quote the regs,


no mention that a 20A is right or wrong.
IET » Wiring and the regulations » "Top-up loft insulation"

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