Loft Insulation - Electrical Cables and Kingspan Therma

[CazDIY]

Hello,

I'm a landlord of an old end of terrace property. The loft was converted into a living space in the 1970s but has stud walls set back in to the room that hide the eaves of the roof.

There is no loft insulation behind these stud walls and we've been having problems with cold bridging on the bathroom ceiling directly below. To cut a long story short, we've installed some Kingspan Therma 70mm insulation in between the joists above the bathroom ceiling (the part that is behind the stud wall in the loft).

There are three electrical cables that are loose in this part of the loft and we've installed the Kingspan underneath these. I'm not sure what the cables are but one could be for the electric shower.

My question is - are the cables ok resting on the top of the Kingspan? Are they allowed to go past the edge of the Kingspan where they go down into the rooms below?

I have photos if they might help.

Thanks in advance

 

[OwainDIYer]

probably. To be certain you'd need to know the size of cable in mm and the rating of the shower in kW.

However, although one side of the cable is in contact with insulation the other is in a roofspace. Also where the cable passes through the insulation it's only for a couple of inches so heat can dissipate along the cable.

 

[CazDIY]

Thanks - I can measure the cable widths, any tips on finding out the electric shower rating?

The cables resting on the top aren't in full contact along the whole lengths with the kingspan as there's quite a lot of slack. It touches it a certain points though.

 

[riveralt]

Thanks - I can measure the cable widths, any tips on finding out the electric shower rating?

The cables resting on the top aren't in full contact along the whole lengths with the kingspan as there's quite a lot of slack. It touches it a certain points though.

You can look up your shower rating online or with the power off remove the front cover of the shower and it state the power rating inside.
The key issue is that for electric showers, normally a 6mm² T&E cable will give you sufficient current carrying capacity (ccc) for 8.5kw (37A) and sometimes higher shower.
But with that size cable there isn't that much ccc leeway available and as soon as it gets involved in insulation the ccc can drop quite quickly.
For example where 6mm² T&E is in contact with plasterboard ceiling or joists covered by thermal insulation not exceeding 100mm the ccc drops to 34A from up to 40A plus.
If you cannot clip it away from the insulation to a joist then I suppose you could always fit some plasterboard between the insulation and the cable.
Edit I have just seen your photograph - you should be able to clip it to the joists without touching the insulation.

 

[ban-all-sheds]

Also where the cable passes through the insulation it's only for a couple of inches so heat can dissipate along the cable.

 

[JohnW2]

Also where the cable passes through the insulation it's only for a couple of inches so heat can dissipate along the cable.

I was expecting someone to wheel that out. However, I have to wonder how many people actual take any notice of that for very short lengths (say ≤100mm), any more than they take any notice of the few inches of 'grouping' of cables that very commonly occurs where they leave a CU etc. A factor of even 0.78 would often have considerable impact, if one applied it.

In any event, in the OP's case, the cable would not be "surrounded by thermal insulation" but would (as I understand the OP's proposal/question) be in contact with a joist on one side - I imagine that would make it Method 100, but I have no idea what de-rating would theoretically be required for an inch or four of that - any idea?

Kind Regards, John

 

[ban-all-sheds]

However, I have to wonder how many people actual take any notice of that for very short lengths (say ≤100mm), any more than they take any notice of the few inches of 'grouping' of cables that very commonly occurs where they leave a CU etc.

The answer to that is the number of people who do cable calculations properly. As for grouping, the loading of cables in domestic environments is very rarely (never?) high enough for that factor to be <1. Also note the caveats in Appendix 4, 2.3 about the relative sizes of grouped cables. In practice, because of the often-noted conservatism in the tabulated ratings and factors, things don't go t***s-up when all the cables leave a domestic CU in a bunch for a short distance, but to comply with BS 7671 no electrician may simply not bother to do the calculations properly just because he finds them too hard.

A factor of even 0.78 would often have considerable impact, if one applied it.

You can't just decide to ignore a particular de-rating factor because you find its impact inconvenient. The different values in that table clearly show that there are limits to how much heat will "dissipate along the cable", and it is just as valid, and must be given just as much consideration, as all of the ones in Appendix 4.

In any event, in the OP's case, the cable would not be "surrounded by thermal insulation" but would (as I understand the OP's proposal/question) be in contact with a joist on one side - I imagine that would make it Method 100, but I have no idea what de-rating would theoretically be required for an inch or four of that - any idea?

Yes, I have a very good idea, and one with which I am quite comfortable.

The factor will of course depend on cable size, which we don't know, but using the 6mm² example mentioned above, I would use:

25mm: 0.97, i.e. an It of 46A
100mm: 0.88, 41A

 

[JohnW2]

... I imagine that would make it Method 100, but I have no idea what de-rating would theoretically be required for an inch or four of that - any idea?

Yes, I have a very good idea, and one with which I am quite comfortable. ... The factor will of course depend on cable size ...

Will it? Most de-rating factors don't appear to be cable-size-dependent.

which we don't know, but using the 6mm² example mentioned above, I would use: 25mm: 0.97, i.e. an It of 46A ... 100mm: 0.88, 41A

They don't sound like unreasonable figures, but if they don't come from the regs, how could one claim that they were compliant? Taken at face value, other than in the case of cables surrounded by insulation (per the table you posted) the implication of the word of the regs seems to be that, for example, 'Method 100 is Method 100', regardless of how short a length of cable is involved. Are you just making up your own 'common sense' quasi-regulations?

Kind Regards, John

 

[ban-all-sheds]

Most de-rating factors don't appear to be cable-size-dependent.

In 4D5, 4 out of 5 are (taking Method C as the baseline).

They don't sound like unreasonable figures, but if they don't come from the regs, how could one claim that they were compliant? Taken at face value, other than in the case of cables surrounded by insulation (per the table you posted) the implication of the word of the regs seems to be that, for example, 'Method 100 is Method 100', regardless of how short a length of cable is involved. Are you just making up your own 'common sense' quasi-regulations?

I have fitted the curve from Table 52.2 to the methods other than 103, which to my mind is the one which equates to "surrounded by insulation".

So 103#, which does not have a short length modifier, is a factor of 0.5 (it's the 1 out of 5 where that is the same for every cable size).

Method 100, 6mm², is 0.72 - fitting the Table 52.2 curve gets 0.97 and 0.88 for 25mm and 100mm.

Do they come from the Regulations? No.

Am I quite comfortable with them? Yes.

Would I say that I had exercised reasonable skill and care in working them out? Yes.

Would I say that to the best of my knowledge and belief those derating factors would give me an Iz I could use when looking at the requirements in Chapter 43? Yes.

 

[JohnW2]

I have fitted the curve from Table 52.2 to the methods other than 103, which to my mind is the one which equates to "surrounded by insulation". ... So 103#, which does not have a short length modifier, is a factor of 0.5 (it's the 1 out of 5 where that is the same for every cable size). ... Method 100, 6mm², is 0.72 - fitting the Table 52.2 curve gets 0.97 and 0.88 for 25mm and 100mm. ... Do they come from the Regulations? No. ... Am I quite comfortable with them? Yes. ... Would I say that I had exercised reasonable skill and care in working them out? Yes. ... Would I say that to the best of my knowledge and belief those derating factors would give me an Iz I could use when looking at the requirements in Chapert 43? Yes.

All perfectly reasonable, BUT, as I said, what about the one question you haven't given the answer to - "Would you say that work which relied on that reasoning and calculation would be 'compliant with BS7671'?" - in other words, would you feel able to honestly sign the declaration on, say, an EIC, if you had relied on those figures? ... after all, there is no mention in the regs of the concept (let alone any figures) of a 'short length modifier' for anything other than ~Method 103.

Kind Regards, John

 

[ban-all-sheds]

All perfectly reasonable, BUT, as I said, what about the one question you haven't given the answer to - "Would you say that work which relied on that reasoning and calculation would be 'compliant with BS7671'?"

I thought I had.

Would I say that I had exercised reasonable skill and care in working them out? Yes.

Would I say that to the best of my knowledge and belief those derating factors would give me an Iz I could use when looking at the requirements in Chapert 43? Yes.

- in other words, would you feel able to honestly sign the declaration on, say, an EIC, if you had relied on those figures?

Without a moment's hesitation.

 

[JohnW2]

All perfectly reasonable, BUT, as I said, what about the one question you haven't given the answer to - "Would you say that work which relied on that reasoning and calculation would be 'compliant with BS7671'?"

I thought I had. Would I say that I had exercised reasonable skill and care in working them out? Yes. Would I say that to the best of my knowledge and belief those derating factors would give me an Iz I could use when looking at the requirements in Chapert 43? Yes.

In terms of pragmatism and electrical common sense, I have absolutely no problem with that, but ....

- in other words, would you feel able to honestly sign the declaration on, say, an EIC, if you had relied on those figures?

Without a moment's hesitation.

As above, I have no problem with that, pragmatically. However, it is surely you who so often rejects pragmatism and speaks of the need for strict interpretation of the wording of the declaration - and I'm far from convinced that you could say that, strictly speaking, what you are talking about amounts to it being "to the best of your knowledge and belief in accordance with BS7671....".

I wonder how far you feel you can undertake such extrapolations from other regs and still feel able to sign a declaration that you believe your extrapolation to be compliant with BS7671?

Kind Regards, John

 

[CazDIY]

Thanks - I can measure the cable widths, any tips on finding out the electric shower rating?

The cables resting on the top aren't in full contact along the whole lengths with the kingspan as there's quite a lot of slack. It touches it a certain points though.

You can look up your shower rating online or with the power off remove the front cover of the shower and it state the power rating inside.
The key issue is that for electric showers, normally a 6mm² T&E cable will give you sufficient current carrying capacity (ccc) for 8.5kw (37A) and sometimes higher shower.
But with that size cable there isn't that much ccc leeway available and as soon as it gets involved in insulation the ccc can drop quite quickly.
For example where 6mm² T&E is in contact with plasterboard ceiling or joists covered by thermal insulation not exceeding 100mm the ccc drops to 34A from up to 40A plus.
If you cannot clip it away from the insulation to a joist then I suppose you could always fit some plasterboard between the insulation and the cable.
Edit I have just seen your photograph - you should be able to clip it to the joists without touching the insulation.

Thanks for the info - I don't have any technical knowledge about electrical installations - but basically if I clip the cable along the joists (assuming there is enough slack), then there should be no issues with the cables going past the insulation just on its way down to the room below?

If I can't clip them fully to the joists, is there any other good way of avoiding contact with the insulation?

What's the best type of clips and method to use?

Thanks

 

[riveralt]

Thanks for the info - I don't have any technical knowledge about electrical installations - but basically if I clip the cable along the joists (assuming there is enough slack), then there should be no issues with the cables going past the insulation just on its way down to the room below?

That is correct - unless you are going to add more insulation clip them to the side of the joists to avoid people walking on them.

If I can't clip them fully to the joists, is there any other good way of avoiding contact with the insulation?

As I said you could put plasterboard between the cable and the insulation - but that seems a bit OTT.

What's the best type of clips and method to use?

Bog standard - but right size - cable clips from any shed.

 

[ban-all-sheds]

As above, I have no problem with that, pragmatically. However, it is surely you who so often rejects pragmatism and speaks of the need for strict interpretation of the wording of the declaration - and I'm far from convinced that you could say that, strictly speaking, what you are talking about amounts to it being "to the best of your knowledge and belief in accordance with BS7671....".

Appendix 4 is informative.

I am informed by it, and other authoritative information, which I use with reasonable skill and care in calculating the Iz of a cable which to the best of my knowledge and belief results in a cable choice which is in accordance with the requirements in BS 7671.

Appendix 4 is informative - nowhere in the regulations are cable sizes wrt load explicitly prescribed in a way which would require them to be strictly interpreted according to any wording.