Is this "surrounded by insulation or on 100mm insulation in your opinion?

[Vortigern]

Just designing and taking into account the factors for insulation and can't decide on surrounded or on top of 100mm. What say you? Cables will be in metal banding inbetween the two layers as in pic. So to be clear it will be under the upper board in pic and above the lower board on top of the lower rockwool (accoustic)

 

[pc1966]

Hmm, tricky. It is not going to be as insulating as enclosed in material as there is an air gap so convention likely to help cool it, but also not really "open" on the non-insulated side. But while that drawing shows an air-gap, it is not clear to me that whoever installs it wont just stuff extra material in because they ahve it, so I would proably err on the side of caution and go with "surrounded" from a thermal point of view.

My other concern is the idea of laying services on the suspended roof - I'm pretty sure that is not acceptable support or likely to meet premature collapse concerns, probably better to have some means of clipping them to the "foil backed plasterboard" or cable tray, etc.

Are any high power circuits going in? Typically if it is just lights then even fully derated 1mm is going to be fine.

 

[Vortigern]

There is an electric boiler circuit at 6Kw and the space they have left means the cable(s) will be tightly pressed against the upper plasterboard from what I can see. Although in contact with the upper plasterboard, that is foil-backed and covered by 100mm insulation. The quandary is if for instance I group cables the ring circuit will be requiring 16mm T&E! Of course that is not going to happen. I find my self imagining it is just on top of 100mm insulation in contact with plasterboard. Which if it is not..... so belt and braces approach is to say surrounded and that means massive cable sizes. So I wont be grouping them! Then a cooker circuit 32a again.
As an aside I use cable calc (Pro certs) and you can't effectively do a RFC calc as it would be 5mm cable. i.e. 2.5mm x 2. So derating on 100mm is 27 > 21a (ref meth 100) > 13a ref meth 103. But... as the cable is 5mm I can double that, agreed?
Again more jiggerypokery, If I do the nominal voltage for 6000w = 27a (max) or 25a at 240v. So by sliding factors around I can get the desired size cable but is it safe?
Then again to be honest I have no idea of the ambient temperature in this scenario. So I guess normal at 30 c. However I could just as easily put 25 and get to a lower csa.

 

[pc1966]

Typically I would look at the OSG and it is saying for method 103 that a RFC is fine in 4mm. Of course you could do 2.5mm for sections not running in the roof space, but maybe not worth the trouble.

Agreed that grouping them is going to make it painful in cable size & cost!

I'm seeing 32A radials as OK for 10mm in the OSG for method 103 so if they can all be run with a good spacing apart then cooker & 6kW heater are fine that way. Not sure if anywhere has a definition of how far apart is "good" but my own gut feeling is at least the 10cm or so of the insulation thickness.

 

[Vortigern]

not grouped I take as separated by the diameter of the cable (flat side) Yes generally agree with your figures but would like to get them 10mm down to 6mm, and wiring in 4mm on ring with any spurs is hell getting three four mils into termination point as well as questionable as to workmanship/safety. I am mulling cable tray under the insulation. As I said the ccc of 2.5mm in a ring I take as 54a > 34a in ref meth 103.

 

[pc1966]

Dropping to 20A RFC would allow 2.5mm cable but my concern is someone later ups it to 32A without reasoning why that was originally used.

My own flat is on a 20A RFC, I think originally due to the high Ze from the old MICC cable feeding it (R2 very low, but R1 is high as it is spec'd to run hot!) but when I replaced the board I just kept 20A as I did not install the cable so don't know if somewhere it is thickly insulated. Which I doubt, judging by how Baltic my flat is in winter!

 

[pc1966]

not grouped I take as separated by the diameter of the cable (flat side) Yes generally agree with your figures but would

If wrapped in a thermal blanket I don't see that as enough to get additional cooling.

In open air / clipped direct that is probably OK.

like to get them 10mm down to 6mm, and wiring in 4mm on ring with any spurs is hell getting three four mils into termination point as well as questionable as to workmanship/safety. I am mulling cable tray under the insulation. As I said the ccc of 2.5mm in a ring I take as 54a > 34a in ref meth 103.

Spurs are the Devil's work!

Do the quick-connect MK sockets take 3 * 4mm in the Wago-like terminals? Not cheap but might save you pain and future trouble if you do have that.

 

[pc1966]

A quick search shows they do take 3 cables of 2.5mm or 4mm CSA, if price and looks both acceptable:

https://www.NoLinkingToThis/p/mk-lo...-socket-white-with-white-inserts-5-pack/313ph

 

[Vortigern]

As I say I assume 54a rating (max) for 2 x 2.5 as cables in parallel or 34a in ref meth 103. So no real need for struggling with 4mm. Although it is good to know the lever MK take 4mm. I think it is all very well massaging scenarios to save on csa but conscience dictates the safest approach which is why I garner others views on essentially is the cable surrounded or on top of insulation. Personally safest is to say surrounded. Which changes the cable size significantly. As to ambient temperature (see pic) in between ceilings with no ventilation and insulated, anyone????

 

[pc1966]

As I say I assume 54a rating (max) for 2 x 2.5 as cables in parallel or 34a in ref meth 103. So no real need for struggling

But it does not work that way for the RFC as you are not guaranteed a perfect balance of loads.

The most "approved" route is to look at OSG Table 7.1(ii) on page 75 (of the new brown AM2 version, can't find my blue plain 18th one just now to check page number) and for methods 101 and 103 they only supported options are 4mm.

with 4mm. Although it is good to know the lever MK take 4mm. I think it is all very well massaging scenarios to save on csa but conscience dictates the safest approach which is why I garner others views on essentially is the cable surrounded or on top of insulation. Personally safest is to say surrounded. Which changes the cable size significantly. As to ambient temperature (see pic) in between ceilings with no ventilation and insulated, anyone????

Without anything detailed in the way of specification or analysis I would say the safe assumption is the cable is basically between two layers of insulation, so like method 103. As for ambient temperature I doubt any domestic room is going to be much above 30C but somewhere like a kitchen might be a touch warmer, and we have seen as high as 40C in the UK this summer!

 

[Vortigern]

I guess you mean Pp 177 on brown on site guide. Just to expand on balanced load viz RFC, I guess the most load would be the kitchen sockets at around 25% of the way around the ring circuit. This is a studio flat with 50 m² floor space. So washing machine, toaster, kettle etc. would be at that point. So you are saying (and I have seen it reference often that the load will mostly (or all?) go onto the run to the kitchen? and the long leg of the ring will take hardly any load. Is there any scientific basis for this or regs. Can't say I have seen any. I just wonder what science this is based on. If that is the case it defeats the whole reason for having a ring then? Might as well do radials (4mm)
Just to beat this to death, the asterisk on the methods points out the cable must be touching the plasterboard in order to conform to the ccc tabulated. I am not positive it will actually contact it very well.

 

[pc1966]

I guess you mean Pp 177 on brown on site guide.

That has the typical CCC for different methods. It is also present on page 84 as Table 7.1(iii) following the "standard circuits" that I was looking at.

Page 75 has the various RFC combinations of OCPD and cable size that seem to be considered "standard". If you look at the table then only 4mm is listed as acceptable for methods 101 or 103. Previously it was the longer length that I looked at when considering 4mm (e.g. 171m instead of 106m on VD limit).

Just to expand on balanced load viz RFC, I guess the most load would be the kitchen sockets at around 25% of the way around the ring circuit. This is a studio flat with 50 m² floor space. So washing machine, toaster, kettle etc. would be at that point. So you are saying (and I have seen it reference often that the load will mostly (or all?) go onto the run to the kitchen? and the long leg of the ring will take hardly any load. Is there any scientific basis for this or regs. Can't say I have seen any. I just wonder what science this is based on. If that is the case it defeats the whole reason for having a ring then? Might as well do radials (4mm)

With method 103 a 4mm radial is only going to be good for 17.5A so with socket outlets then 16A MCB.

You probably would need two 4mm radials for the kitchen if you have more than two ~3kW loads (e.g. washing machine and dish washer perhaps?) as they would really have to be on separate radials, and at that point you might as well do the whole flat as a RFC in 4mm! More so if AFDD are needed...

Also if doing 4mm radials it is no different to a RFC for wiring. Provided you avoid spurs as then most sockets (all in RFC) have only 2 * 4mm cables. Which you should, as they are the Devil's work.

Just to beat this to death, the asterisk on the methods points out the cable must be touching the plasterboard in order to conform to the ccc tabulated. I am not positive it will actually contact it very well.

There are various ways you might look to getting out of using 4mm cable. Going to LSZH version rated at 90C gets you something like a 22% increase in CCC and the accessories are OK as your use of CSA and working current is the same as for 70C PVC, just you are relying on hotter safe running in the thermally insulated region.

Dropping the RFC to 20A is another route to keeping a limit on possible cable heating, but you might find it is getting close to probably total load if the kitchen has a good few gadgets.

But all of those might just bite you if there is any trouble down the line. Sticking to the OSG combinations from Table 7.1(ii) (for RFC, lights, or radials) is by far the easiest route to having a safe and justifiable design for most domestic work!

 

[pc1966]

I guess the most load would be the kitchen sockets at around 25% of the way around the ring circuit. This is a studio flat with 50 m² floor space.

With 4mm VD is really not an issue, you could deliberately make one leg take a longer route so the heavy loads are closer to half-way. They need not be exactly there but if you are looking at, say, 75m of cable in total, then running the short leg an extra, say 20m back, is not going to be an issue.

 

[Vortigern]

Thanks for taking the time to bounce this around @pc1966, appreciated!

 

[Pretty Mouth]

You've probably considered this already, but where the cables run through the insulation, running them in flexible conduit brings you to ref A. This will allow most of the standard circuits in standard sizes, eg 32A/6mm cooker, 32A/2.5mm ring etc.

Surprised that a 4mm ring can be run ref 103. They are very flexible circuits those rings.

 

[Vortigern]

I had not thought of that. A really good thought. Thanks @Pretty Mouth That gets me up to 26a.

 

[pc1966]

You've probably considered this already, but where the cables run through the insulation, running them in flexible conduit brings you to ref A. This will allow most of the standard circuits in standard sizes, eg 32A/6mm cooker, 32A/2.5mm ring etc.

Trouble is you would have to run each T&E in its own conduit or you are penalised by the grouping factor.

I'm not sure the effort of puting in conduit is going to make up for any savings in copper cost or termination effort in going 4mm!

Surprised that a 4mm ring can be run ref 103. They are very flexible circuits those rings.

Indeed, a very good solution for a typical flat/floor.

 

[loz2754]

There is a very informative efixx video that explains the balancing of loads on a ring circuit.

 

[Vortigern]

Thinking about it if I got out of the ceiling and went horizontal along the walls I would be Ref:102 which would assist me and get me up to 27a rating with 4mm.

 

[pc1966]

Thinking about it if I got out of the ceiling and went horizontal along the walls I would be Ref:102 which would assist me and get me up to 27a rating with 4mm.

If you can do that it would be the easiest/cheapest option.