Cable derating reference method - outbuilding/garden room.

[Dale_R]

I have been reading all day to try and figure out what reference method should be used for the following as it doesn't seem to be quite covered in any of the reference methods or on any tables:

I've got an outbuilding with OSB stud walls, the outer OSB sheet has membrane then cladding facing outside the building and the inner OSB sheet has plasterboard and plaster facing inside the room. Between the 2 OSB sheets is a 100mm cavity, attached to the OSB sheet that forms the outer wall is a Rockwool/glass fibre on a roll type of material, this is stapled to the OSB inside the cavity, the OSB board it is stapled to forms the outside wall. 2.5mm T&E comes down from the top of these walls in front of the insulation, it will be touching the OSB wall that faces the inside of the building or have an air gap. The 2.5mm T&E is clipped on at the top of the wall along the upper studs then drop down to where each socket will go, the cable is not clipped inside the wall as these wall panels are prefabricated.

Where I am struggling to work this out is due to a few factors, firstly the regs mention "touching" the inner wall and not "clipped", the 2.5mm T&E is between the inner wall OSB sheet and the rockwool, I cannot say for certain it's touching the inner wall all everywhere it's certainly resting on the wall in places. Some of it might be in free air on one or more sides. I can say for sure that none of it is passing through the insulation.

I'm trying to work out if this would fall under Reference Method 102 or the A - G tables? Reference Method 103 really seems to get the impression looking at illustrations that's for scenarios where a cable would pass through insulation or be surrounded by it on all sides.

This does not seem too dissimilar to Reference Method 100 (But vertical) and speaking from my own experience I've never really seen T&E touching all of the plasterboard when laid on top of ceiling.

There seems to be some ambiguity with Methods 102 - 103 and it leaves a lot to interpretation, then there are the Thermal U values and types of insulation etc.

Any advice would be gratefully appreciated.

 

[EFLImpudence]

Following Note A* under Table 4D5 leads to Installation Method 2 and Reference Method A and I can't see your situation is any worse than that.

Is the 2.5mm² T&E you mention part of a ring circuit and what demand will there be in your garden room?

 

[EFLImpudence]

If any cable actually passes through thermal insulation and is totally surrounded by it that might be a different matter and should be avoided.

 

[Dale_R]

Following Note A* under Table 4D5 leads to Installation Method 2 and Reference Method A and I can't see your situation is any worse than that.

Is the 2.5mm² T&E you mention part of a ring circuit and what demand will there be in your garden room?

Thank you kindly for your reply.

I had it all planned and had the company who built the building run all the cables for me, today I thought I'd crack on with finishing it off and was off to the wholesaler to get some wiring accessories and the CU but for some reason did a sanity check as I needed go get the RCBO's etc and looked at the book and spent a bit of time looking at the 100 series of methods which got me a bit thinking about the "touching" section.

Yes that was run as a ring final, there are a lot of sockets but the demand will never be high. They were run more for convenience (opposed to extension leads) rather than a high load.

I put the the title as outbuilding/garden room so there is some familiarity with the type of construction. It's going to be used mainly as a nice space to do my electronic and industrial electrical equipment repairs, my other half is having a sofa the other end so she can either keep me company or escape from me when I'm in the house.

Nothing will draw any high continuous current, even something I am repairing/working on. I will have a kettle in there that will be the biggest demand.

The plan is/was the following:

Feed to the building comes from a small separate CU that's fed from henley blocks after the main isolator, that small CU currently has a 100A/100ma TD RCD and a 50a MCB which has 10mm SWA feeding this outbuilding. The CU and TD RCD are there for a few reasons, firstly there may be a RCBO going in there to feed a 16A hot tub if we ever get around to buying one, I may decide to go TT at the outbuilding at some point due to a few scenarios where I might end up with extraneous conductive parts referenced to earth (RF Equipment) and it also depends on the readings I get at the building when it's all connected up due to distance and the house being a TN-C-S supply so I thought it won't harm anything putting the TD RCD in if plans do change and it offers belt and braces protection of the SWA if nothing else.

Outbuilding wired in T&E as follows and each line is the RCBO feeding that circuit.

1x 32A Ring final 2.5mm
1x 6A (or 10A) Internal lighting 1.5mm - LED
1x 6A (or 10A) External lighting 1.5mm - LED
1x 20A Electric heater radial, 2x fused spurs 1 electric convection heater and 1 low power tube heater for dew/frost protection in winter. 1x 2.5mm
1x 16A (or 20A) 2x External double sockets radial 2.5mm, nothing permanently connected
1x 16A (or 20A) Spare to whiska box outside if I have a split AC fitted later on

If any cable actually passes through thermal insulation and is totally surrounded by it that might be a different matter and should be avoided.

None of the wiring is surrounded by insulation, I double checked today with the company that installed the building as I wanted to double check, all cables drop down vertically in each wall panel in front of the insulation, technically should be "touching". Horizontal wires are all clipped running across the horizontal beams above each wall.

The outside cables will pass through approx 50-100mm of rockwool which I am factoring in with those cables.

*Edited* 10mm SWA not 6mm.


Apologies for the rambling and thanks again.

 

[EFLImpudence]

It all looks a bit OTT to be honest for a garden room.
There is far more than I have in my two-bedroom flat.

However, I suppose that means the circuits are over-engineered and will only be subjected to low demand.
With a reference method of 20A then no cable is above that.

 

[Dale_R]

It all looks a bit OTT to be honest for a garden room.
There is far more than I have in my two-bedroom flat.

However, I suppose that means the circuits are over-engineered and will only be subjected to low demand.
With a reference method of 20A then no cable is above that.

The problem is with the sockets on the ring final circuit mainly, well come to think of it everything bar the internal/external lighting.
I've been reading the book all day and as much as other sections make sense I keep coming back to ref method 103 which really concerns me. I could understand a cable going directly through insulation or being wrapped in it but without being clipped it seems pretty impossible to get the cable to touch the inner wall surface or plasterboard all over to satisfy method 102.

What baffles me is the company who built the building have built hundreds if not over a thousand different buildings for residential and commercial use with this type of construction and wiring and I assume they have all been signed off at some point.

 

[EFLImpudence]

I've been reading the book all day and as much as other sections make sense I keep coming back to ref method 103 which really concerns me. I could understand a cable going directly through insulation or being wrapped in it but without being clipped it seems pretty impossible to get the cable to touch the inner wall surface or plasterboard all over to satisfy method 102.

I think it refers to fairly solid insulation such that the cable is squashed between that insulation and the inner surface of the wall; not the fluffy stuff.
Even if it does not then if the cable is not touching the wall then it does not qualify as 102.

Also, as you have, if there is a space between the insulation and wall then the cable is not "in insulation" so nothing to do with 102 or 103.
Whether this counts as 'free air' or not I do not know but it is definitely better than Installation method 3 therefore must be better than reference method A (20A).

What baffles me is the company who built the building have built hundreds if not over a thousand different buildings for residential and commercial use with this type of construction and wiring and I assume they have all been signed off at some point.

To be honest, I do not think many people consider derating factors in domestic installations because everything has so much tolerance and safety margin, the figures used do not really apply.

For example what is the actual current that would raise a 2.5mm² conductor in T&E to 70º and a normal 2.5mm² ring final could have a 40A MCB/RCBO; the regulation is still written for a BS3036 30A fuse and its derating factor?

 

[Dale_R]

Thank you for your reply.

I agree with everything you have said there.
There has to be some sort of sensible thinking with it all, in my mind I know its right and just shy over 20A worst case scenario is fine for a RFC, BS 7671 should be a little clearer about the 103 scenario as based on the illustrations and physics those situations would be cables running through insulation and in my mind would be materials like PIR Board or SIPS panels (I think there are even bigger issues to worry about going through any foam based boards with the migration of plasticisers).
And as you say the fluffy stuff is not going to push any cable against a wall. I did have a look online last night regarding posts where Ref method 103 is given for a situation but quite a few I didn't agree with personally. It seems to be a bit of a catch all worst case scenario but some situations may look like it would fall under it as it doesn't fit with a lot of the other methods when in reality it's fine.
Granted the regs keep changing but for example I lived in a 1970's timber framed property a few years ago, the stud walls were OSB on outer facing walls (facing the brick cavity) and inner walls plasterboard, all the original wiring was just dropped down from the attic in front of the insulation and just behind the plasterboard.

I'm sure a lot of re-wires on plasterboard stud walls are just going to be dropped from the attic or drilled at the top of the wall and dropped down in front of the fluffy stuff.

Our current house was built in 2001, the outer walls are brick/block with dot and dab, the inner stud walls are plasterboard with fluffy insulation, no cables are clipped inside the walls and they all drop down from the attic. The CU is in the downstairs toilet and everything runs up through the wall to the attic. All the cables in the attic look as if the builder/electrician threw a drum of T&E around up there and hoped it all fell in the right place, then covered in fibreglass insulation on top, none of it is flat and none of it has been clipped anywhere.
When we moved in I checked everything over and sorted a few little issues, I decided to change all the sockets and switches as a lot weren't working correctly, they were the old Tenby type with the recessed switch, to discover all the dot and dab walls had 16mm back boxes! so had to get the SDS out and replace them all.

I wanted to get rid of the gas hob and single electric oven, pulled the oven out to see if there was a cooker outlet plate behind it for the hob and found 2 single switched plug sockets. I was a bit disappointed thinking there is only a 2.5mm feed there as the original oven was on a 13A plug. and the other socket was used for the hob igniter. So I thought I'd take the sockets off and double check. One of the sockets had a 6mm feed going in to it and the second socket was spurred off it with 1mm T&E going through metal back boxes! I thought if that shorted out there is nothing to protect that cable and unlikely the 32A MCB is going to save it. But yet that was done by the original house builder/electrician. I was pleased however to see a 6mm feed there though as I then put a nice induction hob and double oven in. I ripped it all out and put a Click dual outlet plate in there instead.

My work is all industrial so lots of things have to be taken in to consideration and of course liability if things go wrong, so I'm always a bit cautious about doing things and always try to leave some headroom.

I was at a commercial site the other day and found a Y Shape IEC lead coming out of a UPS going in to something, all 0.75mm. I asked someone why have they fitted this and they said to give more current to the device. I said you have 2 lots of 0.75mm feeding in to a single 0.75mm cable how is that going to help? Not only that the Y consisted of 2x C14 Male plugs, if someone had unplugged one end of that from the UPS the other C14 plug would be live. I pulled it all out and cut it up so someone couldn't use it again and plugged a moulded 1.25mm C13 to C14 lead in instead and surprisingly their load now works absolutely fine.

I was about to go to the wholesalers today to get the CU and RCBO's and noticed Hager still don't do residential B Curve RCBO's with switched 1P+N or dual pole, only single pole. That's a bit odd as their commercial devices do it, I would have expected them to have caught up a bit now especially with bi-directional RCBO's becoming more common.

I noticed Scholmore do some boards under the Elucian name that seems to be reasonably priced with 1P+N that look pretty decent but I have absolutely no experience with them, I may have a read up on them.

I really should crack on with getting this done and I've still got to put the mist coat on the fresh plaster.

 

[EFLImpudence]

I agree with everything you have said there.

There has to be some sort of sensible thinking with it all, in my mind I know its right and just shy over 20A worst case scenario is fine for a RFC, BS 7671 should be a little clearer about the 103 scenario as based on the illustrations and physics those situations would be cables running through insulation and in my mind would be materials like PIR Board or SIPS panels (I think there are even bigger issues to worry about going through any foam based boards with the migration of plasticisers).

Yes, but 103 derates the cables by 50% which is the maximum any cable is ever derated for being completely surrounded by thermal insulation for all its length.

And as you say the fluffy stuff is not going to push any cable against a wall. I did have a look online last night regarding posts where Ref method 103 is given for a situation but quite a few I didn't agree with personally. It seems to be a bit of a catch all worst case scenario but some situations may look like it would fall under it as it doesn't fit with a lot of the other methods when in reality it's fine.
Granted the regs keep changing but for example I lived in a 1970's timber framed property a few years ago, the stud walls were OSB on outer facing walls (facing the brick cavity) and inner walls plasterboard, all the original wiring was just dropped down from the attic in front of the insulation and just behind the plasterboard.I'm sure a lot of re-wires on plasterboard stud walls are just going to be dropped from the attic or drilled at the top of the wall and dropped down in front of the fluffy stuff.

So the cable was either touching the plasterboard or it was in a space.

I wanted to get rid of the gas hob and single electric oven, pulled the oven out to see if there was a cooker outlet plate behind it for the hob and found 2 single switched plug sockets. I was a bit disappointed thinking there is only a 2.5mm feed there as the original oven was on a 13A plug. and the other socket was used for the hob igniter. So I thought I'd take the sockets off and double check. One of the sockets had a 6mm feed going in to it

Yes, people are frequently advised to swap the cooker outlet for a socket when they buy a small oven that comes with a plug.

and the second socket was spurred off it with 1mm T&E going through metal back boxes! I thought if that shorted out there is nothing to protect that cable and unlikely the 32A MCB is going to save it.

Well it would.
It cannot overload and the 32A will clear a short-circuit or earth-fault current.

But yet that was done by the original house builder/electrician. I was pleased however to see a 6mm feed there though as I then put a nice induction hob and double oven in. I ripped it all out and put a Click dual outlet plate in there instead.

Maybe back as it was when built.

My work is all industrial so lots of things have to be taken in to consideration and of course liability if things go wrong, so I'm always a bit cautious about doing things and always try to leave some headroom.

As I said, I think headroom already included.

I was at a commercial site the other day and found a Y Shape IEC lead coming out of a UPS going in to something, all 0.75mm. I asked someone why have they fitted this and they said to give more current to the device. I said you have 2 lots of 0.75mm feeding in to a single 0.75mm cable how is that going to help? Not only that the Y consisted of 2x C14 Male plugs, if someone had unplugged one end of that from the UPS the other C14 plug would be live. I pulled it all out and cut it up so someone couldn't use it again and plugged a moulded 1.25mm C13 to C14 lead in instead and surprisingly their load now works absolutely fine.

A widow-maker.

I was about to go to the wholesalers today to get the CU and RCBO's and noticed Hager still don't do residential B Curve RCBO's with switched 1P+N or dual pole, only single pole. That's a bit odd as their commercial devices do it, I would have expected them to have caught up a bit now especially with bi-directional RCBO's becoming more common.

I noticed Scholmore do some boards under the Elucian name that seems to be reasonably priced with 1P+N that look pretty decent but I have absolutely no experience with them, I may have a read up on them.

I am a bit out of date about such things; have been retired here for ten years.

I really should crack on with getting this done and I've still got to put the mist coat on the fresh plaster.

Good luck.