Discuss Which Reference method in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

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Blacktail

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Hi, designing my first small installation which i will be using for my first assessment with Elecsa/NECEIC etc ( any thoughts on which way to go appreciated). Installation is in an oak wooden framed and mostly glass conservatory with instruction of no visible wiring please! The obvious way of running the majority of the cables is under the floor. Floor construction is concrete pad then approx 100mm insulation 25mm ply, 25mm solid oak boards. Initially was thinking of channelling out top of insulation and running cables under ply/oak . Would 25 mm of ply and then 25 mm Oak flooring be considered a thermally conductive surface (ref 102). Given the nature of the room i can imagine very high ambient temps at floor surface. If i lay cables under insulation, as the builder would prefer due to time constraints, would the pad be considered a thermally conductive surface. It is just conventional poured concrete over compacted hardcore. I guess this will reduce ambient temp factors as well?
 
S

sjm

I would run the cables in 25mm conduit then the builder can pour his concrete on top of this. I think this should make the install method B. You could use a DECENT flexible conduit to speed up installation.
 

Guitarist

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That's a brave one to do as a first assessment. Not sure if you are planning to feed it through conduit, but I wouldn't dream of just laying T&E under a floor without mechanical protection. As sjm says... if you do, then ref B.
 
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Blacktail

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Thanks Guys for the reply. The concrete pad is already poured. It is on this i was going to lay the cable. On top of this is going 100mm of insulation then 50mm of boarding. I was going to run the cable sandwiched (well actually in a cut out slot) between the concrete and the insulation giving 150mm of total covering to surface hence not considering ducting, but from you reactions think this was a mistake. Table 4A2 No.60 shows multicore in conduit in concrete - Ref B but is stating thermal resistivity not greater than 2K.m/W. If i do it as i have suggested, sandwiched, with one side of the cable/ducting touching the concrete and the other side of the cable/ducting now encased in insulation greater than 100mm i dont think this is B? I was concentrating on Installation methods for T+E in thermal insulation as this seemed most relevant. In which case should i be considering the concrete pad as "the wall". Also if in ducting is it touching the wall? Hope this makes it a bit clearer.
 

Guitarist

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I can only reiterate that I would not simply lay the cable on a concrete floor with a "notch" cut out of the insulation. Once you put the cable in conduit then you will be creating an air gap around it.
Do you know the thermal resistivity of the insulation which you are planning to cover the cables with?
 
S

sjm

Put the cable in conduit, allow for space factors etc. IMHO what the conduit is then covered with, as regards insulation, doesn't matter and the installation method remains B.
 

Guitarist

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Put the cable in conduit, allow for space factors etc. IMHO what the conduit is then covered with, as regards insulation, doesn't matter and the installation method remains B.
Have to agree here. Just laying it on a concrete floor, covered in notched insulation would not even be an option in my book.
 
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Blacktail

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Thanks Guys, Ok so we are saying so long as space factors/grouping etc is ok then lay the cable in conduit and treat as method 60 on 4A2 = Ref B. Makes sense. My confusion with that was the the Insulation going on top would have a thermal resistivity greater than 2 K.m/W. The foot note " o " on page 264 states that thermally insulating materials are excluded and as the conduit will be covered on three sides by insulation, thought this would be relevant. The insulation has a thermal conductivity of 0.023 W/m k which im pretty sure is the reciprocal of Thermal resistivity so i am guessing is greater than 2 K.m/W. Sorry for banging on about this but want to get it correct.
 

Guitarist

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Have you considered notching the insulation on top and laying the conduit above the insulation, rather than below it, under the flooring?
 
E

Edd

Always go on the worst case, If your not sure that it will be classed as thermal insulation then asume it is. The NICEIC will look and see that you have taken the conservative path and your refrence method and calculations can be shown.
If he thinks your method is wrong it will be wrong in the right direction, but you can then ask the questions, and converse with him in the way you were thinking. If you guess the wrong way, then he will rip you a new one. .... Only joking it will be fine:)
 
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B

Blacktail

EF Member
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Thanks Edd, I can feel the confidence seeping away by the minute!! Using the worst case approach it would be method 102 poss 103. 102 if laying it as Guitarist suggest which is starting to sound like the best idea. As far as i can see the OSG does not even list ring cicuits in 103 in its Final Circuits so im guessing they really dont like the idea and I dont fancy 6mm t+e in shallow backboxes.

If laying on top of insulation, 25 mm pvc conduit sufficient for mechanical protection. Really appreciate your input guys.
 

Guitarist

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Just as a side (and I realise many don't agree with this), it's worth testing your cabling before the floor is actually laid. Just link your cables in choccy block and carry out cont and IR tests at the ends. This way, you know that nothing has been nipped and that the cabling is good to begin with.
 

topquark

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Dumb question time, is the oak boarding to be floating or fixed? If it's floating then you could run the conduit within the ply layer (not if fixed though).

I don't think LABC will be happy with you channeling out the underfloor insulation TBH, unless it's been over specified (depth wise).
 
S

sjm

Reference method 100 to 103 are used for T+E with at least 1 side of cable in contact with insulation material. Therefore if the cables are in conduit they are not in contact with the insulation material so reference method A will apply to the cables. Use table 4D5 column 7 of appendix 4 for CCC of cable.

IMO I would fix my cables in conduit on top of the concrete pad under the insulation. Perhaps you could install a few radial circuits for sockets, instead of a ring, to cover the lower CCC of cable in insulation.
 

Guitarist

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If you are going under the insulation, even in conduit, then agree method A, so radials may be a better solution. I can't really see an inch notch of insulation really affecting the insulation rating of the building tbh, but BC can be funny at times over negligible things. Maybe best to ring them up and ask what they would be happy to see.
 

topquark

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I've had it once before Guitarist. They don't like the reflective foil being breached. If it's not spec'ed with aluminium foil for the joints then they prob won't be too worried, as you say though best to ask.
 
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Blacktail

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Sorry seem to be having problems posting and you have all got ahead of me!! , but reassuringly you seem to have come to same conclusion that ref method A is the way to go. And radials look like a better option. The insulation is over 100mm and so dont think there should be a problem but take on board what you are saying. Inspector is on site this pm so will get builder to ask.

Guitarist, sjm, topquark thanks for the input didn't think i would fall quite so heavily at the first fence in my design, hopefully not bother you all again!!
 

pmcsparks

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Managed to get conduit laid in/under the concrete sub-floor at the last conservatory I did.
So method B60 worked for me. Ran two radials for sockets, tails run up in conduit to back-boxes.
Lighting was all in ceiling, clipped to joists and surrounded by insulation, so method 101 there.
 
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