Conduits in insulated walls

[jamesph]

Hello All.

I have searched this and several other forums and found lots of advice but nothing that addresses my problem.

I am presently constructing a new house using Insulated Concrete Forms (ICFs). – Just in case you don’t know, these are basically polystyrene Lego bricks with a hollow core which is filled with concrete. The thickness of the polystyrene on the inside is 57mm. This will eventually be covered with plasterboard.

I understand that it is not advisable (or perhaps not permitted) to place pvc cable in contact with polystyrene.

Consequently I have to provide conduit embedded in a chase in the polystyrene insulation for the vertical drops to sockets and switches – max drop length about 2.5m. These will only be from a point above a suspended ceiling where the cables can be surface clipped on timber. The cable runs are fairly long and I don’t want to run long conduits or trunking above the ceiling with singles.

I am weeks away from appointing an electrician but I need to install the conduit drops soon. It has already been decided to do most of the circuits in 4mm² t&e in radial rather than final ring mains.

What I can’t find is clear guidance as to the size of the conduit. I am assuming that the 4mm² t&e would drop down to the socket then rise again to go horizontally to the next drop – this would require two t&es in the conduit.

Or

It could it drop to the first socket then travel horizontally in conduit to the next. This would only require one t&e in the conduit.

Or

Two conduit drops per 13A socket.


Is there another way?

Would it be appropriate to apply the 45% rule as the conduit would be embedded in insulation?

Your advice will be appreciated.

 

[SimonH2]

Can't help with the specifics, but have you considered radials with 2.5mm² cable (with 20A or 25A (if you can get them) MCB) ? It's a lot easier to work with, probably a lot cheaper, and except for a couple of exceptions is likely to be more than adequate in a modern house.
Outside of kitchen and laundry, most homes probably wouldn't come anywhere near 20A on a socket circuit.

 

[jamesph]

Thanks for your response Simon.

I have considered 2.5mm² cable but the cable runs are quite long – possibly 30m for the longest – and I think there would be some de-rating because of the conduit in the polystyrene wall.

But I think I could accept a lower rated MCB because these circuits (not in a kitchen where we will probably do a ring) will mainly be convenience sockets for vacuum cleaners and the like. But who knows what will be connected in future?

 

[ban-all-sheds]

I am weeks away from appointing an electrician but I need to install the conduit drops soon.

HTH did your project manager allow you to get to the point where real electrical installation design work is needed and yet you are weeks away from appointing an electrician?

Whoever he is, he is a useless, incompetent t**t who could end up costing you money/derailing your schedule/both.

Somebody is going to have to sign this:

I being the person responsible for the design, construction, inspection & testing of the electrical installation (as indicated by my signature below), particulars of which are described above, having exercised reasonable skill and care when carrying out the design, construction, inspection & testing hereby CERTIFY that the said work for which I have been responsible is to the best of my knowledge and belief in accordance with BS 7671:2008, amended to 2015 except for the departures, if any, detailed as follows:

And if you don't have an electrician yet, and someone unqualified to do design is making decisions on circuit topologies, cable sizes, conduit sizes, installation methods etc, just who is going to sign that?

What are Building Control expecting? What was said to them about who would be doing the electrical work, and ensuring compliance with Part P?

It really does beggar belief that the guy in charge has allowed this situation to arise. 09:00 Monday at the latest you need to grasp this idiot warmly by the throat, pin him to the wall and tell him that you need an electrician on board before the end of the day/week, depending on how long you can have things on hold.


Unbelievable.

 

[jamesph]

Thank you ban-all-sheds for your most helpful and informative post.

I will be having a strong word with the “useless incompetent t**t” first thing on Monday.

Have you personally had any experience in installing T&E in conduit in walls in a passive house constructed in ICFs? If so I would still value your professional advice.

 

[ban-all-sheds]

No, but if I had to, I would know how to design it.

And there's the rub - nobody here can sign the EIC and nobody here can do whatever your BCO is expecting.

You have to have the person who is going to sign the declaration(s) actually do the work he will be signing for. You need your electrician now, not in several weeks time.

 

[jamesph]

Thanks again ban-all-sheds.

You really do need to leave me to worry about who signs what. If you have no constructive advice in relation to a question which was simply about cables in a conduit then perhaps you should leave further posts to your more helpful colleagues.

You know nothing about the circumstances surrounding this build or the qualification of the people involved. The circumstances of conduits in polystyrene walls are a bit unusual so if you have nothing constructive to say about the technical aspects, please refrain from further posts – you are not been helpful - just disrespectful.

 

[SimonH2]

Jamesph ... Just bear with him, he's really irritating like that and does put a lot of people off, but (more irritatingly) he is mostly right on this. Someone needs to be designing this installation, and whoever that person is needs to have quite a bit of specialist knowledge. Asking ad-hoc questions from "some blokes on an internet forum" is one thing if you mostly know what you are doing and need to check a few details; it's not going to do the job if your level of knowledge isn't sufficient to know what questions to ask.
For example, you've asked about conduit sizes - but you've not asked about the cable rating for this installation method.

It's not clear from your original post who is responsible for what with this build. Is it something YOU are personally doing, employing contractors as required, or is it something you have employed someone else (an architect ?) to manage ? The difference is that in the first case it is you who is the “useless incompetent t**t” and you need to be sorting yourself out, in the latter case it is whoever you have employed.

A couple of answers to the OP do come to mind.

Polystyrene and PVC cable absolutely do not mix. The polystyrene "sucks" plasticisers out of the PVC - the polystyrene "melts" and the PVC goes brittle.

For a situation like this, I don't think anyone worries about "fill factors" for the conduit. Most people working with cable all the time "just know" which size to use for drops like this. From memory, I think 2off 2.5mm² T&E will fit comfortably in a 25mm oval conduit - I seldom work with 4mm² cable so off-hand couldn't say what size you'll need.

 

[RF Lighting]

A straight drop of 25mm probably would be ok, but to cover all bases I'd be installing 2no. 25mm steel conduit drops to each 35mm box. You'll also need to use box extenders to allow the plasterboard to fit.

 

[ban-all-sheds]

You really do need to leave me to worry about who signs what.

Indeed.

But my duty of care to you means that I have to make sure I do what I can to alert you to the problem that somebody is going to have to sign certificates, and that you can't.

If you have no constructive advice in relation to a question which was simply about cables in a conduit then perhaps you should leave further posts to your more helpful colleagues.

If you end up with a completed house and no certificate from Building Control you'll realise just how constructive my advice was.

You know nothing about the circumstances surrounding this build or the qualification of the people involved.

I do know that there is no qualified electrician doing the things for which a qualified electrician is required.

And I know that whoever is in charge of this project has behaved in an extraordinarily unqualified manner by allowing it to get to this stage without the involvement of a professional electrician.

The circumstances of conduits in polystyrene walls are a bit unusual

Not to anybody competent to design electrical installations.

so if you have nothing constructive to say about the technical aspects, please refrain from further posts – you are not been helpful - just disrespectful.

Disrespectful to whom? Your benighted project manager who is defrauding you and leading you into trouble? I'm all broken up about that.

 

[jamesph]

Ban-all-sheds:

Whilst I applaud your desire to make sure that I get this installation right and within the appropriate regulations I can’t understand why you should viciously berate the person that you call a “useless incompetent t**t.”

This was unnecessary, pointless and it undermines your authority. You have assumed that this project will not be designed and signed off by a competent person – you can’t know that. You can only incorrectly infer it from what I have written. You would be advised to ask questions before jumping to conclusions. And if you think I am stupid enough to complete a project without the right certifications then you are wide of the mark. I have full contact with BC and with LABC who are guaranteeing the build.

As a retired Civil Engineer with a further, albeit minor, qualification in electrical engineering I could design this installation but I am not qualified to sign it off. What are your qualifications?

For reasons that I will not bore you with I simply need to install a few lengths of conduit in advance of the final design and installation of the system. I just needed a bit of reassurance. I don’t need your preaching or your patronising. I would have thought that the language and content of my first post would have indicated a reasonable level of understanding. Why could you not deal with the question? I would not have objected to a gentle, respectful reminder of the need for competency and certification.

Your first post was unforgivable – I have jumped to the conclusion that you had had a bad day or as the time of the post was late Saturday night you were probably just the worse for drink. I have as much justification for that view as you had for yours. Actually had you thrown in a few smiley faces I would have seen your post as comedy!

Your last post was not much better, being an attempt to justify yourself.

You have made a lot of noise but you have significantly failed to offer any advice on the actual question raised – You say you know how to design the conduit drop but have not given any evidence that you can.

You have no duty of care to me.

You say that this conduit design would not be a problem for “anybody competent to design electrical installations”, well review this thread and you will see that the three helpful posts (for which I thank the contributors) are not exactly in agreement in their advice so there is room for interpretation or uncertainty about the correct solution or at least an acceptable solution. Obviously I would have thought that in order to maintain your good standing on the forum you would be the first to propose THE UNIQUE CORRECT solution rather than just claim to be able to do the design.

So there it is. Please be aware that I will not respond to any further posts from you, Ban-all-sheds, but I would welcome any further assistance from your friendly colleagues.

Kind regards

 

[cjard]

I would still value your professional advice.

While certain emphatically made points on DIYN have some merits* I would hasten to add that they are not necessarily made by electrical professionals..


*Aside from the fact that they frequently depart from the entire raison d'être for this site, that being to help people do things themselves

 

[cjard]

.
For example, you've asked about conduit sizes - but you've not asked about the cable rating for this installation method.

To be fair, he did make a statement in one of his posts hat led me to the conclusion he's aware of the concept of derating..

 

[SimonH2]

Ban-all-sheds:

Whilst I applaud your desire to make sure that I get this installation right and within the appropriate regulations I can’t understand why you should viciously berate the person that you call a “useless incompetent t**t.”

...

You are best just "tuning out" to BAS's abrasive posts - no rational discussion has ever persuaded him to change so far. We have our suspicions as to the reasons, but ....

For reasons that I will not bore you with I simply need to install a few lengths of conduit in advance of the final design and installation of the system.

It could be argued that the design should come before the installation. What if, when you come to someone else finalising the design, they turn round and say something like "if only you'd ...." ? Also, you are putting someone in a tricky position if they arrive to find the conduits in and covered up - with only your word that they are (for example) thermally in contact with the plasterboard surface. As BAS correctly points out, someone will have to sign a certificate - and if they can't see first fix then they are having to take your word for something they are signing a legal document about.

 

[cjard]

You are best just "tuning out" to BAS's abrasive posts - no rational discussion has ever persuaded him to change so far. We have our suspicions as to the reasons, but ....

BAS runs the London Fire Brigade