routing ring main, celotex & underfloor heating

[meadowhog]

Hi All

Due to a floor to roof window I have to route the ring main conduit down through 75mm of tiled wet underfloor heating screed and 100mm of celotex. To avoid derating, can I chase a channel in the slab and lay under the insulation? Only the slab is down.

Out of interest 17th edition states that derating is a problem if the route through insulation is 400mm or more, is that 400mm unbroken or 4 lots of 100mm? The reason I ask is, I have on the same ring, another patio door where I could go over it if I cant go under it.

 

[ericmark]

Reading the 17th Edition 433.2.1 refers to "a reduction in current-carrying capacity" not "a change in cross-sectional area" so if for example 2.5mm with a current-carrying capacity of 21A ref method 100# was connected to some 6mm with a current-carrying capacity of 23.5A ref method 103# this would not really be considered as a reduction in current-carrying capacity.

Any connection which used screws must be accessible for inspection and there could be a problem in using thicker cable in some socket outlets although MK does seem to have bigger holes then other makes.

However using a cooker connection unit or other junction box designed for heavy cable you could still comply with regulations.

The 17th Edition does have some errors and is open to interpretation plus it is a recommendation not a law although it can be quoted in a court of law to show negligence. But some times you have to use some common sense and ask yourself is this a problem or not.

All cable however thick must generate some heat when current flows and only when one is on site can one often see when problems may arise. If the cable can get hotter without out damage to what it is buried in maybe one of the XLPE (Cross-Linked Polyethylene) or LSZH (Low Smoke Zero Halogen) cables should be considered with 90 deg rather than 70 deg rating also cables like the Ali-Tube to BS 8436 do have better heat conduction properties than twin and earth.

However it is you who must sign the installation certificate and you must decide what is OK. What I have said may not fit in with the situation you have.

 

[ColJack]

can you not route through the ceiling and down the wall? why do you have to go through the floor?

 

[ColJack]

eric? was that meant as a reply to this thread or did you post it to the wrong one by mistake?

 

[chapeau]

I'd say it was a extremely simple and inspired answer, so simple that it is not obvious, but so simple that it is very clever.

 

[meadowhog]

MMmmm, Eric.

Coljak, its a bungalow with a vaulted floor to roof window and patio door with 150mm celotex in the roof. If I go roof route I cant avoid insulation.

Thanks Chapeau, plastic conduit and chased into slab, it is then.

 

[ericmark]

eric? was that meant as a reply to this thread or did you post it to the wrong one by mistake?

Sorry yes put simply I was advocating using heavier cable than standard in area where there is high insulation.

However I was being careful to point out.

1) There is nothing to stop one changing up and down in cable size.
2) Even with heavier cable there can still be problems.

Where a kitchen for example is a distance from a consumer unit I have voiced the option that using heavier cable from CU to kitchen then a lighter 2.5mm on the local ring main does no strictly comply with 433.2.1 in BS7671:2008 because the two cables are of different current-carrying capacity and some time in the future some one may be unaware of the change in cable size and feed with inappropriate MCB size. Even though electrical it would cause no problems and cure volt drop problems. However up and down grading cable physical size because of the conditions which it meets is not an change in current-carrying capacity. However lightly reading the BS7671:2008 it is easy to miss the distinction between current-carrying capacity and cross-sectional area.

Seems I failed first time to get my meaning across is it plain this time?

In short I am saying why not just use thicker cable?

As a P.S. if the cable is clipped to something able to transmit the heat away there is not a problem.

In

100 to 102 it can be seen the cable is touching a beam or ceiling which will remove the heat only with

103 does the cable run where it is not in contact with a surface able to remove heat which explains the massive drop in current carrying capacity.

 

[ban-all-sheds]

Due to a floor to roof window I have to route the ring main conduit down through 75mm of tiled wet underfloor heating screed and 100mm of celotex.

Would having 2 radials instead of a ring make life easier?

 

[meadowhog]

Thanks guys,

2 radials would be ideal but the extension is 85m square so would not work. I also thought of extending the existing ring main but for the same reason plus the windows, it wouldnt work.

Now I get you Eric, I'd prefer twin and earth and if I run up and down the wall to the sockets as your PS I'm sorted.

Cheers

 

[ban-all-sheds]

2 radials would be ideal but the extension is 85m square so would not work.

I don't follow that...

 

[meadowhog]

Ban-all-sheds you've made me think and yes you're right, I could run radials. I could fit 3, and 2 spurs from existing ring to get to other side of windows.

Thanks all, I've now got options.