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Thread: Running conduit under concrete floors

  1. #1
    Automated Home Jr Member
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    Default Running conduit under concrete floors

    Hi All,

    I have a general question about running conduit under a screeded floor around the house and wondered how others have managed.

    My Node - 0 is in a rack in the cellar. I'm presently working on a "media" room that is above the cellar. This needs quite a bit of wiring and just happens to be the best room to work on at present.

    I'd ready to complete the room, which includes fitting underfloor heating and adding a screed. The problem is that I have 2 further rooms which are connected to this and I'd like to make sure I can run cables into these as required.

    I plan to run two 3" downpipes under the floor and into the adjacent room, use one for power cables and the other for data. This allows me to get cables into the first room, and from there into the second one.

    However, I'm not sure what's the best way to distribute within a room itself, or in fact, how much conduit to run in the floor/walls. For example, should I run a downpipe along the centre of the room and 'tee' off in the middle to provide a feed to the centre of each wall, or is this a bit over the top (my wife certainly thinks it is!). Alternatively, do most people just get cable into the room and put "enough" in there, screed the floor and hope for the best!

    Any thoughts appreciated!

  2. #2
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    I like the conduit idea but thinking practically, once it is in there and buried, could you really get extra cables into it.

    I used a cable rat for many years as a push through, tape on and pull back, and it is great for relatively straight routes, how would you get round the 90 degrees?

    Regards,
    Peter.

  3. #3
    Automated Home Legend chris_j_hunter's Avatar
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    some rules of thumb we're planning with that might be useful :

    filling conduits more than 40% with cables can be tricky ...

    multiply first estimate of how many cables you need by three ...

    be careful not to bend cables at radii less than ten times their diameter ...

    keep power & signal cables 300 or more mm apart & cross them at right angles ...
    Last edited by chris_j_hunter; 30th July 2009 at 10:11 AM. Reason: added fourth

  4. #4
    Moderator toscal's Avatar
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    Also never have more than 2 bends in a conduit, as it gets really really hard to pull the cables. Something I have done is to use a vacuum cleaner on one end of the conduit. Then make a small plug attached to the cables lubricate plug and cable with washing up liquid insert plug into the conduit at the other end and then get someone to turn on the vacuum cleaner. It works quite well over the longer straight runs. Or if you know someone with a compressed air supply you can use this, on the same end as the cable is entering, instead or as well as the vacuum cleaner. I've used the compressed air method for running new electrical supply cables from an appartment on the 6th floor to the basement where the electricty meter was. Worked very well. mind you could have killed the chaps who pulled out the original cables without putting in a rope or something first. It took us all day, ( due to rules of the community we could only work from 9:30am to 2:00pm).
    Also something we do is to leave a string in the conduit so when it comes to adding more cables its quite easy. And after every second bend we put a junction box. On one install we used square trunking as this had a lower profile.
    Last edited by toscal; 29th July 2009 at 07:22 PM.
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  5. #5
    Moderator Gumby's Avatar
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    "I used a cable rat for many years.. ..how would you get round the 90 degrees? "

    Err ... stronger cheese?

    Sorry
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  6. #6
    Automated Home Guru JonS's Avatar
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    I've used conduit in several places including in the loft rooms to get cables to the roof void side of the rooms where there were more than 2 bends and I used a vacuum cleaner to suck some string through - worked a treat! Pulling cables through the same conduit (~45mm waste pipe) was much harder though - will have to try washing up liquid next time! IME you just won't get a T-junction to work as waste pipe has a direction of flow so getting cables to go into the wrong side of the T is nearly impossible (I tried!). Also where I used a T in 20mm conduit in the walls, it was impossible to get the cables through the T without removing the inspection cover. Fine for first time installation but useless as far as allowing future cable updates - even swept elbows were tricky as there are edges on the elbows that the end of the 2nd cable would invariably catch on:-(

    For cables within the screed - have a think about floor loading. If you are putting a 75mm pipe through 100mm of concrete screed - you won't have a lot of concrete to support the piano, washing machine, party-dance-floor, kids jump zone or what ever gets put on top. Maybe worse would be the flex it may offer to the top screed and UFH pipes. That said I do have a toilet waste pipe in my sub-based and that's ~100mm, with concrete, insulation and top screed on top, so maybe you need a bigger pipe designed for significant load set deeper rather than a cheap drain pipe?

    In the one place I needed to do something similar I tried to use waste pipe but at the edge of the room wherethe UFH pipes were not going to be. The problem was the pipe floated on the screed as it was being poured, in the end I had a u-shaped channel which was good enough for the HDMI cable I put in under the wooden floor.

    One final thought re mains cable bundle in UFH screed - get an electrician to spec the cable thickness. IIRC you have to de-rate for bundles and probably have to derate again for a heated environment in much the same way as is necessary for running in insulation. Also flat cables don't run well in round conduit , though at 75mm it may not be as bad as running in 20 mm conduit which isn't good - should have used oval!

    Have fun and let us know how it goes.
    Last edited by JonS; 29th July 2009 at 11:42 PM.
    JonS

  7. #7
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    Hi All,

    Thanks for that.

    The pipes in this case are in a part of the house that have timber joists. We're battening the floor and screeding over that, so I have room to run them under the joists/insulation.

    I had thought of doing the same as JonS and running some box-section around the outer edge of the screed, but didn't think I'd have enough room. I was also trying to split power and data and it appeared to be a bit messy/cumbersome.

    > multiply first estimate of how many cables you need by three...

    Thanks Chris, but that's part of the problem for me. Do you mean add three times the amount of cable now, for use later? If so, I've already allowed for quite a lot, so I'm not sure where I'd route the additonal ones to or where to leave them ready for use

  8. #8
    Moderator Gumby's Avatar
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    It may be worth asking this question in the general forum, or I could move the thread now all the Idratekkies have had their 2p.
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  9. #9
    Automated Home Legend chris_j_hunter's Avatar
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    >move the thread ...

    maybe just a cross-reference would do the job ... might be good for drawing people in, too?

  10. #10
    Automated Home Legend chris_j_hunter's Avatar
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    >add three times the amount of cable now, for use later ?

    yep - evidence seems to be that many (not all) wish later they'd put more in, cable's not all that expensive, and adding more later can be a pain ... OTOH, if you're well up to speed on these things & have done this sort of thing before, it may not be necessary !

    in our case, we've tried v.hard to define what we'll need, and do believe our estimates are good, but we'll still be putting-in rather more - Cat-5e, CT100, and power ... and leaving space for fibre to be added later (not confident enough to do it sooner) ...

    BTW, some of the Cat-5e might have to be screened - eg: if want to use Idratek intercom options ...
    Last edited by chris_j_hunter; 30th July 2009 at 11:44 AM.

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