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Cable/colour coding for a volt free connection?

  1. Jun 1, 2013 #1


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    This is a more a question about rules/regulations than actual EE, but I figured this was still the best sub-forum.
    We are about to start renovating a house, and we will have to change a few things in the central heating system.
    One problem is that we somehow need to get a cable from the wiring centre (downstairs) to the boiler which in a cupboard upstairs. This is a volt free connection that turns the boiler on/off.

    Now, I will let an electrician connect and check everything that is live. But since getting this cable from the wiring centre to the boiler will be messy and time consuming I figured I might save money by doing that part myself (and again, it is volt free so there isn't any danger).

    However, I haven't been able to find any information about what type of cable to use, or even if there are any rules/regulations or even recommendations about this in the UK. Does anybody know?

    For a volt free connection I could from an electrical point of view just use whatever 2 conductor cable I wanted. But I figured I should use something with sensible colour coding etc. to make sure I don't violate any rules (I will obviously avoid any colours that could make someone belive that this is live cable). This will after all be a permanent installation in a wall.
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  3. Jun 1, 2013 #2


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    A "volt free" connection is an open circuit, so no wire is needed.
  4. Jun 1, 2013 #3
    I'm not familiar with U.K. wiring code (law) but I would like to mention that there is a class of cable called "plenum cable" that is recommended/required for wire running through ducts and other communicating spaces. The key feature of plenum cable is the chemistry of its insulation: it is somewhat resistant to burning, and the products of combustion are less toxic to humans than those of normal cable.

    Surely, a true "volt free" connection is a short circuit, made with high-ampacity wire :-)

    All kidding aside, I'm not familiar with the trade expression "volt free", and it may have an established meaning in the U.K. and not in the U.S. so I'm reluctant to comment. In terms of control wiring, the usual thing to do is use low voltages. In the U.S., the trade expression for this is: "reduced voltage" and, by ancient convention, is often 24 volts, AC or DC.

    This type of system is governed more by law than physics, so I'll leave this now in the hope that someone familiar with U.K. wiring code will chime in with specifics.

  5. Jun 1, 2013 #4


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    Actually, yes, that probably is a better definition. I was thinking more along the lines of hey, if you are never going to put a voltage on it, what is the point of having it?
  6. Jun 1, 2013 #5
    Hard to fault your reasoning! A voltage-free circuit is the EE's version of: "If a tree falls in the forest, and there's no one there to hear it, does it make a noise?"
  7. Jun 1, 2013 #6
    There are a number of "best practices" that a good electrician or HVAC contractor will follow when installing control wiring. An example is fastening wires in the center of studs, rather than against a wall, to prevent someone from hitting the wire with a nail when they are hanging a picture.

    This may not be best forum, maybe a HVAC forum.
  8. Jun 1, 2013 #7

    jim hardy

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    You may be putting him in an untenable position - legally responsible for somebody else's work.

    Ask your electrician if he'd appreciate a helping hand with that cable pull and exactly how he wants it done.

    You'll not save a cent if he has to re-do your work because you didn't ask him how to do it.
  9. Jun 2, 2013 #8


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    Well, yes:tongue:

    However, in this context is simply means that that there is no EXTERNAL voltage applied across the connection, all your cable (with a switch at the other end) should do is to close a circuit; not e.g. actuate a relay. For some reason this is known as a volt free connection.
    Also, I don't think this is UK only, I've come across it before in control applications.

    I agree that it is a stupid name: but it is what it is called,
    Last edited: Jun 2, 2013
  10. Jun 2, 2013 #9


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    I thought of that, and I might just ask what cable to use once I know who we will be using.
    I will use an electrician for connecting everything to mains. However, there will probably be a couple of weeks or so between pulling this cable and the rest of the work, and I don't want to pay £150-200 for something I can do myself (they tend to bill by the hour, and most have a minimum call-out charge)

    Since this is not mains wiring there does not seem to be any legal requirements at all so I doubt an electrican will complain as long as I am reasonably sensible.
    Also, the rules here in the UK merely states that domestic wiring has to be done by a competent (not qualified) person. The only thing an amateur CAN'T do is to inspect work.
    Due to my work I am certainly competent enough for control wiring.

    Last edited by a moderator: Jun 3, 2013
  11. Jun 3, 2013 #10
    Actually this seems like you need "thermostat Wire" - this is for the control of the boiler? Here in the US we would refer to this a low voltage wire, or energy limited. Color coding would be regional - but if you are connecting to a boiler control - I would look that up, there are relatively standard color codes here for HVAC control.

    Volt Free is really misleading - since if there is NO voltage ( or current) - then the wire is not doing anything. ( Like saying I want to install an empty pipe)
  12. Jun 3, 2013 #11
    I'm guessing that "volt free" is what we call "dry contact" here in the US. When contact closes it does not apply any voltage. Far end equipment recognizes the dry contact closure, by whatever means.

    If this is thermostat wiring then, as Windadct mentioned, there are color code conventions. There are numerous online sources for this, not sure how much national dependency there is. Or take a look at the literature that comes with the thermostat.

    FYI, if you indend to notch or bore through load bearing studs to route this wiring there are a bunch of rules in the building codes for this too.
    Last edited: Jun 3, 2013
  13. Jun 3, 2013 #12


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    This does sound like simple low voltage thermostat wiring.
    You could put the wire in, but if its the wrong one, not gonna help.
    Also, the electrician is going to have several long speciality tools for getting thru top plates, walls and joists. Long pulling tools and long drills for those pesky spots.

    So once you buy all the tools you need and all the labor...it probably won't add up. Also, what may take you 4 hours may take him 15 minutes. Good luck either way.
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