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Using mains cable for control wiring, what about colour coding, and then a question about armour

Will D

Hello everyone,

Two questions for a personal project:

1.

I'd like to run some extra cable between my house and our heat pump for possible future use for controlling 3-way valves. My understanding is that BS7671 asks for 3-core control wire to have the neutral/mid-wire be blue, and the other two wires to be one of a range, but not yellow/green like an earth wire. Our heat pump asks for these wires to be 0.75mm2 CSA and as they're for mains voltage they'll have a suitable voltage rating too. However, I've searched around and can't find any 3-core cable which complies with all of these requirements.

The main sticking point is the colour-coding. I can only find 3-core cable in either standard mains colour-coding (brown, blue, yellow/green) or in 2x black and 1x yellow/green. I thought of running a cable with many cores, as I need to run a bunch of 2-core as well for other possible future uses, however all the multi-core I can find also follows the black+yellow/green coding. I can't find any suitable 3 or multi-core wire that has any blue cores!

So, I was wondering what's acceptable, or what's the common practice. Just use either the mains or the black+yellow/green multi-core wire and sleeve it to make it clear what's doing what? Or do I not need to do anything at all?

For reference, the existing wiring from the install done a year+ ago uses a black+yellow/green multicore to run out from the wiring centre to the heat pump, and mains-coloured 3-core inside the house from the wiring centre to the connection box with the 3-way DHW valve. This suggests to me that black+yellow/green multicore is acceptable without sleeving, however it's the fact that BS7671 seems to ask for a blue core which has me bamboozled.

2.

As said, the cables need to run out from the house to the heat pump, it's about 7m total with about 1m actually underground. The cables are in trunking the whole way, even when underground. I don't think the existing cabling has armour, so I wasn't going to buy armoured cable for these new runs, but as I was already writing a question I thought I'd ask about this too.

3. ;)

Not a question, but for complete info I also need to run a 5-core cable for connection of the heat pump's wired controller which will follow the same route. It doesn't need 0.75mm2 and doesn't carry mains voltage. The heat pump manufacturer told me that it doesn't need to be screened/shielded and even Cat6 cable would be fine. I was just going to buy a 5-core multicore cable for this.

Cheers Slight smile

  • 0 in reply to Will D
    Will D said:
    then what does it mean by "mid-wire"?

    It's to cover "neutral like" cases in some less common supply arrangements, e.g. the "M" wire in this diagram (from BS 7671 section 3 - the "coils" represent supply transformers):


       If this is a fairly common type of cable-need, then why can't I easily find this kind of cable in the UK?

    Oversleeving the ends is the usual solution. The only case I can think of where "special" core colours are available is for "twin brown" T&E - used for switch drops - but even then the vast majority of installations still use brown/blue and oversleeve the blue with brown sleeving. It's just easier to have one reel of cable and use it for everything - and it's normally significantly cheaper too (because the way the wholesale system works higher volumes mean lower prices).

    Also once you get two or more cores the same colour it becomes difficult to work out which is which (brown and black oversleeved brown are immediately distinct at both ends - whereas two browns means you'd have to get a meter out and start continuity testing or have numbers printed on each core to be able to connect them correctly (other then in the simple switch drop case where it doesn't matter if they're reversed). Which is probably why you don't get 3-core all brown - even though the vast majority of 1.0 or 1.5mm² 6243Y is used for two-way switching on single phase lighting circuits.

       - Andy.

      

  • 0 in reply to Will D
    Will D said:
    I'm just surprised that a 3-core cable which follows the colour coding for control wiring isn't readily available,

    Really? (and by that I mean 'identification requirements of both BS 7671 and BS EN 60204-1 - which doesn't have to be identification by colour).

    What about here: https://www.superlecdirect.com/6940-10x-1-5mm-10core-bs5467-xlpe-swa-pvc-cable-numbered-black/

    That cable is available in 2, 3, 4, 5 and larger numbers of cores ... all with numbered cores like a 'traditional' indoor control cable use in machinery installations, like YY, CY or SY ...

    Here's the rub, though,   ... it might be a lot more expensive, and/or you might need to buy very long lengths.

    ... but I've worked on jobs where literally 10s of km of multicore control cables with either numbered cores, or varied colour cores (including bicolour arrangements), with overall armour, and sometimes twisted pairs (with each pair individually screened) have been installed.

    Power cables are used a lot more frequently, and more often in shorter lengths (by shorter, I mean purchased in no more than 100 m reels) ... it's really all driven by market forces as to what is regularly stocked in wholesalers, and whether you can buy it by the metre, reel ... or alternatively on the huge wooden drum with a minimum length of run.

  • 0

    Hi  ,

    Thanks for the suggestion, I do see that Superlec has a wide range. However I still can't see a three core cable with one blue and two either "Brown, Black, Red, Orange, Yellow, Violet, Grey, White, Pink or Turquoise", one of the cores is always yellow/green for earth instead. I didn't see any 4-core with blue, but there is 5 core with blue, yellow/green and then three cores which can be used for live. So, I could use this and earth both ends of the yellow/green and just not use one of the cores. Seems like a waste of copper to me.

    PS - for some reason I can't see any more posts beyond my reply to mapj1 7 days ago, I think the webpage isn't loading properly. I only saw gkenyon's because I got an email notification about a suggested answer.

  • 0 in reply to Will D

    I was pointing to a suitable cable that meets the requirements. But that doesn't mean you can buy it in small quantities

    The reason you can't find what you want is likely the issue of simple "supply and demand" ... in that, in my career, I've ordered many km of conformant cable ... but on a day-to-day basis, such cable if in sparse demand and wholesalers will usually stock what's normally required ... which is most often power cables.

    Will D said:
    PS - for some reason I can't see any more posts beyond my reply to mapj1 7 days ago, I think the webpage isn't loading properly. I only saw gkenyon's because I got an email notification about a suggested answer.

    I'm sure that, when the third-party suppliers fix the issue on the Forum, the currently "unloadable" posts will be available.

  • 0 in reply to Will D
    Will D said:
    one of the cores is always yellow/green for earth instead.

    IN a power cable that will always be the case. For machinery installations, earthing (well it's called protective bonding in that standard) is usually needed to conform.

    a lot of MVs seem to need a CPC, certainly the metal ones, but I can see that there are plastic ones with double insulated on them.

    BS 7671 does require the CPC to be taken to each point in wiring normally, so are you sure you don't need the CPC?

  • 0 in reply to gkenyon

     no I'm not sure I don't need the CPC and if it's there I'll use it. I was expecting to find 3-core without, just as 2-core without is available. Of course it's no problem to use a cable with more cores, earth the yellow/green core, and not use any spare cores. Or use a multi-core black+yellow/green and sleeve all the cores I do use with appropriate colours, also earthing the yellow/green core.

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