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Radial Voltage Drop Calculation?... By Load or MCB Rating?

Nano Brewery
I'm doing the calculation for total voltage drop.  I have 14 radials/ways going out.  As I understand it, that would equal a max allowance of 0.285% voltage drop per radial/way.  Or some sort of mixture throughout not exceeding the 4% drop allowance overall.  Firstly, is this correct? 


Secondly, should I be doing my calculations based on actual Load or the MCB ratings of the individual radials/ways? 


Thanks in advance!
  • 0
    Re Cable choice - what options would you personally go for?

    Main cable runs to power sockets and/or isolators around perimeter on tray and clipped direct, all above 1m from ground - NYY?  SWA?

    Secondary cable runs from Isolator (1m above ground) to Element (30cm above ground) -  CY?  NYY?  SWA?


    Cheers!




    Very difficult to say - I'm not able to see the overall situation remember.


    If the robustness of SWA isn't necessarily needed, I might consider BS 8436 types (which goes under various brandnames, but often have "shield" in the name) - much easier to dress and gland off than SWA, but you still get some decent protection from shock if the cable is damaged (unlike all-insulated types), easy to get a decent IP rating using simple stuffing glands, and you get some reasonable screening thrown in too. But that's just thought to add to your list - rather than a recommendation.


    If the heaters are similar to domestic immersion heaters, there's likely to be a requirement for heat resisting cabling - so heat resisting (HOFR) flex would be my first thought - but again that can't be a recommendation. Hopefully the heater's installation's instructions give you a clue to what's actually required. Likewise the heater's boss might constrain your choice (is it fitted with a 20mm threaded inlet or a cord grip that'll only accept flex for instance?)


       - Andy.


  • 0
    Mind you the one time H07 and similar non shielded 'soft' flex are used (and used by the mile)  are the film industry, and events like pop festivals and concerts, where it gets a real beating, being run over and knotted and quite often pulled about with fair robustness during installation as well  -  and it is in fact surprisingly robust. But yes, you do expect to see ferrules, and the right sort of cord grips at each end.

    You may well want something of that sort at least for the last hop from isolator to immersion heater. But I'd certainly agree, it is not common for fixed wiring, where something with a harder jacket and less finely stranded cores is expected.

    Mike.
  • 0
    mapj1:

    Mind you the one time H07 and similar non shielded 'soft' flex are used (and used by the mile)  are the film industry, and events like pop festivals and concerts, where it gets a real beating, being run over and knotted and quite often pulled about with fair robustness during installation as well  -  and it is in fact surprisingly robust. But yes, you do expect to see ferrules, and the right sort of cord grips at each end.

    You may well want something of that sort at least for the last hop from isolator to immersion heater. But I'd certainly agree, it is not common for fixed wiring, where something with a harder jacket and less finely stranded cores is expected.

    Mike.




    Cheers Mike.

    What options would you personally go for?


    Cable choice 1 - For main cable runs to power sockets and/or isolators around perimeter run via tray and/or clipped direct, all above 1m from ground - NYY?  SWA?  Something else?


    Cable choice  2 - For secondary cable runs from Isolator (1m above ground) to Immersion Elements (30cm above ground) -  CY?  NYY?  SWA?  Something else?

     


  • 0
    AJJewsbury:
    Was wondering if H07RN-F would also be suitable option?

    Flex isn't often used for fixed wiring - partly because it's more expensive and harder to dress neatly, but also because the finely stranded conductors may need special preparation (e.g. ferrules) for use in some types of terminals. It's more usual to use a rigid conductor system for the fixed wiring and then transition to flex for the final span to the appliance where flexibility or heat resistance is required. Immersion heater normally require heat resisting flex for example.




    Cheers Andy.


    The H07RN-F I'm looking at has rigid conductors:
    https://www.superlecdirect.com/cables-and-accessories-c1/h07rnf-flex-mains-and-control-cables-en6033212-s185/


    I see that NYY is compatible with 20/25 cable glands though, so may be a neater and more IP rated job all round?
    https://www.superlecdirect.com/cables-and-accessories-c1/iec605021-vde-0276603-nyyj-cable-s11/



     


  • 0
    The problem of being too specific, is that without seeing  the parts, advice is likely to be wrong.  Do the terminals of the heaters get hot ? how hot?  If they get hotter then PVC would like then rubber is better. Do the makers recommend a particular flex- is the cord grip  expecting a cable without a screen like H07 x , or if there an obvious place to terminate with a screen, then CY maybe - is the cable hole  circular or oval ?

    how long is the hop from heater to isolator, and is it in a place where it will get scuffed as folk wall by or push a trolley past ?

    Same applies to the fixed stuff - If it can be tied back out of the way almost anything that fits the terminals will do OK, but if it might sustain knocks and bumps then something like SWA is safer, but dearer and more wilful to fit. Think of NYY as looking like SWA but without the armour, so easier to fit, but needs routing in safe zones, or protecting, much as flat twin and earth would, but NYY is better than T and E for UV and for glanding to be watertight.

    Mike.
  • 0
    The H07RN-F I'm looking at has rigid conductors:

    It shouldn't have - the -F bit of the designation specifically means a fine stranded conductor.


    H= Harmonized

    07 = 700-ish volt rating (specifically 450/750V)

    R = rubber insulated

    N = Chloroprene rubber sheathed

    F = fine stranded


      - Andy.
  • 0
    Nano Brewery:

    The H07RN-F I'm looking at has rigid conductors:
    https://www.superlecdirect.com/cables-and-accessories-c1/h07rnf-flex-mains-and-control-cables-en6033212-s185/



    The page you link to seems to contain only H07RN-F, and the -F in the harmonised designation indicates "fine wire flexible cable" - so I'm not sure how they can have rigid conductors.
  • 0
    The website tells all
    Please note: The Superlec Direct team are not qualified as electricians or any other tradesmen. If you require advice ..

    (ask someone who actually does know, in so many words)

    I think you are being fooled from that web effect where the same images and captions are re--used in the description pages of many different products - it will be flexible, and it will achieve that flexibility by having the right number of fine strands.

    The clue is in the bending radius of 15 diameters for repeated flexing - this sort off stuff finds itself supplying portable tools and in machinery that moves repeatedly back and forth and the cables coil and uncoil every day for hundreds of cycles.

    Eventually in those applications the cores work-harden and snap, but if strain relief has been designed right that is many years down the line.

    M.
  • 0
    Thanks all!  Apologies, I misunderstood.  No mention of finely stranded conductors in description.  Need to school up on cable specs!!


    Immersion elements have M25 entry - https://processheatingservices.com/immersion-heaters/screwed-immersion-heaters/industrial-heaters-piirange/


    Elements are approx 30cm up off floor - They are facing into centre of room/work area, so could potentially be knocked IF a trolley was to be used or by a barrel being rolled around - So maybe SY or SWA for this?


    Sounds like HO7 (at £1.80/metre) may still be an option for Cable choice 1 - For main cable runs to power sockets and/or isolators around perimeter run via tray and/or clipped direct, all above 1m from ground?  Alternatively NYY (at £1.36/metre)?
  • 0
    So maybe SY or SWA for this?

    It's nigh on impossible to say at a distance. We still don't know about the temperature requirements for connection to the immersion heaters (either at the terminal or ambient next to the tank), nor other environmental conditions (wash down for instance) or 101 other factors - as we can't see the situation.


    The steel braid of SY might give you a little extra mechanical protection, but generally it's not substantial enough to act as a protective conductor, so can't be relied upon to offer shock protection if the cable is damaged through to the live cores.  Neither SY nor SWA have any particular heat resisting qualities. It might be that some heat-resisting cable plus some additional protection (say in flexible conduit) might be more appropriate. Or maybe something more substantial is needed to protect from impact damage (e.g. steel crash barriers) - if large heavy hard thing with sharpish corners are moved about carelessly. We just can't tell from here.


    Unfortunately cable selection isn't trivial - especially once you move out of the common domestic/office type settings. They actually write text books on this kind of thing (e.g. https://shop.theiet.org/guide-to-cables-and-cable-management) - and even those only tell you the sort of parameters to consider rather than saying "type x cable will be OK".


       - Andy.

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