Sub main & 3%

Discussion in 'Electricians' Talk' started by seneca, Sep 14, 2014.

  1. seneca

    seneca Screwfix Select

    Hi all,
    If a sub main is feeding an outbuilding with sockets and some lighting does the 3% volt drop have to be satisfied by the sub main? I've always assumed it does but as the sub main is not specifically a "lighting" circuit I started wondering!
     
  2. JP.

    JP. Screwfix Select

    Hi Sen. From my way of thinking the sub main is limited to the 3% volt drop whatever happens - thus the final circuit would be used to carry out any other VD calcs which would incorporate the sub VD in the calculations.
     
  3. Lectrician

    Lectrician Screwfix Select

    Yes, it's a factor of it - Your sub-main would not be allowed to drop the full voltage anyway - You need to factor in the sub-circuits from the sub-main. So the sub-main may drop around 2%, leaving 1% for your lighting circuits.

    Voltage drop is not per circuit, it is "between the origin of the installation and the point of load", or similar wording. Your 3% for the lighting circuit in the shed is 3% over the sub-main and the final circuit.
     
  4. seneca

    seneca Screwfix Select

    Thanks chaps, yes that's the way I've always understood it.
    The reason I started thinking more into it is that I have a job to do with a very long sub main, 220 metres which with a 60 amp supply will need a 95mm swa! (3%) The client says he'll never use more than 40 amps at the outbuilding so I could fit a 45 amp fuse in the switch fuse at the origin, in which case 70mm swa will be ok. (still about 3 grand's worth though!) I just don't want him coming back and blaming me for using too small a cable in the future if he decides to want to use more power.
     
  5. Lectrician

    Lectrician Screwfix Select

    For long runs, use some calc software, or consider the below quotes. For long runs, where an increase in cable size is seen, re-calculate using the correction for operating temp calculation - a 95mm cable is never going to get near it's operating temperature with just 60amps.

    "Where the wiring systems of the installation are longer than 100 m, the voltage drops
    indicated above may be increased by 0.005% per metre of the wiring system beyond lOO m,
    without this increase being greater than 0.5%."

    "The values of voltage drop assume that the conductors are at their maximum permitted normal operating temperature."

    "The direct use of the tabulated (mV/A/m)r or (mV/A/m)z values, as appropriate, may lead to pessimistically high
    calculated values of voltage drop or, in other words, to unnecessarily low values of permitted circuit lengths. For example, where the design current of a circuit is significantly less than the effective current-carrying capacity of the
    chosen cable, the actual voltage drop would be less than the calculated value because the conductor temperature
    (and hence its resistance) will be less than that on which the tabulated mV/A/m had been based."

    In saying all that, my software shows 60amp/90mm and 40amp/70mm :)
     
    Last edited: Sep 14, 2014
    FatHands likes this.
  6. seneca

    seneca Screwfix Select

    Thanks for that Lectrician, the cable is going to be directly buried at about 0.6m for the majority of it's length so not likely to get much heating. Would you go for 70mm if you were doing this job?
    I used Cablemate for my calcs. btw.
     
  7. FatHands

    FatHands Well-Known Member

    Hi Sen,
    Would you recommend this software? I see its an application and not a spread sheet!

    Edit: I've used the calculator on the TLC site up until now.
     
    Last edited: Sep 15, 2014
  8. seneca

    seneca Screwfix Select

    Hi Fats,

    I usually use the TLC volt drop calculator but it doesn't cater for this length of cable, I find the Cablemate one is pretty good, it has a lot more variables shown.
     
  9. FatHands

    FatHands Well-Known Member

    hiya,
    does it account for a combination Sen, i.e (and this is a bit pedantic i guess!), 10 metres in 10mm T&E in surface mounted trunking and then 20 metres in 10mm 3 -core SWA buried?
     
  10. seneca

    seneca Screwfix Select

    You'd probably have to do that as 2 separate calculations Fats.
     
  11. FatHands

    FatHands Well-Known Member

    Got it. Cheers Sen. :)
     
  12. iandaviot

    iandaviot Member

    Megger have iphone app that seems to be quite good for quick cable calculations.
     
    FatHands likes this.
  13. seneca

    seneca Screwfix Select

    Lec, would you go for 70mm if you were doing this job?
     
  14. Lectrician

    Lectrician Screwfix Select

    I think so, yes. Still with a 60 amp fuse. You can base the VD on anticipated load rather than OCPD. A two core would be fine to save money, TT'ing the shed if SWA not man enough (not able to calc at the minute).

    Is this a farm, sitting right on its own TX? Is it a small TX? While you shouldn't consider the DNOs volts, it's comforting to know you're not at the end of a long line when running circuits tightly to their max volt drop.
     
  15. seneca

    seneca Screwfix Select

    Thanks for that Lec. Whilst it's not actually a farm as such there is a lot of land there, and he now tells me he has 2 barns to be supplied from the sub main. The DNO's cable comes straight of the pole which is close to where the cut-out and meter is fitted, about 15m away i'd say. I can't remember seeing a TX on that pole but they have given him a 100 amp supply so I imagine it's probably not too far away. Yes I will use 2 core swa, I've put a 30m/a rcd at the supply end because that's what I had at the time but I will change that for 100 m/a later. One other thing, i'll probably put garage type c/units in the barns, say 20a mcb for sockets and 6a for lights, i'm wondering whether to connect all the earths together so that they're all served by the rod which I've put in at the supply end and maybe another rod at the far end but i'm thinking maybe it would be better to isolate the sub main armour at the barns and put separate rods in for that end, do you think that would be better than connecting them all together Lec?
    None of this will be connected to the DNO's earth of course, in fact i'm surprised they've given him a PME supply!
     
    Last edited: Sep 18, 2014
  16. Lectrician

    Lectrician Screwfix Select

    If it's all TT, I connect all the rods together, via the armouring. The lower the better :)

    TN-S is always preferred, but PME is often presented where TN-S can't be. Occasionally, if a TX and distribution is feeding a few sites, they always keep the same earthing system through out that distribution. This sometimes means a supply being PME where you would prefer a TN-S.
     
    FatHands likes this.
  17. FatHands

    FatHands Well-Known Member

    Thanks for posting that Ian, I just put that on my phone - its like the TLC one only doesn't depend on a connection :)
     
  18. seneca

    seneca Screwfix Select

    Thanks Lec, I was thinking it best to connect them all together which will give the best possible Ra, thanks again.
     

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