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Single rcbo in shed

  • 15-08-2018 4:40pm
    #1
    omega42
    Registered Users, Registered Users 2 Posts: 517 ✭✭✭


    Hi folks,

    Theoriticlly if someone wanted to bring power out to a shed, would the following be an option.

    4mm cable from house cu out to shed cu. The house side would be protected with a b32 rcd. Shed is about 10m from house cu. Cable will be run through a pipe buried in ground.

    I am wondering would a single rcbo be enough to run a few plugs and 3 led lights (two ceiling and a security light) or should a second rcb/mcb. I also have a second rcbo that could be used for lights if it would help.

    Thanks for your input

    Seamus


Comments

  • #2
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,613 Mod ✭✭✭✭2011


    For a start a 4mm sq. cable is too small for a 32A RCBO.


  • #3
    omega42
    Registered Users, Registered Users 2 Posts: 517 ✭✭✭omega42


    2011 wrote: »
    For a start a 4mm sq. cable is too small for a 32A RCBO.

    Thats what I had lying around, so if I got a 25A rcbo would I need another mcb along with the Rcbo or would it be possible just to use the rcbo?

    Thanks


  • #4
    [Deleted User]
    Posts: 5,238 ✭✭✭ [Deleted User]


    2011 wrote: »
    For a start a 4mm sq. cable is too small for a 32A RCBO.


    Is it?
    Depends on how long it is surely?


  • #5
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,613 Mod ✭✭✭✭2011


    Sir Liamalot wrote: »
    Is it?

    In general, yes a 4 sq. is too small.
    It is only large enough a cable in exceptional cases.

    I don’t think you will find a REC that will feed a 4 sq. circuit from a 32A MCB.


  • #6
    [Deleted User]
    Posts: 5,238 ✭✭✭ [Deleted User]


    4mm² is standardfor 32A single phase flex cable I quite often plug downstream distros (3x16A) into with onboard RCBO.
    Just a cowboy me.


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  • #7
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,613 Mod ✭✭✭✭2011


    Sir Liamalot wrote: »
    4mm² is standardfor 32A single phase flex

    I disagree.

    There are plenty of cookers fed from 32A MCBs up and down the country.
    Let’s be honest: Standard practice is to wire these with a 6 sq. or larger, not 4 sq. isn’t it?

    I very much doubt that any REC will wire a 4 sq. circuit and protect it with a 32A MCB. Just because it “works” doesn’t mean it is ok. Now maybe you can show that on short cable runs it is ok, but a shed is typically a long cable run.

    Besides, a 20A supply is more than enough for most sheds.


  • #8
    Risteard81
    Registered Users, Registered Users 2 Posts: 2,567 ✭✭✭Risteard81


    To be fair to Sir Liamalot there are reference methods whereby 4mm^2 live conductors are adequately protected by a 32A device. Whilst it's common for 6mm^2 conductors to be used for 32A radial circuits, very often they are not actually required.

    In fact many in England wire 32A radial final socket circuits with 4mm^2 T&E.


  • #9
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,613 Mod ✭✭✭✭2011


    Risteard81 wrote: »
    To be fair to Sir Liamalot there are reference methods whereby 4mm^2 live conductors are adequately protected by a 32A device.

    Sure, hardly standard practice in Ireland though.

    Whilst it's common for 6mm^2 conductors to be used for 32A radial circuits, very often they are not actually required.

    Agreed, for a short cable run a 4 sq. cable may be ok.

    However based on the information provided by the OP it would seem highly likely that the cable run is not that short (as the cable would has to be buried). Therefore I would it is unlikely that a 4 sq. would be sufficient.

    Will it work? Probably.
    Will the maximum volt drop and disconnection times be exceeded as per ET101? Maybe.

    When advising someone and the information is that limited I would err on the side of caution by recommending a larger cable size (or protective device with a lower rating).

    In my experience these cable sizes are generally selected in well wired domestic installations in the ROI:

    1.5 mm sq for 10A MCBs
    2.5 mm sq. for 16 or 20A MCBs
    4 mm sq. for 25A MCBs
    6 mm sq. for 32A MCBs
    10 mm sq. for 40A MCBs

    Would this align with your experience the ROI?

    That is not to say that smaller cable sizes are not permitted,
    In fact many in England wire 32A radial final socket circuits with 4mm^2 T&E.

    ...in Italy they don't really believe in earthing, in the Netherlands they wire sockets and lights on the same circuit, in France they have ordinary sockets in the bathrooms.

    Not really relevant as we are in the ROI.

    Edit: I accept your point that the laws of physics apply here as the UK.


  • #10
    [Deleted User]
    Posts: 5,238 ✭✭✭ [Deleted User]


    I run sheds from solar powah! :p
    But yeah 4mm² won't cut it there either.

    I was talking about event cable.. Big blue plugs and sockets, rubber box distros, H07RN cable. 32A single is 4mm² (and sometimes 2.5mm²)
    I don't ever recall seeing 3 x 6mm² for this.


  • #11
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,613 Mod ✭✭✭✭2011


    Sir Liamalot wrote: »
    I run sheds from solar powah! :p

    :)
    But yeah 4mm² won't cut it there either.

    There you go.
    I was talking about event cable.. Big blue plugs and sockets, rubber box distros, H07RN cable. 32A single is 4mm² (and sometimes 2.5mm²)

    This is a pricey, high quality cake that has superior current carrying capacity than a “normal” cable of equal size. Also you a are describing a temporary installation so obviously not as important.

    I don't ever recall seeing 3 x 6mm² for this.

    I do.


    Note: The shed is 10m from the house so the cable run could easily be more than 20m.


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  • #12
    [Deleted User]
    Posts: 5,238 ✭✭✭ [Deleted User]


    2011 wrote: »

    There you go.


    Not quite I was thinking extra low voltage primary cables.

    10mm² is a small cable.


    2011 wrote: »
    This is a pricey, high quality cable that has superior current carrying capacity than a “normal” cable of equal size.


    I callit standard, but hey I did say I'm a cowboy...
    I don't think it has a higher current rating we just don't bury it in walls.


    2011 wrote: »
    Also you a are describing a temporary installation so obviously not as important.

    I think that's subjective.


  • #13
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,613 Mod ✭✭✭✭2011


    This data sheet for H07RN cable states that the mV/A/m = 12

    If we assume a volt drop of just 1% between the meter point and the distribution board (which would be typical) this means when fed from a 32A MCB a 4 mm sq. cable of 18m will exceed the maximum permissible volt drop of 4% (as per ET101). As the shed is 10m from the house I would assume that 18m is a length that would be easy to exceed to the furthest point in the circuit. Obviously it could be a shorter run, but without knowing I would recommend going for a larger cable or a smaller protective device.

    However I do accept your point that it can be done using a 4 sq. I just wouldn’t recommend it for the reasons given.


  • #14
    micks
    Registered Users, Registered Users 2 Posts: 840 ✭✭✭micks


    2011 wrote: »
    This data sheet for H07RN cable states that the mV/A/m = 12

    If we assume a volt drop of just 1% between the meter point and the distribution board (which would be typical) this means when fed from a 32A MCB a 4 mm sq. cable of 18m will exceed the maximum permissible volt drop of 4% (as per ET101). As the shed is 10m from the house I would assume that 18m is a length that would be easy to exceed to the furthest point in the circuit. Obviously it could be a shorter run, but without knowing I would recommend going for a larger cable or a smaller protective device.

    However I do accept your point that it can be done using a 4 sq. I just wouldn’t recommend it for the reasons given.

    he says in OP that the shed is 10m from house cu


  • #15
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,613 Mod ✭✭✭✭2011


    micks wrote: »
    he says in OP that the shed is 10m from house cu

    You are correct, my bad. I read that as shed 10m from house.
    Even at that it would not be hard to use 8m of cable in the shed to the furthest point. Wiring to the shed in a 6 sq means that the sockets can be wired in a 2.5 mm sq. with is easier for connecting (assuming a 20A MCB in the sub board).

    As Sir Liamalot says it can be done in a 4mm sq. cable in some cases, I just wouldn't recommend it.

    Regardless, it would be better not to feed the entire shed from an RCBO, an MCB would be better. The sub distribution board in the shed (that the OP states is being installed) can then contain an RCBO for the sockets and an MCB for the lights. This more standard approach means that the chances of lights tripping is dramatically reduced and if the sockets trip they can be reset locally.


  • #16
    [Deleted User]
    Posts: 5,238 ✭✭✭ [Deleted User]


    2011 wrote: »
    As Sir Liamalot says it can be done in a 4mm sq. cable in some cases.

    I said no such thing. I was just inquiring why you deemed 4mm² unsuitable for a 32A supply.


  • #17
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,613 Mod ✭✭✭✭2011


    Sir Liamalot wrote: »
    I said no such thing. I was just inquiring why you deemed 4mm² unsuitable for a 32A supply.

    Well that begs the question, would you do it or not?


  • #18
    Risteard81
    Registered Users, Registered Users 2 Posts: 2,567 ✭✭✭Risteard81


    2011 wrote: »
    Risteard81 wrote: »
    To be fair to Sir Liamalot there are reference methods whereby 4mm^2 live conductors are adequately protected by a 32A device.

    Sure, hardly standard practice in Ireland though.

    Whilst it's common for 6mm^2 conductors to be used for 32A radial circuits, very often they are not actually required.

    Agreed, for a short cable run a 4 sq. cable may be ok.

    However based on the information provided by the OP it would seem highly likely that the cable run is not that short (as the cable would has to be buried). Therefore I would it is unlikely that a 4 sq. would be sufficient.

    Will it work? Probably.
    Will the maximum volt drop and disconnection times be exceeded as per ET101? Maybe.

    When advising someone and the information is that limited I would err on the side of caution by recommending a larger cable size (or protective device with a lower rating).  

    In my experience these cable sizes are generally selected in well wired domestic installations in the ROI:

    1.5 mm sq for 10A MCBs
    2.5 mm sq. for 16 or 20A MCBs
    4 mm sq. for 25A MCBs
    6 mm sq. for 32A MCBs
    10 mm sq. for 40A MCBs

    Would this align with your experience the ROI?

    That is not to say that smaller cable sizes are not permitted,
    In fact many in England wire 32A radial final socket circuits with 4mm^2 T&E.

    ...in Italy they don't really believe in earthing, in the Netherlands they wire sockets and lights on the same circuit, in France they have ordinary sockets in the bathrooms.

    Not really relevant as we are in the ROI.

    Edit: I accept your point that the laws of physics apply here as the UK.
    Sorry 2011,

    I tried replying to this last night but it didn't seem to go through. Certainly what you have written would indeed align with my experience in the 26-Counties, and as such I refrained from advising the 4mm^2 approach. I simply pointed out that in very many circumstances it will comply with the Wiring Rules, although it isn't a standard approach here.

    Certainly I have always felt that it is very often best to adhere to the standard approach unless there is very good reason not to, as this otherwise leads to confusion and indeed accusations (even where the accusations are not actually justified!).

    So for these reasons I agree that 6mm^2 may be the better option (notwithstanding issues of volt drop etc. with 4mm^2 live conductors).


  • #19
    [Deleted User]
    Posts: 5,238 ✭✭✭ [Deleted User]


    2011 wrote: »
    Well that begs the question, would you do it or not?


    Depends on how long it is 2011. :pac:


    I work to 3% drop to final circuits with mains.
    0.03% parallel/series interlink between batteries.

    0.4% with extra low voltage between charge regulator and battery.
    1% between solar panel and charge regulator.
    Max load for inverters.
    1.5% ELV radials and fixed appliances.
    < 5% for ELV lighting circuits.



    If yer in a field 3% for motors. Resistive loads & SMPSs ya get what yer given. ;)


  • #20
    [Deleted User]
    Posts: 5,238 ✭✭✭ [Deleted User]


    ...and another thing I'd put a 40A, 45mA RCBO in the main CU and a 32A 30mA locally in the shed.

    err...ah trick question...:o

    I'd hire a REC to do that for me..whistling.gif


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