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How to supply a 20A Distribution cct?

Farmboy
Sorry if this is too simplistic a question but I'm wondering what you think, given I've had conflicting views from a trade assoc tech dept and Hager tech dept.


A 65m, SP+N, 20A (estimated, but could be subject to some increase) distribution cct, fed from within a commercial setting (an osteopathy/acupuncture service provided in a converted ground floor flat), is to supply a shed/summerhouse with a couple of sockets (for a kettle and TV) and a couple of LED bulkheads (via cb's in the shed CU), sited at the end of the garden. How would you supply the distr. cct at the meter position?

- From a henley block in the existing tails:

              - Its own modular enclosure containing DIN rail mounted main switch and 20A fuse carrier.

              - A rotary handled 20A fused switch disconnector.

- From a 20A cb in the existing CU.


From the trade assoc., one said from the CU, another said it's got to have its own main sw, another said no problem with the modular encl and fuse carrier. While Hager said its got to be the rotary type, not the fuse carrier.


F
  • 0
    A B20 at the origin and a B16 M.C.B. at the outbuilding garage unit should just about discriminate on overload, but not on a dead short or high earth current fault where they may operate together. Although if the house M.C.B. is warm and the outbuilding M.C.B is colder they may operate in a similar  way under overload conditions, and trip off together.


    Z.


  • 0

    Zoomup:

    A B20 at the origin and a B16 M.C.B. at the outbuilding garage unit should just about discriminate on overload, but not on a dead short or high earth current fault where they may operate together. Although if the house M.C.B. is warm and the outbuilding M.C.B is colder they may operate in a similar  way under overload conditions, and trip off together.


    Z.


     




    Yes, which is why I'm wondering about selectivity with the fuse.


    F

  • 0

    will need 10.0 or 16.0mm2 for Volt drop limitation at 20 Amps over 65 metres.






    No it won't - 16mmsq is more than you need to supply a small house or flat !!

    It's not a lighting-only circuit, so the larger volt drop figure applies. You may need 6mmsq, not more.


    Selectivity between 20A and 16A is not guaranteed, and wont be for fuses either,  though if you accept the overload case being limited by the breakers at the load end, then the source end protection only needs to handle short circuits in the cable, and disconnect in 5 seconds or less for a short at the far end just before the load end breakers, so the source end MCB  could be 32A.


  • 0
    Many suggestions above, but what is the cheapest compliant solution? That's what the customer appears to want.


    As for selectivity - how often are you going to get a fault in such a position that it would trip two protective devices; and if it did, would it bother the customer?
  • 0

    hertzal123:

    Believe Hager do a fuse unit for their consumer units.

                                                                                   regards,

                                                                                                 Hz




    My wholesalers don’t stock them to sell individually and I didn’t want to order a boxful to get one for domestic fire pump circuit.


     Andy 

  • 0

    Chris Pearson:

    Many suggestions above, but what is the cheapest compliant solution? That's what the customer appears to want.


    As for selectivity - how often are you going to get a fault in such a position that it would trip two protective devices; and if it did, would it bother the customer?




    Give the customer the option/choice. Bigger cable for flexibilty to deal with potential increased load and with selectivity designed in, or smaller cable on a breaker. Explain the limitations and let them choose.

  • 0

    Chris Pearson:

    Many suggestions above, but what is the cheapest compliant solution? That's what the customer appears to want.


    As for selectivity - how often are you going to get a fault in such a position that it would trip two protective devices; and if it did, would it bother the customer?




    The potential customer sounds like a like a waste of time, what plans are there for heating the shed?


    It realistically needs a 32-amp supply with a consumer unit in the shed with a B6 for the lights and a couple of socket circuits each protected by a B16 MCB to stand a chance of achieving the discrimination between the protective devices that you are hoping for. That will also allow a fan heater to be used along with the kettle along with few other things like a sound system.


    Andy 


     

  • 0

    mapj1:




    will need 10.0 or 16.0mm2 for Volt drop limitation at 20 Amps over 65 metres.






    No it won't - 16mmsq is more than you need to supply a small house or flat !!

    It's not a lighting-only circuit, so the larger volt drop figure applies. You may need 6mmsq, not more.

    ..........................................................................................................................................................................................................................................6.0mm2 as an absolute minimum, 10.0mm2 preferably.

    Well, a house does not normally have a 65 metre run for its internal main supply cable.


    The outbuilding will have lighting so the 3 percent max. Volt drop will apply in my opinion. Also if 10.0mm2 or 16mm2 is installed for a 65 metre run rated at 20 Amps. it limits Voltage drop and it allows for future extra loads.


    With a 20 Amp load and a 65 metre run  using 6.0mm2 cable I calculate that the Volt drop will be just short of 10 Volts. Doesn't that create a loss of just under 200 Watts?


    Z.


     



     




     

  • Former Community Member
    0 Former Community Member
    Z,


    As this isn't a final circuit or a lighting circuit exclusively, then I'd be inclined to call it a distribution circuit which would allow for a max. 5% volt drop. Admittedly, on a bit of 6mm² cable, you wouldn't have much room for expansion before you reach the cap of 5% and very limited on what you can power, but just playing devils advocate. I'd stick it on a 10mm².


    Just wait until they want to run a hot tub off of it!




    We aren't talking about a spherical cow in a vacuum here.

  • 0
    Potentially this shed provides additional work space and somewhere to escape to whilst the main work space is in use.


    65 metres of 6.0 mm SWA runningall the way with a B32 MCB in dedicated CU upfront with a DP main switch supplying a three way CU in the shed with a B6 and two B16 RCBOs would be what I would use if the maths works out okay with 100 mA RCD protection added to the distribution circuit if needed using a time delayed RCD. 


    Otherwise it's probably a B20 MCB at the supply and a 4.0 mm SWA with the lights connected using a SFCU in the shed along with a couple of double sockets. If the MCB trips he'll have to walk the 65 metres to and from the supply to reset it.


    He cannot have a highly engineered job at no cost. 


    Andy

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