MCBs in series: Discrimination

Discussion in 'Electricians' Talk' started by IIABDFI, Sep 24, 2010.

  1. IIABDFI

    IIABDFI Member

    Please could someone point me towards something that explains how to design two MCBs in series but provide adequate discrimination.

    The scenario, is you guessed it, a shed. 16A MCB in Shed.

    I think the answer is 32A but the cable sizing only just manages that so would prefer to have 20A upstream.

    Or reduce the MCB in the shed... 20A upstream and 10A downstream but unnecessary restrictive (although within expected demand) and 10A into a double socket doesn't feel right.

    I can't make head nor tale of the Wylex MCB chart; i'm sure it's trying to tell me something.

    Thanks in advance
     
  2. Joelp1

    Joelp1 New Member

    c20 in house, if that still complies, and b16 and b6 in shed?

    Or, b16 in the house and b20/b6 in the shed.

    In fact theres loads of way to sort it.
     
  3. IIABDFI

    IIABDFI Member

    Or, b16 in the house and b20/b6 in the shed.

    Not sure about that one.
     
  4. spark&half

    spark&half Active Member

    What size cable have you got feeding the small DB?

    Length of run?

    As mentioned 32A at house and 16A/6A at small db end sounds good to me.

    ;)
     
  5. seneca2

    seneca2 New Member

    Agree with s & h provided the cable can support it, ie size-length of run.
     
  6. Lectrician

    Lectrician Screwfix Select

    Even with a 32 and 16 you are likely to find the 32 pops with a short circuit type fault.
     
  7. IIABDFI

    IIABDFI Member

    25m of 2.5mm2 3-core SWA. Underspecified for the task which is the problem.
     
  8. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    If using earth wire and SWA as earth, why would 32A at house not be ok for 25m ? You won't be able to overload it, except with fault.

    As 'lectrician' said, most likely pop both ends anyway.

    If 10A is with shed design, I would go for that, best chance. If it pops, it pops.


    'Course, I ain't an electrician(I've heard that somewhere before....!!!)


    Mr. HandyAndy - Really
     
  9. sparky001

    sparky001 New Member

    what about 16A in house supplying sockets, run through 20A DP switch if isolation required then install fused connection unit for lights.
    That's what i'd do
     
  10. seneca2

    seneca2 New Member

    That's quite a good way of doing it sparky, have done a few like that myself.
     
  11. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    ....or straight from the DB(16A+6A) in the shed to a 13A FCU for sockets, 3A FCU for lights ? 16A max. 20A in house.

    ??


    Mr. HandyAndy - Really
     
  12. Removed 4

    Removed 4 New Member

    'Course, I ain't an electrician(I've heard that somewhere before....!!!)


    At this rate, dear, you're likely to hear it again....


    Lucia.
     
  13. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    Ah, that's sounds like you have some misgivings about the above suggestion!

    Which are...?


    Mr. HandyAndy - Really
     
  14. Removed 4

    Removed 4 New Member

    Not at all, Andy dear. I think that all your ideas are absolutely wonderful. I don't know why Screwfix don't have a separate Mr. Handyandy - Really Talk Forum - really.....


    Lucia x
     
  15. larnacaman

    larnacaman New Member

    Why so complicated?? Straight forward installation as far as i can see. RCD protected 20A breaker at the house, supplying a garage DB with a 16A mcb for the socket and a 6A mcb for the lighting. In this instance no other local isolation is required, as the DB breakers are both local and in line of sight....
     
  16. fooman

    fooman New Member

    Not at all, Andy dear. I think that all your ideas
    are absolutely wonderful. I don't know why Screwfix
    don't have a separate Mr. Handyandy - Really Talk
    Forum - really.....


    Lucia x



    :^O brilliant lucia :^O
     
  17. Lectrician

    Lectrician Screwfix Select

    Why so complicated?? Straight forward installation as
    far as i can see. RCD protected 20A breaker at the
    house, supplying a garage DB with a 16A mcb for the
    socket and a 6A mcb for the lighting. In this
    instance no other local isolation is required, as the
    DB breakers are both local and in line of sight....

    QUOTE^^^^^^^



    And the title of this thread is????
     
  18. larnacaman

    larnacaman New Member

    Lectrician,

    He's probably not going to achieve total discrimination, with any MCB set up of the same type is he, under short circuit conditions?? So what are you trying to say??
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice