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Overload protection for swa

Discussion in 'Electrics UK' started by soloheadbeg, 1 Feb 2021.

  1. soloheadbeg


    14 Feb 2011
    Thanks Received:
    United Kingdom
    I have a granny flat fed via 6mm2 three core swa with the live core connected to the load side of a 63A neozed fuse in the house consumer unit. (In Ireland the live tail from the meter is connected to a neozed fuse ( as referred to above) as opposed to live and neutral entering via a double pole switch.) The granny flat contains a sub consumer unit which originally had two 20A socket radials and a 10A lighting circuit. The swa from the house cu runs for about 6 m resting on joists then runs externally through steel conduit for 3 m, underground for 3 m and then enters the granny flat and runs about 3 m behind plasterboard (probably behind a couple of inches of fibreglass insulation) and into the sub unit in the granny flat. I would have thought that the 6mm2 swa should be overload protected by something closer to 50 A mcb at the house end. The logic, I imagine, at the time was that the granny flat was protected by a total of 50 A (2x20 + 1x10) anyway at the granny flat end so the 6mm2 swa shouldn’t be under pressure. In the meantime two 20A mcbs have been added to the granny flat sub unit to supply two sheds. Potentially the granny flat might now pull far in excess of the 50 A. I understand about diversity and in reality I can’t imagine a situation where anything over 30A would ever be drawn at the granny cu for any appreciable time.

    My question is should I consider having the live feed to the granny flat from the house cu moved from the 63A neozed fuse to a 50 or 40 mcb to ensure that the swa run from the house is definitely overload protected.

    Many thanks
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  3. EFLImpudence


    7 Jul 2010
    Thanks Received:
    Retired to:
    You seem to understand the situation.
    There are various options to achieve the desire outcome - lower rated fuse?

    I am sure you won't have 10A of lighting in the flat so that is some reduction for a start.
    • Thanks Thanks x 1
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    3 Sep 2019
    United Kingdom

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