Fusing down at cable changes

Discussion in 'Electricians' Talk' started by lensman, May 30, 2004.

  1. lensman

    lensman New Member

    There've been a few posts regarding fusing down when going from one cable size to a smaller one. How is one supposed to do this when going from 6mm to 4mm, since the latter is good for 32A and I haven't seen a normal FCU of this size.

    Reaons for asking is that I've got a couple of rooms of my house on a 4mm radial. However the rooms are (as the cable runs) about 15m away from the CU, so I ran the 15m section of cable to the first branch in 6mm, then did all the smaller sections in 4mm. The circuit is on a 32MCB at the CU, so everything would be fine by the regs if it was all in 4mm. But... am I still supposed to insert a fuse at the point where the 6mm changes to 4mm?
     
  2. Lectrician

    Lectrician Screwfix Select

    4mm is good for 32amp as is 6mm.

    a 4mm radial can feed upto 50m/sq noadays, only need to increase to 6mm if the volt drop wil prove excesive.

    with a 6mm starter, you are sortd i would think
     
  3. lensman

    lensman New Member

    I'm happy that the circuit is technically safe. I was wondering if the regulations about fusing down at a change of cable applied here - even though I think it would be pointless.
     
  4. HappyDayz

    HappyDayz New Member

    In any situation with mixed cable sizes as long as the overcurrent device is selected so that it protects the cable with the smallest c.s.a. then that will be fine.

    One regular example of this is garden lighting where a large c.s.a. cable is often needed, because of the voltage drop on long runs, and yet wires internal to the lamps will only be of "normal" sizes. The overcurrent device, be it a fuse or breaker, must reflect this and so, typically, will be 6amp.
     
  5. Lectrician

    Lectrician Screwfix Select

    HD is spot on...
     

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