Reduce cable size after a fused connection unit

Discussion in 'Electricians' Talk' started by SmugDictator, Jan 14, 2020.

  1. SmugDictator

    SmugDictator Member

    Hi, just looking at some electrics which have been installed.

    Is it possible to reduce cable size on a radial circuit for a load following a fused connection (FCU) unit according to the regs?


    20A MCB ==2.5mm=> [13A FCU] ==2.5mm=> [3A FCU] ==1.5mm=> [Light]

    The first 13A FCU :
    SUPPLY: The 2.5mm cable supply and looped out with 2.5mm cable to the next FCU.
    LOAD: Feeds a fixed appliance requiring up to 13A with 2.5mm cable.

    The 2nd 3A FCU:
    SUPPLY: Just the 2.5mm cable supply which was looped out of the first FCU.
    LOAD: An external light fitting on 1.5mm cable.

    Is it acceptable that the cable size has been reduced from 2.5mm to 1.5mm for the load from the 2nd FCU? It would seem the 20A MCB is appropriate for the 2.5mm cable and the 3A fuse is fine for the 1.5mm cable. The risk is someone could put a 13A fuse into the 2nd FCU, but then there are many other places in a domestic setting where someone could make this mistake.
  2. Hans_25

    Hans_25 Screwfix Select

    Looks OK to me - the fuses are protecting the cable so applying that rule, all looks good. There might be some confusion by people who don't know the setup, may be put a kettle onto the 3A circuit but then the fuse will do its job and blow. You could out a label on sockets on the 3 amp circuit saying "3 amps max" or some such.

    I have a similar setup on my boiler circuit: 13 Amp FCU which then supplies two additional FCUs for the boiler (3 amp) and shower/bath pump (5 amp). 2.5mm is used through out in my case, but I could have used 1.5mm cable for the two low power circuits.
  3. terrymac

    terrymac Screwfix Select

    1.5mm² cable from the load side of the FCU to an outside light fitting is perfectly ok ,even if someone puts a 13 amp fuse into the FCU.
  4. Philip Hyde

    Philip Hyde Active Member

    3183y flex

    Attached Files:

  5. seneca

    seneca Screwfix Select

    Yes that's fine.

Share This Page