Feed from 6 amp MCB to Boiler

Discussion in 'Electricians' Talk' started by IanGO, Sep 22, 2021.

  1. IanGO

    IanGO New Member

    I have a Solar system with a Givenergy battery and hybrid inverter. The system has an emergancy back uo circuit which has a small CU with RCD and two MCB's - One feeds a double socket near the system in my attached garage. The circuit is live all the time and can feed 2600 watts. I would like to use the 6 amp RCD to keep my oil fired boiler running during a power cut.
    The boiler is cirentlly fed from a metal clad swiched spur connected to the garage & extension ring circuit from the main CU. I have ran 1.5mm T&E from the emergency CU to the boiler, so the question is should I use a standard switched spur with a 3 amp fuse, use a 5 amp round pin plug and socket or just use a junction box to connect to the boiler?


    By the way I used to wire control systems in Nuclear power stations and was a full member of the IEEE
  2. Tony Goddard

    Tony Goddard Screwfix Select

    As you are feeding from a 6 Amp MCB all of the methods you describe would be fine, my preference of them would be an FCU with a 5A fuse, I would probably mark it to indicate that it is live even in a power cut, in hospitals they use a red letter E on any fitting that stays 'hot" under power cut conditions.

    As to whether it is notifiable, well your nuclear power experience I'm afraid counts for nothing in the weird, perverse world of part P, you could have personally designed the whole electrical infrastructure of sellafield itself - but try to add a new circuit for a doorbell at home!!! - i'll put on my tin hat, but I would say crack on!!
    ElecCEng and seneca like this.
  3. The Happy Builder

    The Happy Builder Screwfix Select

    An oil boiler needs a 5 amp fuse, it probably wouldn’t actually be wrong to connect it straight to a 6 amp MCB, but it would be far better to connect it to a SFCU with a 5 amp fuse as a local isolator.
  4. seneca

    seneca Screwfix Select

    The 6 amp mcb would probably trip before the 5 amp fuse would blow!
  5. ElecCEng

    ElecCEng Screwfix Select

    But adding a FCU is allowable under Part P, bonkers no?

    @IanGO, direct off the MCB with a local isolator near the boiler (for maintenance) is the standard way of doing things. However if you’re DIY’ing this, then you’re limited to adding to the socket circuit.

    You also need a label on your ‘small CU’ stating it has multiple sources of supply and where to safely isolate. Somewhere between your solar setup and the CU should be an isolator for this very purpose.
  6. The Happy Builder

    The Happy Builder Screwfix Select

    Almost certainly, but the heating engineer can take the 5 amp fuse out and put it in a pocket.
  7. Bazza-spark

    Bazza-spark Screwfix Select

    Or just lock off the MCB.
  8. ElecCEng

    ElecCEng Screwfix Select

    How many heating engineers have you met who carry a lockout kit??? ;)

    BillyBobToo likes this.
  9. Bazza-spark

    Bazza-spark Screwfix Select

    How many have you met that wouldnt lose the fuse? :D
    ElecCEng likes this.
  10. The Happy Builder

    The Happy Builder Screwfix Select

    The heating engineers are generally trained to remove the fuse from the SFCU and secure the fuse holder with a cable tie, ideally with a warning sign attached.
  11. Bazza-spark

    Bazza-spark Screwfix Select

    Ah, a nice secure HSE etc approved method then.
  12. spinlondon

    spinlondon Screwfix Select

    If you intend feeding your boiler from your solar power system, then you would need an interlock device, which would disconnect the grid supply to the boiler before allowing the solar power to be connected.
  13. IanGO

    IanGO New Member

    Hi All Thank you for the replies, The emergency feed from the solar system is "live all the time" the cmall CU us on the opposite side of a double garage. I have decided to move the existing SFCU to the emergency circuit (to allow local isolation). If the inverter fails and I loose the power then I will have to quickly add a plug top to the boiler and plug in in. Thanks All for your time and comments.

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