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MCB size for dedicated fused spurs

Johno12345
A little thought, when installing a new circuit for a fused spur to supply an alarm or something similar, just about everyone uses a standard 2.5mm radial circuit, but the MCB size seems to vary from 16a down to 6. 


part of me (the larger part) thinks, just keep it standard 16a, and another part says to size the breaker to the load 


(I'm a specifier, not installer)
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    I'll sometimes run a 1.00mm from a B6A rather than a 2.5mm for a FCU, especially when room is tight and the load demands only a 3A fuse at the destination end.

    No point in running a bigger cable to provide capacity which most likely will never be used.
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    Surely it depends on the size of the fuse fitted in the downstream connection unit ... if you are bothered about selectivity, that is?


    Certainly, there's not much point supplying an FCU with a 5 A BS 1362 fuse, from a B6 mcb, or one with a 13 A fuse from a B10 or B16 mcb. - might as well fit a flex outlet plate as the fuse is highly unlikely to go first even for lower current faults  ! And for faults of 100 A or more, it's likely a B20 will go well before a 13 A BS 1362 fuse!
  • 0
    whjohnson:

    I'll sometimes run a 1.00mm from a B6A rather than a 2.5mm for a FCU, especially when room is tight and the load demands only a 3A fuse at the destination end.

    No point in running a bigger cable to provide capacity which most likely will never be used.


    Regulation 524.1 (via Table 52.3) requires a minimum 1.5 mm2 for power circuits (i.e. not lighting circuits or auxiliary/control circuits) in sheathed cable. Yes, a reduction for a specific appliance is permitted in Table 52.3, but that's only in flex to the appliance itself, not from the mcb to the appliance.


  • 0
    gkenyon:
    whjohnson:

    I'll sometimes run a 1.00mm from a B6A rather than a 2.5mm for a FCU, especially when room is tight and the load demands only a 3A fuse at the destination end.

    No point in running a bigger cable to provide capacity which most likely will never be used.


    Regulation 524.1 (via Table 52.3) requires a minimum 1.5 mm2 for power circuits (i.e. not lighting circuits or auxiliary/control circuits) in sheathed cable. Yes, a reduction for a specific appliance is permitted in Table 52.3, but that's only in flex to the appliance itself, not from the mcb to the appliance.




    Hi Graham, it could be argued that "an alarm or something similar" would be classed as a "signalling and control circuit", and the minimum CSA of 0.5mm2, or even possibly 0.1mm2 since most alarms are electronic.

    Andy has commented about table 52.3 in this old thread https://www2.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=54291&STARTPAGE=1&FTVAR_FORUMVIEWTMP=Linear

     


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    OlympusMons:

    Hi Graham, it could be argued that "an alarm or something similar" would be classed as a "signalling and control circuit", and the minimum CSA of 0.5mm2, or even possibly 0.1mm2 since most alarms are electronic.

    Andy has commented about table 52.3 in this old thread https://www2.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=54291&STARTPAGE=1&FTVAR_FORUMVIEWTMP=Linear

     




    A supply to an alarm system or control system is definitely a power circuit. The function of the circuit is definitely not not "signalling and control".


    The switched live to a timed fan, or the link wire in mains smoke alarms, may well be a control or auxiliary circuit, as would the secondary (alarm circuits themselves) of the fire or intruder alarm.


    If you install the connection unit on a lighting circuit, then I can see an interpretation that the circuit is mainly for lighting, so 1.0 sq mm is OK ... but if it's a dedicated circuit for something else, I think that argument peters off somewhere.


    However, linking back to the original post, I remembered this recent thread: https://communities.theiet.org/discussions/viewtopic/1037/26230


    Which got me thinking that FCUs to BS 1363-4 are not necessarily suitable for termination, without special preparation, 1.0 sq mm (or 1.5 sq mm for that matter) - the manufacturer may choose to state otherwise, of course.


  • 0
    Graham,

    "special Preparation" of say 1.0 mm2 or whatever , would you be inclined to put a cable doubled doubled doubled ......... etc into that category? In an ideal world (which does not actually exist) such doubling (or twisting if you must) will take the conductor effective diameter up towards the terminal hole diameter thus making decent electrical contact and possibly mechanical soundness too.
  • 0
    gkenyon:
     


    A supply to an alarm system or control system is definitely a power circuit. The function of the circuit is definitely not not "signalling and control".

     




    I see what you are saying here in that the part of a circuit from the CU to the FCU has a solely power-supplying purpose, much like a distribution circuit. And that the part of the circuit past the FCU, which, because it has its own OCPD and can be considered to be a circuit in its own right, can have the reduced minimum CSA of 0.5 or 0.1mm2. But you mentioned in your earlier post that the fuse in the FCU would/could be redundant and if manufacturers' instructions do not specify an FCU, then it could be omitted, making the whole circuit from the CU a "signalling and control" circuit.

    Also, the definition of a circuit in BS7671 states that more than one OCPD can be incorporated in a circuit, therefore irrespective of whether an FCU is used, the purpose of the whole circuit from the CU could be defined as a "signalling and control" circuit.

    I am not nit picking, just pointing out apparent inconsistencies in BS7671, I agree with Andy in the thread I linked to earlier, it would make sense if BS7671 defined power circuits etc but it doesn't.

     


  • 0
    An oven and cooker hood usually have a luminaire, which can be used solely for the purpose of providing light. The oven can also cook, the hood also extracts. When used for lighting purposes, the circuits can have min CSA 0.5mm2. They would also need RCD protection.

    Apologies to OP for a bit of thread drift.
  • 0
    OlympusMons:

    An oven and cooker hood usually have a luminaire, which can be used solely for the purpose of providing light. The oven can also cook, the hood also extracts. When used for lighting purposes, the circuits can have min CSA 0.5mm2. They would also need RCD protection.

    Apologies to OP for a bit of thread drift. 


    oops, min CSA 1mm2


  • 0
    What you need is a B3 MCB with 25mm terminals

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