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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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    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.


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  • 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.


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