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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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    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!
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  • 0
    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!
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