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sizing of cables for adjustable short-circuit threshold

Huw Pullen
Hi 

i have been searching the regs and concluded that there is no reg covering this.


I have a 100A MCCB but adjusted overload to 40A. I been asked to size the cable to the adjusted overload setting (40A) and not the max MCCB size.

Its not something i want to do as I've always sized to the max fault current. is there any guidance or reg that I can throw back at them to stop this sizing to the adjusted setting and potentially under sizing the cable and under certain conditions, a fault current may exist on the upstream LV panel for a longer period of time than expected resulting in damage to equipment and cables

Thanks for your input

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  • 0
    I think you are worrying about the wrong thing. If it is a concern, then an anti-tamper sticker over the setting switch/knob or what ever it is and a sticky label 'Must  Be Set to 40 amps' would be the sort of thing.

    Some designs have places you can wire seal the covers in place after setting, then to open it and twiddle requires tools and is then in the same league as opening the box and bypassing it with  jumper cables and we do not try to design for that. As noted above the whole point is that you can make it into a customised breaker with characteristics that match your wiring, and there is some sense in letting a large one loaf along and run cool if you can afford the space and cost.
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  • 0
    I think you are worrying about the wrong thing. If it is a concern, then an anti-tamper sticker over the setting switch/knob or what ever it is and a sticky label 'Must  Be Set to 40 amps' would be the sort of thing.

    Some designs have places you can wire seal the covers in place after setting, then to open it and twiddle requires tools and is then in the same league as opening the box and bypassing it with  jumper cables and we do not try to design for that. As noted above the whole point is that you can make it into a customised breaker with characteristics that match your wiring, and there is some sense in letting a large one loaf along and run cool if you can afford the space and cost.
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