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Multicore cable calculations

Martin Coward

I am instaling a piece of equipment that has a total loading of 4.5kW, but am constraint on space to run cable from the starter to the equipment. I intend to use a 7 core cable, of which only three cores are for the 3phase power the remainder are for start / stop and indicator light. Power is supplied via an MCCB and 2.5mm cable is large enough to run the cable even with the derating factor of multicore cable. My problem is the impedance, (Zc) value is adjusted by the 7 cores meaning a 4mm cable is needed as opposed to 2.5. Is it acceptable to calculate using 3 cores as these are the only cables carrying power or is 4mm cable required.

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    step back one,
    if this was 3 cables
    1 x 3 core
    2 x 2 core  (and at 24v would be light load anyway - could have been 1.0/0.5mm  lighting "flex" for the current needed ?)
    How would you rate / calculate it ?

    That they are all in one 'outer' case ("flexible conduit" with little air space...)
    is kinda academic.
    At a pragmatic level, one might even look to put 1 Hot  - 1 cool - 1 hot - 1 cool -  (if not for colours chosen) to allow any heat generated to be next to a cooler "spacer" - giving further practical allowances.
    Then note in the installation cert / documentation that cable usage rated for 3 cores max only, and 4 cores for low current ELV auxiliary use. - so that later maintainers do not try to change use of the other cores, or negatively assess it at inspection time ?

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  • 0

    step back one,
    if this was 3 cables
    1 x 3 core
    2 x 2 core  (and at 24v would be light load anyway - could have been 1.0/0.5mm  lighting "flex" for the current needed ?)
    How would you rate / calculate it ?

    That they are all in one 'outer' case ("flexible conduit" with little air space...)
    is kinda academic.
    At a pragmatic level, one might even look to put 1 Hot  - 1 cool - 1 hot - 1 cool -  (if not for colours chosen) to allow any heat generated to be next to a cooler "spacer" - giving further practical allowances.
    Then note in the installation cert / documentation that cable usage rated for 3 cores max only, and 4 cores for low current ELV auxiliary use. - so that later maintainers do not try to change use of the other cores, or negatively assess it at inspection time ?

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