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What would the group rating factor be for more than one layer of many multicore cables laid on a cable ladder system?

Okechukwu Ezeaba

Considering many distribution circuits where many multicore cables would be on laid on a cable ladder system all touching with more than one layer, what will the group rating factor be? As per Table 4C4 of BS 7671, I observed that values were given for just single layer of cables up to 9 cables. Please note that some of the cables would be used for intermittent loads and they are not all of the same sizes.

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    Really you need airflow - the whole point of a basket, ladder or well perforated tray is that air can circulate between the cables. As soon as you prevent that, or one cable is being heated by the hot air from the layer beneath, you lose most of the advantage and you may as well just screw the cables direct to the wall, or ceiling or whatever. Of course if the cables are sized for volt drop reasons, rather than load, and are not running near full load, then layers are fine.

    but yes, treat as a bundle of cables.

    Mike.

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    Really you need airflow - the whole point of a basket, ladder or well perforated tray is that air can circulate between the cables. As soon as you prevent that, or one cable is being heated by the hot air from the layer beneath, you lose most of the advantage and you may as well just screw the cables direct to the wall, or ceiling or whatever. Of course if the cables are sized for volt drop reasons, rather than load, and are not running near full load, then layers are fine.

    but yes, treat as a bundle of cables.

    Mike.

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