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Grouping Factor - How to apply

gullanedave

Hello,

We have a 12-way 630A MCCB panelboard, with 7x outgoing 4-core XLPE/SWA (de-rated to 70 degrees) installed on perforated tray. None of these cables can be discounted for grouping consideration, as all are loaded at >30% of cable current carrying rating.

Upon leaving the MCCB panelboard, these 7x cables are on the same section of vertical tray for ~1m or so, then split, with 3x going left on their own new section of horizontal tray, and 4x going right on their own section of tray.

These runs supply the 7x downstream MCB dist boards, ranging from 30m to 120m away, all located within the property.

My question is;

Do these cables need to be calculated for grouping based on the ~1m section rising from the MCCB panelboard where all 7x cables are installed on the tray side-by-side, touching, or, can they be calculated for grouping based on the overwhelming majority of the cables' length grouping scenario? (e.g. a 120m cable, where 1m of run is group with 7 cables, and 119m of the run is grouped with 3 cables, or less).

Thanks.

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    well strictly the only bit that sets the rating is the bit that will run hottest, so where they are closest and/ or least well cooled. 
    However many years experience tells us that some designs have been exceeding the book values for years without ill effect, and the regs have for very good reasons, to assume the worst cases for everything.

    I suspect if you do calculate it you will find you have to split the cables apart to meet the regs - but it may well not actually run hot unless the load is pretty much continuous and simultaneous on all branches.

    Mike

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  • 0

    well strictly the only bit that sets the rating is the bit that will run hottest, so where they are closest and/ or least well cooled. 
    However many years experience tells us that some designs have been exceeding the book values for years without ill effect, and the regs have for very good reasons, to assume the worst cases for everything.

    I suspect if you do calculate it you will find you have to split the cables apart to meet the regs - but it may well not actually run hot unless the load is pretty much continuous and simultaneous on all branches.

    Mike

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