Many Cables on Perforated trays

Horizontally, most of the influence on a cable comes from its closest immediate neighbours, and progressively less from those further away, By the time you get to 4,5,6 cables wide the ones in the middle may as well be part of an infinite array of cables. If you look at the tables you see that the effect of one more cable when you go from 5 to 6 is quite small.

This does not work vertically, as the heat from the cables below rises and pre-heats the ones above. If you do it properly the lower cables are not derated as much as the ones at the top of the stack. The cables at the top get all of the heat of all of those in levels below. Ideally minimise vertical stacking, and arrange some offset or arrange enough air flow sideways that they are more or less independent,

Realise that the formulae in the regs are an approximation, to a small sub-set of all possible wiring layouts, and to a degree you are on your own. There are also reports by ERA that you may be able to get hold of that cover a few more arrangements.

Mike.

To mapj1

Thank you for your well-reasoned explanation. According to your logical explanation, if we know the nature of the circuits which has less load, these cables should be installed on lower trays. And group rating factors applied to lower tray cables are smaller than cables on top tray. Also thank  you for your information about ERA.

eizo

'group rating factors applied to lower tray cables are smaller than cables on top tray. 'msut be correctd. Smaller must be larger.

'group rating factors applied to lower tray cables are smaller than cables on top tray. 'msut be correctd. Smaller must be larger.