Many Cables on Perforated trays

Indeed - lightly loaded stuff at the bottom saves heating the cables above- unless of course the top layer is jammed against the ceiling.  The regs assume your tray is on an infinitely high wall and the risk of a hot air trap at the top is overlooked - but there is a reason for the regions of bootscraper style mesh flooring on cable troughs on some industrial sites and the positioning of vent louvres at the sides of connection boxes.   It is the same reason we use perforated tray or basket and not solid shelves. All the hot air has to go somewhere, ideally out. I have in the past walked along on service trays near the ceiling of  factory hanger, and after a few mins I was dripping sweat, even though the doors were open and floor level was decidedly chilly. And no, not fear of heights or rusting support studs, just that the top 6 foot or so of the space to the ceiling was essentially a heat trap.

And the assumption that a cable diameter spacing has the same de-rating factor for both thick and thin cables is also a bit off, in practice the way chimney convection currents start to form eddies in larger gaps, means that rule of thumb  tends to over-cook thin cables while fat ones run a bit cooler than expected. The saving grace is that very rarely are all cables at full load at the same time. Indeed quite often we know the total supply to the building or the dis-board or whatever, and can know an upper bound of how many can be fully loaded at once.

For the original query, look at how similar 6 is to 5, add a similar bit more for luck, or just round to the next cable size, and that will cover you for 7-10, and probably beyond. ;-)

Mike.

Indeed - lightly loaded stuff at the bottom saves heating the cables above- unless of course the top layer is jammed against the ceiling.  The regs assume your tray is on an infinitely high wall and the risk of a hot air trap at the top is overlooked - but there is a reason for the regions of bootscraper style mesh flooring on cable troughs on some industrial sites and the positioning of vent louvres at the sides of connection boxes.   It is the same reason we use perforated tray or basket and not solid shelves. All the hot air has to go somewhere, ideally out. I have in the past walked along on service trays near the ceiling of  factory hanger, and after a few mins I was dripping sweat, even though the doors were open and floor level was decidedly chilly. And no, not fear of heights or rusting support studs, just that the top 6 foot or so of the space to the ceiling was essentially a heat trap.

And the assumption that a cable diameter spacing has the same de-rating factor for both thick and thin cables is also a bit off, in practice the way chimney convection currents start to form eddies in larger gaps, means that rule of thumb  tends to over-cook thin cables while fat ones run a bit cooler than expected. The saving grace is that very rarely are all cables at full load at the same time. Indeed quite often we know the total supply to the building or the dis-board or whatever, and can know an upper bound of how many can be fully loaded at once.

For the original query, look at how similar 6 is to 5, add a similar bit more for luck, or just round to the next cable size, and that will cover you for 7-10, and probably beyond. ;-)

Mike.