derating multiple trefoils

There are a number of things that affect the rating, and make the choice to use one or other 'book value' more or less inaccurate. Ultimately it is all about the cooling. (well, assuming the run is not long enough that it is all about voltage drop instead)

Where are these bundles of 3 or 4 cables going to be mounted? I assume some sort of tray, ladder or basket but not too close to a wall or ceiling that will trap hot air.. Are the bundles spaced sideways by more than a bundle diameter apart, so air can flow between ?  Are the 2 layers  arranged so that the top layer is heated by the hot air rising from the cables below, or are they offset so that does not happen?

In general bundles side by side are better cooled than the same spacing one above the other,  but in real situations any ventilation or lack of it can have a huge effect.

I'm assuming these are not armoured cables, sa it looks like your 607amp  figure comes from Table 4E1A ,but if it had been AWA and table 4E3A moves you to 612A per core before grouping factors, again for 30C ambient air and  copper at 90C. (the core to core spacing increases with the AWA, so it is fatter and cools marginally better...)

The maker's figure of 775A for starters makes me wonder if their assumptions are different - perhaps they assume an allowance for a lower air temperature (20C perhaps) or a different mounting method.

There are some tables by ERA that consider cable ratings that are based on a load of tests don in the 1970-s and 80s and these do suggest that many of the IET regs values  can safely be pushed a bit. Certainly the next to nearest neighbour effect in a horizontal direction is far less than vertically, and for larger cables can be safely forgotten Smaller cables at smaller spacing not so...

I think your 6 bundle assumption is conservative and safe, and will run cool at full load, the makers 4 bundles may be a bit more gung ho, though it may well work out in practice, perhaps check their assumptions against the actual situation to see how much slack there really is.

Mike.

There are a number of things that affect the rating, and make the choice to use one or other 'book value' more or less inaccurate. Ultimately it is all about the cooling. (well, assuming the run is not long enough that it is all about voltage drop instead)

Where are these bundles of 3 or 4 cables going to be mounted? I assume some sort of tray, ladder or basket but not too close to a wall or ceiling that will trap hot air.. Are the bundles spaced sideways by more than a bundle diameter apart, so air can flow between ?  Are the 2 layers  arranged so that the top layer is heated by the hot air rising from the cables below, or are they offset so that does not happen?

In general bundles side by side are better cooled than the same spacing one above the other,  but in real situations any ventilation or lack of it can have a huge effect.

I'm assuming these are not armoured cables, sa it looks like your 607amp  figure comes from Table 4E1A ,but if it had been AWA and table 4E3A moves you to 612A per core before grouping factors, again for 30C ambient air and  copper at 90C. (the core to core spacing increases with the AWA, so it is fatter and cools marginally better...)

The maker's figure of 775A for starters makes me wonder if their assumptions are different - perhaps they assume an allowance for a lower air temperature (20C perhaps) or a different mounting method.

There are some tables by ERA that consider cable ratings that are based on a load of tests don in the 1970-s and 80s and these do suggest that many of the IET regs values  can safely be pushed a bit. Certainly the next to nearest neighbour effect in a horizontal direction is far less than vertically, and for larger cables can be safely forgotten Smaller cables at smaller spacing not so...

I think your 6 bundle assumption is conservative and safe, and will run cool at full load, the makers 4 bundles may be a bit more gung ho, though it may well work out in practice, perhaps check their assumptions against the actual situation to see how much slack there really is.

Mike.