The way I read the standard and subsequent derate requirements, if there is _any surface_ within 1 (insulated) cable diameter, it's not free air. Especially if the surface is conductive, magnetic, or both.

Converting energy to motion for more than half a century

According [for information only]
ANSI/NEMA WC 51-2009/ICEA P-54-440-2009
AMPACITIES OF CABLES INSTALLED IN CABLE TRAYS
3.4 CORRECTION FACTORS FOR TRAY COVERS
The following table provides ampacity correction factors for trays employing covers. The adjustment factors represent worst case conditions where the air gaps between the cable mass and cover are greater than ½ inch. For an air gap spacing less than ½ inch the thermal resistance of the air gap decreases dramatically resulting in an increase in the multiplier.
From Table 3-3 the derating factor is about 80-85%. Ventilated Flanged may be 82-87% but it depends on many other conditions and NEC is a better source, in my opinion.

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thanks Gr8blu and 7anoter4!

@gr8blu: which standard are you referring to?

@7anoter4: I went through the NEMA standard, it looks like all the equations/etc assume a fully stacked (touching side-by-side) and no adjustment factors for 'spacing' between cables. or am i misreading it?

Indeed, nor NEC neither ANSI WC51 is referring to spaced cables and only solid cover is considered.
If we take 1 copper conductor 600 V/90oC insulation 40oC NEC[full 24" width solid bottom tray] 25 cables, not spaced, according to Art.392.80.2.b,Tb 310.15.B.17, Tb 310.15.B.2 uncovered:414 A;solid unventilated cover: 382 A and ANSI WC51 uncovered:427 A ; covered: 333 A. However, NEC is the law.

More than 90% of one row cables NEC does not permit and for less does not care.
ANSI WC51 consider more than one row but not less.

Sorry. I forgot the NEC article 392.80.1.c and 2.c. which considers ampacity with maintaining space of not less than one diameter between conductors. But without any cover.
However, maintaining spaces in solid bottom cable tray is a problem -we need a frame all 3 feet to maintain the distances.