Perforated Cable Trays

To get to more than 30% holes is quite see-through, and not in the realms of the sort of thing that can easily be punched from flat sheet without it all falling apart, where the holes remove  typically less than 25% of the weight and as such are mostly blocked from airflow by the cables they are supporting.

You may look at cable ladder tray style tray

if you want something more beefy and less easily distorted  than baskets or as it is also called mesh tray

Mike.

That's what I'm thinking but how many of us have assumed that the "standard"  perforated cable tray is not what BS7671 define as a perforated cable tray.

true - it probably ought to say 'basket or ladder' really, to avoid confusion. I'm not sure how common it is for installations to be that close to the edge though - once you start filling  basket the airflow is not that much better.

M.

That table 4 looks to be rather unhelpful - a tray with exactly 30% missing would seem to fall into category C rather than D, but still (just) meet BS 7671 requirements.

   - Andy.

Thank you for pointing that out. I never noticed that and I know city and guilds questions often use such a situation in the design erection and verification exam. However, I would have thought that a  swa operating at 70 or 90 degrees c pinned to a metal tray in an ambient temperature of 30C would dissipate heat better than in free air, the tray acting as a heat sink. But there you go, I guess not!

The heat sink effect is very small due to the thermal resistance of the cable sheath and the very low cross section and material of the tray. A tray should look similar to "clipped direct" to timber or similar if it is full, and a bit better if there are significant gaps between each of the cables (or at least those that may run close to ratings). Ladder with touching cables is no better, again the air spaces are very important.  See tables 4C5 etc.