Well BAS, as I already pointed out, by the time you have made and fixed the spacer, you've done most of the work anyway. So for the sake of a couple of minutes to drill out some holes with a hole saw, and then run the nibblers along the slot - job finished as I've done it.
Your way: You still have to make the spacer, you still have to cut holes in it to match the holes in the enclosure, you've
MORE work to do to fix it since many materials will sag and open up a gap (IP4x gone) if not fixed as close intervals, and then you have to fix two short pieces of trunking on an uneven wall that's a PITA to fix things to (no, you really can't drill drill a hole and pop a plastic plug in) and make sure you get them aligned.
That latter step would probably more than lose any saving made earlier. My way, I had everything bolted together, so just put the trunking back into place, mark the holes for the CUs, then sort out the fixings. I admit to having cheated a bit on the fixings ...
The traditional way of fixing stuff into this sort of wall is to : dig out the plaster and mortar till you get back to some larger solid stones, hammer in a wooden wedge, make good the mortar and plaster (couple of days by the time it's all set etc), and then screw the whatever to it. There's a bit of incentive to avoid unnecessary fixings
Since I have a rigid assembly with 10 fixings in total (the trunking is done as above - wood wedged between larger stones), there's a lot of mutual support. So I drilled deep holes to get well down into the mortar, squirted in some "no nails" type of stuff, then fed in wooden kebab skewers to fill the hole (hammering in the last ones till things were solid. The combination of the sticky stuff, and spreading the wood when a screw is driven home, makes for a reasonably secure fixing. I'd be wary of doing just 4 of those for a CU on it's own - I've seen plenty of stuff pulled off this sort of wall, taking a lump of mortar with it. But with the three items all bolted together, I'm happy that the ten fixings will keep them in place - especially as a CU isn't something being pulled away from the wall in normal use, unlike (say) a socket or a TV bracket with a heavy TV sticking out from the wall.
I hope you use trunking which is better than IP3x if you're going to put unsheathed cables in it.
Wake up at the back
IP4x kits in use for that very reason. Standard trunking and fittings are IP3x, add the IP4x kits and they become IP4x. The IP4x kits also cover up the joins in the lidding (so everything looks a lot neater, and hold the ends of the lidding which is useful when (as is normal) using other than full lengths.
So what do people do when they have to cut trunking shorter?
Clean up the cut edge, then attach the trunking to a fitting. There's no exposed edge (two sheets butted edge to edge, or overlapped and bolted together with the manufacturer made edge showing) sticking out to drag cables across or to cut into insulation due to cable movement. Your way, there's an exposed edge that the cable need protecting against.
It's true that I was assuming that the holes in a CU would be adequate to accommodate the cables expected to enter a CU.
You know what they say about ASSUME - it makes an ASS out of U and ME
I've seen installations where they'd started off with what looks like generous space for cables, and by the time they've finished the bushes are crammed tight - god help anyone trying to add a circuit later, but the original installers don't g.a.s. as they'll be long gone
I think that would be the case with these - if you tried to use just one side and the pre-punched knockouts, you'd be struggling before you got it fully populated. In our case, it may look waaaay too big, but we've got 7 ways of an available 18 already in use. In the near future I can see a call for another 3 or 4 ways - leaving 8 or 7 spare. If some plans come to fruition building wise, then I can see a call for another 3 or 4 ways, so potentially down to just 3 ways spare. That's why the boards and the trunking are so oversized for what's in them - to avoid a repetition of the "board full, need to add a second board" issue that's so common.
There's six off 25mm knockouts top and bottom, using proper glands you can get 2off 2.5mm² T&E cables through one hole - so one hole per RFC. For 1mm² T&E, you can get 2off through one 20mm hole (so need a reducer for the gland). It's a 20 way board if fitted with a standard 2 pole main switch - you do the math. In this application, you could just fit brass bushes & nuts through the holes, so would get more cables in (especially through the larger ones which I think are 32mm on the top and bottom) - but still a LOT less capacity than (two off) large slot that's something like an inch wide and 6 inch long.
And maybe one of those reasons is that they invent problems which don't really exist.
Or maybe, they know what the problems are, and the ways to handle or avoid them. It's easy as an armchair expert to say what should and shouldn't be easy - like those people who watch someone struggling with something and say something like "all you have to do is take that bolt out", and then get offended to get a reply along the likes of "I'm trying to get that ****ing bolt out you moron, now p**s off"
or in this case, an armchair expert saying something along the lines of "just put the cables through those little holes"
