Square or rectangular, and deep.
One of my complaints about the boxes typically used in British installations is that they're not really deep enough.
The 16mm box might be just about acceptable for a single switch with a single cable entering, but that's about all. I've seen triple 3-ways crammed into boxes this shallow, and 5/8" just isn't enough room for all the wiring.
The common 25mm box used for sockets is barely big enough for two sets of 2.5 T&E cables on a ring or radial circuit.
I'd like to see deeper boxes specified as certain minimums, although the construction of walls in this country poses a small problem. While we're at it, I'd like to see timber used a lot more instead of brick.
