What size back box do I need for a Shelly?
There are three standard depths of back boxes in the UK: 25mm, 35mm, and 47mm.
In either case, most Shelly’s are typically around 39mm x 36mm x 17mm in dimension, which means they’ll fit comfortably in a 35mm or 47mm deep back box.
If you have a particularly busy back box with many wires and connectors, you may benefit from first replacing it with a 47mm back box to give yourself a bit more room to work.
Can I use a metal or plasterboard?
Galvanised steel and plasterboard back boxes are both sufficient for installing a Shelly. It is equally as easy to install a Shelly in a metal back box as it is to install one in a plasterboard back box.