Hello,

Why is it that metal back boxes come in 25mm, 35mm, and 47mm sizes but
surface mount pattress boxes only come in 25mm and 47mm sizes?

I would like to wire a spur and I wasn't sure that a 25mm box would
give me enough room for the cables; a 35mm box would have been better.
Will a 25mm box be a tight fit?

I see Toolstation sell 32mm boxes but they are the same size as
Wickes' 25mm boxes so perhaps one measures internal depth and the
other external depth?

Thanks,
Stephen.