Aldrich wrote:

I am rewiring the upstairs lighting circuit of our house. I have chosen to
use 1.5mm 6242Y cable. I want to use the junction box method rather than
the loop-in method because I find it a pain trying to get three cables into
a ceiling rose. The junction box method requires 4 cables (switch, light,
loop-in and loop-out) per box and the books I have read suggest that it is
typical to connect 4 cables to a junction box.

However, I have just discovered that the "standard" 20A junction boxes that
I have bought are only rated for 3 x 1.5 cables. One solution is to use two
junction boxes, one to handle the loop-in/out and the other to handle the
local lighting circuit. However, that seems a little inelegant and costly on
boxes.

You didn't mention how many screw terminals there are in your junction
boxes. If there are only 3 (which sounds likely if it specifies only 3
cables can be connected) then you've got the wrong jct boxes; if there
are 4 (ie, which you need for the jct box method) then I'm stumped
because you that would normally presuppose you will have 4 cables
entering the jct box (unless it's the final one on the circuit, when
there would only be 3).

There are presumably cut-outs on the side of the junction box enabling
different numbers of cables to enter, depending on the orientation of
the lid; there are usually little diagrams showing the orientation for
1, 2, 3 or 4(?) cables entering - do you have this?

David