This seems to be hotly debated subject on the various forums.

Doing first fix on rewire. Solid ground floor so ground floor circuits run in the ceiling and drop down the walls to the circuits/light switches etc.

The existing outlets were done in the 60's and the drops appear to be in 20mm equivalent (imperial) round steel conduit with two cables in each (for ring sockets) although I haven't measured it yet.

Is it still permissible to use 20mm conduit with two 2.5mm cables in, given grouping factors etc. It is only for the couple of metres where the circuit drops from the ceiling.

The remainder of the circuit will be clipped to the joists in the ceiling void.

Plan was to use a single 20mm or 25mm round conduit for each socket drop (Using 35mm boxes with dual knockouts)

There is also a couple of sockets where the conduit will be in an insulated stud wall - again only for the drop from the ceiling.

Thanks