"DavidD" wrote
| I need to rewire some bits of my cellar. I want to use conduit so I can
| run power, phone and lan.
| I think I'll be able to squeeze it all into 20mm,

You can't "squeeze" cables into conduit at all. There are rules on the
physical capacities of conduit and you should calculate, for every length of
conduit, the unit capacity of the conduit and of the cables therein. You
will also have to calculate any derating factor due to mounting method and
grouping for the cables themselves.

Owain