Ok as I am in temp accommodation at the moment and don't have access to my bible I have been at my mates and trying to figure this out but the more we look at it the more confused we are becoming, for a 3 meter length the correction factor for 6mm is 88 but over 3 meter it drops to 58 i would have thought this would have been the other way around but there you go never mind that but if the op could have kept the run between his boxes to 3m instead of 4 then he could in fact run all 5 conductors needed in a 20mm conduit and not break a single rule but the extra meter makes it not work but its been a nice little refresher for me thanks guys but only conduit I use these days is to sleeve an outside light cable and I am sure in the multitude of regs changes since I needed to know they have changed this the same as everything else so I stand corrected sorry for doubting anyone but it was all over a meter