Suggestions for best way to wire underground supply please

[Wilko]

Why would buried in the ground be considered more onerous for current rating than clipped direct/'free air' . Doesn't make sense to me.

I know what you mean! Looking into it a bit (feedback appreciated)
Table 4B3 gives rating factors if we know the soil thermal resistivity on the site. Otherwise we use the tabulated ccc of Table 4D4A column 6 (say) which are worked out for 2.5K.m/W. Looking on internet (no warranty there) for thermal conductivity and using 4B3 for rating factors :
- dry soil 3.3 - rating factor 0.9
- damp sandy soil 0.5 - rating factor 1.88
So local conditions could change the rating quite a lot, and it looks like BS7671 has conservatively punted for dryer soil conditions.

 

[Guitarist]

I've just done the exact same job, except from a detached garage on an old property. Gas central heating, so used 16mm 3-core swa with a 60amp switch fuse.