Help sizing cable for Farm

[ian.settle1]

I disagree with the advice you were given, VD starts at the point of origin of the supply not at the end of a sub-main.

City & Guilds use this formula in their design courses, also the ECA give this advice.

Agree with Lenny, when calculating the VD the maximum permitted VD anywhere in the installation must not be greater than either 3% or 5% (depending upon type of final circuit) of the voltage at the main point of supply into the building eg the DNO cut-out.

Therefore for a lighting circuit it cannot have a volt drop greater than 6.9v from the DNO cut-out to it's furthest point, not as you think 6.9v from the DB supplied by the sub-main.