BS 7671 v.d. figures normally only apply to the installation covered by BS 7671 - i.e. start at the origin.

I've not heard of a DNO suggesting they could guarantee a v.d. as low as 1% (that would imply a L-N loop impedance of less than 0.023Ω for a 100A supply) I think they just normally quote the -6/+10% of the ESQCR.

  - Andy.

Thanks for the response with this, that’s makes sense the VD applying from the origin re BS7671. 

I’ve had DNO VD quoted to as low as 1.5%, although for a dedicated 200kVA 3-phase supply. 

it gets more complex with private transformers and larger sites with in effect more than one origin. The DNO rules look easier as the problems of flicker and so on are reduced when you have a lot of other loads on the same street main or transformer as your largest step load is a small fraction of the total. The same considerations may be applied to private networks, sometimes.

If you were to consider the worst case DNO voltage, of 230V minus 6% then a BS7671 compliant installation would simply be impossible!

This does mean that in some odd cases the same cable installation may both comply and fail depending who put it in. As an example  consider a farm with the supply cable coming a long way up a track, might comply if the DNO put the cable in, and the electricity meter is at the farm house end , but not comply if the meter box is at the main road end of the track, and a cable of the same weight and installation method belongs to the farm.  The voltage drops and real performance are in both cases are the same of course, just the problem is considered to be someone else's.,
Mike