Hi Canoeboy,
"The DNO quote worse case scenario and it's never ever correct unless their transformer is the other side of your wall"
I had to laugh at the bit about "unless it is on the other side of your wall" !!!!
Yes, it is exactly right what you say, up to a point, The figure the DNO quote, is at the point that your service cable joins the main, soooooo, knowing the impedance of the service cable, it would be quite easy to calculate what will be left by the time it "gets to the end" as it were.
This was a new supply though, so all i could do is go on the quoted figures minus whatever i calculated for the service impedance. No idea what that would be though, cos i cannot be bothered to look up the service cable impedance right now, as it is after one o'clock in the morning as i type this!!
I could i suppose, have gone and measured the PSCC next door, but then my nice new service cable is probably lots bigger than their 80 year old one, so that would not have given me much to go on, and besides, it is very MUCH preferred, to go on the supplied figures from the DNO, as who knows what will happen in future... They might well come along next week and reinforce their supply by installing a substation next door for all i know [i wish they would!] so, i would think it wise to always protect yourself and go by the DNO design figures, otherwise, if anything ever did go wrong, you will not be in the position of having to tell a coroner the electrical equivalent of "I knew the brake servo on my car did not work, but thought it ok as i only go at 20MPH!!!"
john...
ie 2 doors away :^O 
