Ok so when the engineer says the VD drop was calculated using 2% and 80% of the OCPD( 320A) in the case of 400A breaker and panel that would give you smaller wire when you want to go the other way..larger . It’s not clicking in my brain ..1) Yes over sizing the feeders for the future possible loads is the responsible way to design. The expense is minimal and the probability of the panel being used to its capacity is fairly probable.
2) Yes, those are correct steps but they are starting points only. You need to make sure those steps match your specific situation, like when you have a 600 foot run. I personally would have requested an additional utility transformer because a 600 foot feeder isn’t really the ideal case.
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well actually ultimately I’m trying to mKe sure the feeders are sized properly for the situation as I was told the electrician/estimator is responsible for verifying feeder sizes are correct or not. So to
Do that verification in the example I posted above . 400a ocpd, 400a panel, (1) 4” conduit w/4#600 thhn and keeping in mind VD has to be below 2% for the chosen feeder(I assume) How would you calculate that?