I think that the NEC is missing the boat with it's FPN regarding voltage drop because it is a recommendation and it is not enforceable. To calculate voltage drop I use vd= 2xLxRxI/1000 I can not find anything in the NEC telling me what to use for I. Is it the size of the service 80% of the service or the calculated load of the dwelling ? Section 230.31 says service lateral conductors shall have sufficient ampacity to carry the current for the load calculated. So i guess it is the calculated load. But can the AHJ require a voltage drop calculation on a code recommendation ? I don't think so. I don't know why the NEC uses this FPN rather than carve it in stone.