Always took this as 3% to the sub. I'm somewhat new to this though so I could be wrong...8-102 Voltage drop (see Appendix D)
(1) Voltage drop in an installation shall
(a) be based upon the calculated demand load of the feeder or branch circuit;
(b) not exceed 5% from the supply side of the consumer’s service (or equivalent) to the point of
(c) not exceed 3% in a feeder or branch circuit.
(2) For the purposes of Subrule (1), the demand load on a branch circuit shall be the connected load, if known;
otherwise it shall be 80% of the rating of the overload or overcurrent devices protecting the branch circuit,
whichever is smaller.