Is it at all possible that you slightly misunderstood or misread the question??
The only rules I can find that refer to maximum voltages between conductors are the following:
12-502 Maximum Voltage. Nonmetallic-sheathed cable shall not be used where the voltage exceeds 300 V between any two conductors.
12-1608 Maximum Voltage. The voltage between conductors contained in surface raceways shall not exceed 300 V unless the raceways are marked for a higher voltage.