Since we don't know how the 12 single-core cables are running [in groups of 4-each group of all phases and N, for instance- or in conduits or in cable tray or else] so we don't now the distance between cables. It is important in order to calculate the resistance [proximity factor] and the reactance. Also we don't know the power factor, the frequency and the conductor temperature.
In order to get 7 V voltage drop we have to take-for instance-groups of 4 cables laid in straight line ABCN, NCBA.. and so on, power factor of 0.99 , 60 Hz and 75oC.
I agree with kingpb. For 2 cables per phase the voltage drop will be 11.82 V [2.46%] less than recommended by NEC [Art.215.2 Minimum Rating and Size. (A) Feeders Not More Than 600 Volts.(b) Informational Note No 2.]