13800 & 4160 in duck bank over 3 circuits Detail 3 in code required for conductor rateings (310.61)Table 310.77 If 6-350 mcm wires in same conduit rating them dn an additional percentage 310.15(b)(2)(a)4-6 80%.
thus use table 310.77 Detail 3 then an additional derateing for 6 current carrying conductors in conduit.
350mcm 90c =315x2=630x80percent=504 amps
So parallel 350 in a single conduit with 4 other hv circuits in the ductbank .Rateing of conductors = 504 amps
Remember this is 4160 volts
