NEC 310.2 Definitions.
Electrical Ducts. Electrical conduits, or other raceways round in cross section, that are suitable for use underground or embedded in concrete.
A definition in National Electric Safety Code:
NESC Section 32. Underground conduit systems
NOTE 1: While it is often the practice to use duct and conduit interchangeably, duct, as used herein, is a single enclosed raceway for conductors or cable; conduit is a structure containing one or more ducts; and conduit system is the combination of conduit, conduits, manholes, handholes, and/or vaults joined to form an integrated whole.
In my opinion a duct is a bigger conduit . A duct bank it is something else.
Nor NEC neither IEEE 835 presents any ampacity for buried conduits-only for direct buried cables and in duct bank.
However, maintaining a controlled distance between ducts and use a reduced thermal resistance concrete it is possible to increase the cables ampacity [4-6%]. For instance:
Cable 3*500 MCM 75oC copper conductor 3 ducts in a bank NEC Table B.310.15(B)(2)(6) [RHO 90] 320 A ampacity.
Calculated according to Neher and McGrath in duct bank 319A
Calculated according to Neher and McGrath in buried conduits at same distances 317 A
Calculated according to Neher and McGrath in buried conduits with no clearance 307 A