1.- OKONITE is good place to start with ampacity values for "typical" configurations. Typical meaning a common set of values.
2.- In addition you should use the IEEE books Volume 1 & 2 ( Copper and Aluminum ) 70's vintage and/or the new book by Anders ( find it in the IEEE Online bookstore ).
3.- Over the years IEC 287 Standard has proven a better solution for me, as it basically gives you the main formula to calculate ampcity and then works with each module that makes up the equation. The modules make up for the configuration and installation details selected along the way.
4.-The NEC requires that if the U/G cable lenght is > than the O/H lenght by 25' the ampcity for the U/G prevails.
5.- Please remember that a cable installed U/G has a lower ampacity availability than the same cable installed overhead. This is due to the fact that in order for the cable to dissipate it's heat it will have a series of "thermodynamic resistances " to overcome:
a) All resistance units part of the components that make up a cable ( insulation,shield,armour,jacket,etc )
b) Resistance of the installation components ( concrete,conduit,air in conduit,fill,sorrounding fill).
c) Installation components such as the depth on the duct bank,the deeper the worst dissipation model.
Several techniques are use to improve ampcities:
a) Separation between circuits.
b) Leaving empty ducts between circuits.
c) Using the outer conduits always for circuit installations.
d)The installation of chemically treated sands in lieu of air for all conduits that hold current carrying conduits.
e) Cooling water pipes.
f) In Europe the use of direct buried cables is very popular.