When using Non-Metallic Raceways, the NEC allows the Parallel Feeders to be run either way -

*Option #1: A,B,C,N in each Duct,
or
*Option #2: All "A"s in one Duct, all "B"s in another Duct, all "C"s in a third Duct, etc...

Although I have never done it, nor ever specified it on Drawings, "Option #2" is valid for Non-Metallic Raceways, but you would need at least Three (3) for a 3 Wire Feeder - as in 3 Phase 3 Wire System,
and Four (4) for a 4 Wire Feeder - as in a 3 Phase 4 Wire System.

The EGC - if to be installed, will need to be the "Full Size According To Article / Table 250.122" in _EACH_ Duct.
So, if the Genny is _NOT_ to be an SDS (using a 3 Pole ATS), each Duct will need a 4/0 cu. (or 350 MCM al.) Equipment Grounding Conductor.

For an SDS, the EGCs (Equipment Grounding Conductors) will actually be GECs (Grounding Electrode Conductors), and therefore will need to be terminated at both the Genset + its local Electrode, and to the Building's Electrode system.

The size will reflect what is found in Article / Table 250.66.
In this case, you would be looking at a maximum size of 3/0 cu, or 250 al.

Good luck!

Scott35

p.s. I rarely deal with any back-up Gensets over 100 KW, so if the information is lacking, feel free to fire away!!!
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Dodging the flames...

Scott

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!