Medium voltage devices and equipment are all 100% rated. No need to increase the cable by the 125% like is done with lower voltage systems (480V power circuit breakers are also 100% rated) Therefore, the current is 121 A. This does not necessarily affect the cable size, because for 15kV voltage class, #2AWG is typically the smallest cable manufactured. As you probably saw from the Southwire catalog.
Keep in mind, that Southwire, like others, do not manufacture a MV-90 cable anymore. They have standardized on MV-105. So the current ratings you see in their catalog are for 105 deg C, not 90 deg C.
As far as 310.77, unless your going to put concrete around the conduit, then technically, it does not apply. This is because it is usual and customary to put concrete around MV Conduit.
EPR is more flexible, however not quite as durable as XLPE. The PVC jacket will help protect the insulation during pulling.
"Just because you're paranoid, doesn't mean they're not out to get you"