You should be fine. You could actually use 300KCMIL and use the round-up rule (240.4). You are right, the nuetral probably won't be a current carrying conductor, but even if it were you would be alright because for derating you can use the 90 degree column.
One thing, however: I notice that you talk about the connected load. 230.42 requires that you use the calculated load for the purpose of sizing service conductors. HAve you done a load calculation?
