I promise to let this thread die after this. I checked Magnum’s 2000 W inverter installation manual and they too recommend 4/0 (0000) wire for installation. Looking at ABYC E-11 Table IV (http://www.pkys.com/Reference.htm )which is apparently applicable to inverter installations they list the following “Allowable Amperages” for wires run inside engine rooms:

1/0 – 242 amps
2/0 – 280 amps
3/0 – 327 amps
4/0 – 378 amps

The appendix of Xantrex Prosine 2.0 manual says nominal current at full load is 200 amps. You think they would have listed the Full Load Amps when the thing starts up if they are telling you to use 250 to 350 MCM wire. FLA not listed anywhere. With info they provided, 1/0 wire should be fine if not for mandate that 300 amp breaker be used in which case I guess you have to size wire at 3/0 which is still a big difference from 250 MCM.

I think the problem with both Xantrex and Magnum installation manual is that they took worse case installation scenario – even though that scenario not explicitly stated ( appears they assume permenant mount in building that requires wire run in conduit) - and give wire size for ONLY that type of installation. If this the case, then they should also give Full Load Amps so installer can apply proper codes based on particular installation (ABYC for marine) or, better yet – give an extra paragraph in their 50+ page manuals that gives wire size for buildings, boats, and RV installations based on applicable codes and save the user the trouble.