What service conductors did he spec out? 3/0 is the right size for 1500kcmil USE (625A ampacity) or similar. If multiple smaller cables are used, 2/0 may be acceptable, but you'd end up having to derate for # of conductors in a raceway, which would put you right back at 3/0. Seems like a reasonable requirement to me.
You could always call the engineer and ask.
