Whoa...you're trying to build your own transfer switch??!! Whatever you build is not going to be listed for the application, and won't be as relaible as a purpose-built transfer switch. This is not something you want to mess around with! Besides, you will have a hell of a time finding a double throw, 250A relay!
May I suggest a real, listed transfer switch, a reliable company is ASCO, check out what they have here
http://www.ascopower.com/Products/ATS/atsmenu.html be safe!
mike
