As for the question when an EU aircraft has to declare an emergency, the answer is whenever either applicable domestic (e.g. EU-OPS) or foreign rules (e.g. FAR) require it to do so.
Yes, thank you
hvogt again that's common sense and we all agree with that. But regarding my last question:
i.e., can we plan to fly to the States, with only 30 minutes of reserve fuel?
what's your opinion, having in mind that, under certain circumstances, one would have to declare a fuel emergency when reaching the 45 minute mark?