I'd like to see a reference to an US or EU standard that lists in a table the suggested wire gauges for AC/DC and volts / amps.
I cannot speak for the US system. In Europe there is not one system, It is not a continent wide electrical code. Every country has its own wiring regulations and code. With that said there has been large efforts to harmonize systems from country to country. Also in Europe we do not use the AWG (American Wire Gauge) cables are graded/categorised by there cross sectional area. i.e 2.5mm or 4mm etc. The following is directly from the BS7671 which is the wiring regulations code for the entire of the U.K while other European countries may be slightly different it would be very similar.
I should also point out it is very hard to have just one table as there are literally 40 pages of tables. Because you have to take into a large amount of factors including. Conductor material, Type of sheath composition. Thermal abilities of the cable and the environment it will be in. The type of containment/fittings. and then there are applied factors such as correction for heat and density of cables and voltage drop. The lists go on and on.
Depends largely on the application. Are you talking only house wiring? What environment the wires will be in, open air, conduit, sheathed, etc. Most common house wire is 14AWG used on 15A circuits, and 12AWG used on 20A circuits. Depending upon application, you may need to upgrade a size or two for length of run.
It really depends on the type of wire, along with the application. #2THHN is good for a 100 amp service, 240 volts. #2 welding cable is rated for 200 amps. Wire length adds resistance to the wire and so the further you go, the larger the wire (larger wire = less resistance). While @Tester101 gave you a NEC table 310.16 will give you the most common, I think finding all the ratings on one page probably won't happen. You will have to basically Google your application to find the correct wire and then Google for the amp chart.
For residential AC wiring in the US, look at this. http://www.ask-the-electrician.com/electrical-wire.html
The applicable table is actually from NEC 240.3. In short:
Amps Gauge General Use --------------------------- <=15A 14AWG Branch lighting/receptacle circuits 20A 12AWG High-demand lighting/receptacle, dedicated appliance branches 30A 10AWG Heavy-duty 120V, light-duty 240V 40A 8AWG Medium-duty 240V 60A 6AWG Medium-duty 240V, small sub-panel feeder 100A 4AWG Heavy-duty 240V, sub-panel feeder 150A 1AWG Large sub-panel feeder, small main service 200A 2/0AWG Standard main service
You can virtually always be more conservative than this, running fewer amps through thicker wire. For instance it's perfectly fine to run 12/2 instead of 14/2 for a normal 15A circuit. However, there are upper limits to the gauge of wire a wiring device like a plug or switch will accept, so don't go TOO conservative (in almost all cases you can go one step up in wire gauge, i.e. from 14 to 12, and a device rated for the thinner gauge's amperage will take it). These are the minimums that will be approved by an inspector should you do any major electrical work.