What's the suggested standard gauge wire for a given current?
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.

http://www.batt.co.uk/upload/files/table4d1abs6004bs6231bs6346_1220253954.pdf

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.