If you're interested in an outside view... most European codes require derating for bundling when the cables are spaced closer than their own diameter (i.e. if you leave once the cable diameter in between two cables it's not bundling).
For wiring run in insulated walls there are no derating factors but in fact whole different ampacity tables, called installation methods. These cover wiring in conduit in non-insulated walls (either masonry or stud walls), insulated walls (type of insulation not specified), direct burial of cables in masonry walls, direct burial underground and wiring in free air (overhead lines), giving different ampacities for each method and cross section.
Usually the difference is pretty small, but when said difference is hovering around a breaker size it can be important (like if you have an ampacity rating of 15.7A you can't use a 16A breaker any more, but with 16.3 you can).
Since the maximum allowed fuse/breaker rating under normal conditions is specified a lot more conservative by the NEC than by any other code I know derating might not be as much of an issue anyway, but that's just an educated guess.
