In open air the wire is able to dicipate heat where s wire that is not able to the heat escalates because to wire is not able to use the open air around it to radiate heat. 90degC rated wire when encased within Romex will end up getting hotter then that same wire in open air. The run that Romex through walls and in attics laying within insulated increased the wires inability to dicipate heat which increases the risk of a fire.
a 1A current on 14ga wire could cause wire to get very hot, all depends on how heat can be removed.
14ga copper in STP w/ no insulation will melt around 1980F, and will fuse at just about 166A, which is well beyond what a 30A OCD would allow.
could a 30A fuse on 14ga wire cause a fire? sure, but under some rare conditions. as mentioned already, wiring is not a homerun from A to B, there's pigtails in there, connections to devices like lights switches and outlets.
here's an example of hazard. last year i redid my kitchen backsplash with some nice tile, had to pull out some outlets (still connected to wires), did the tile, pushed outlets back, then i heard a bzzzzzz, bzzzzzz, sound, it was faint, but after investigating more closely i found that all five outlets has loose wire screws and one outlet so loose that there was small arc from screw to wire which was making that notorious 60Hz bzzzzz sound.
2nd example of hazard. a few months back i pressed the test buttons on four afci breakers in my panel, the 3rd one simply went "hummmmmmmmm" and smoke started to come out of the breaker, i quickly turned off main. the breaker was faulty and the lever didnt even move after i pressed test button. yanked that breaker out and it was already fairly warm (all that within ~5sec timeframe). this is why you should test these items per manufacturer recommendation, but since the breakers are "hidden" in panel they go forgotten, not seen like smoke detectors, etc.
