Without consulting the code, it seems that everyone "just knows" that
14 gauge is used for 15 amps, 12 for 20 amps, etc. My question is -
how much margin of safety is there in this? I'm assuming that the
temperature of the wire at load was used to actually establish the
code. Presumably, though, 14 gauge could carry 16 amps without
creating a hazard. How about 17 amps? How about 18? ... (How many
hairs does it take to make a beard?). In a pinch, if you were
building something temporary - something you were going to tear down
in a week or two - would you use the 14 gauge you have on hand for
your table saw, or would you make a special trip to the store for 12?
At what point does the heat create an actual fire hazard? I'm not
considering over-fusing anything - I'm just curious. In Grandpa's
day, it was standard homeowner practice to simply insert a bigger
fuse when one kept blowing. No doubt the elders who wrote the
various versions of the code were aware of this practice and took it
into consideration when deciding on the numbers.