well, i am not sure they can..... where's the list of all possible scenarios?
Its built into the numbers and codes themselves.
NEC has some restrictions and allowances in the code, but i dont see a list of "all possible scenarios".
Look at it like this. Wire can be installed almost anywhere under any conditions even when taking other code rules into account. Table 310.15(B)(16) doesn't get to specific other then ampacity for a given wire size and insulation type while article 310 doesn't go much past saying you adjust for ambient and de-rate for bundling. Therefore, they must assume wore case scenario. 3 current carrying conductors in a raceway in an 86*F environment can be anything from conduit in moist earth, to conduit attached to a concrete wall, to conduit in direct sunlight, to NM in very dense thermal insulation for 200 feet. All of those 4 scenarios will have radically different current capacities from the ability (or inability) to dissipate heat. Thus 310.15 (B) (16) must take the worse case scenario, that being cable or conduit in dense thermal insulation. The table must also consider an adverse conditions the code protects against, such as the wire under full load in thermal insulation when a short circuit occurs. The brief (yet large) short circuit current along with the time it takes to open the breaker will add additional heat. The wire must be able to withstand this worse case scenario without melting or setting something on fire.
Just to give you an idea regarding heat dissipation IEC based codes let you select ampacity based on condition of use. 2.5 mm2 (a bit larger then our 2.08mm2 #14) twin and earth cable (The euro version of Romex) has different ampacities based on where or how it will be used. If the wire is not covered in thermal insulation it is rated 27amps. In a wall touching thermal insulation 20 amps. If embedded in dense thermal insulation then it goes down to 13.5 amps.
Such a table could be developed for the NEC (and I believe it should be evaluated), however any CMP member can always argue conditions today will change tomorrow. An example would be a basement with open rafters and beams. In theory we could easily load #14 to 25 amps. I wouldn't loose any sleep. It will dissipate the heat well and not reach questionable temperatures. However if someone buys the home and wants to finish the basement adding thermal insulation, things change. The electrician has to either come back re-pulling all new wire or the cable simply gets covered over when the HO does not know any better. Suddenly you end up with a much larger waste of copper or the owner now lives with a potential fire hazard. Therefore a good argument exists that for ease and safety 310.15 should simply assume wore case scenario right off the bat.
who's to say a compliant romex installation (a bc bound to 60C column) in scenario #3,456 doesnt pose a hazard?
It could, but when going by the worse case scenario the probability sharply goes down.
see, i dont think every scenario can be accounted for. this is a limit model which brings things back down to zero, hence, zero current is the safest for all known and unknown wiring scenarios. but, we cant live in a world of zero, we have to accept some risk, etc.
You are correct, but we can get very close to zero without actually going to zero. In the real world fires from fixed building wiring itself is rare. In fact nearly every electrician working in the field will tell you about the only time they see melted or damaged dielectric insulation is with code violations, loose connections or a short circuit where the OCPD failed to trip. Damaged wire insulation from overheating in code complaint installs is virtually unheard of.
a local street crossing where i used to live was always somewhat dangerous, no traffic light there for 30+ years. then one day a person was hit by a car and then suddenly a light went up, however, this did not prompt the locale to put up lights at every crossing similar to this one.
We can not control people and how they drive, however we can set common sense rules greatly reducing those hazards especially when its building practices. Picture something along the lines of this. Bridges often have a very high degree of safety built into them along with more safety factors added on top of that. Same goes for most buildings. The theory is its best to spend a bit more upfront and have confidence under all scenarios (and even some of those that might be unforseen) then to have a bridge fall down or in the case of the NEC a home burn down.
