Sorry to say, but for some reason this thread is full of amateur opinion & mis-information. Unfortunately, the OP doesn't know who the professionals are here & who to trust. Little I can do about that.
said by 05451874:
Possibly but then why didn't his 12/2 ALSO melt? If 14/2 melts and 12/2 doesn't let's apply some logic instead of non-sense. An a/c unit could function fine on a 15a circuit as required yet when the compressor kicks on it can draw more then 15a's under some conditions.
Nonsense? Now who is trolling. Its obvious from your post that you don't understand this subject the way a professional does. You say a compressor can draw more than 15A under some conditions? No. It draws more than 15A under all conditions. Its a short term condition called in-rush and will not heat up the wire. Meeting the NEC code requirements for breaker size and wire size is more than good enough - its safe under all proper wiring installations - with a safety margin to boot. Circuit breakers are designed to work with their respective wire sizes, to trip under overload conditions, before the wire is damaged. This includes when the wires are grouped together as allowed, inside of thermal insulation. As quite a few people here have said there was something else wrong with the circuit you described; perhaps a poor connection or bad breaker.
said by 49528867:
More so when you consider the fact that 14 AWG copper can carry 20 amps until the cows come home without a problem.
Pretty much accurate. 14AWG wires can carry 20A without issue. However, the NEC limits 14AWG wiring to 15A, because of certain issues, mostly poor residential installation practices, where the crappy installer doesn't follow the other provisions in the NEC. (Think a few wires bundled together in an attic reaching 130 degrees F in the summertime. The NEC would require this to be derated. I have yet to see any typical homebuilder take that into account.)
said by Fronkman:
20A devices are 500% more expensive.
Then you are buying your materials in the wrong place. The cost of both 20A receptacles and breakers are usually the same price or just slightly more than 15A devices. And there is almost no need for 20A receptacles on 20A circuits, with the exception of a few appliances that require dedicated 20A circuits, anyway. Seriously, I think you should start to consider the use of a qualified electrician. Sorry to say, but your questions make me wonder how qualified you are to do the actual wiring.
said by 05451874:
Then why is 12 AWG required by code not just use 14 AWG for all 20 amp circuits...
Its not. 14AWG is permissible in quite a number of circuits, just not receptacles wired with type-NM cable.
said by sk1939:
Remember that for datacentres all circuits are de-rated; 15 amp is de-rated to 13A, 20 to 16A, 30A de-rated to 24A. It's not all that outlandish really, especially if your running multiple circuits in conduit, where you have heat dissipation to worry about.
There is no 'de-rating' as you discuss in data centers or anywhere else. You are mistakenly confusing the requirements for continuous vs. non-continuous loads. This is not de-rating the circuit. It also has absolutely nothing to do with the wire - just the breakers. Where thermal-magnetic breakers are used - which are not 100% rated for continuous loads - the load must be multiplied by 125% in the calculation. This is for the breaker to work properly. If you could buy a 100% rated breaker, the wire would not have to be increased. Individual wires in conduit have much better heat disipation than type-NM cable. Any multiple circuits have to have de-rating applied whether its in conduit or cable. This has nothing to do with the other points discussed here. Sorry, but there is nothing in your statement that is not wrong.