#14 copper can conduct a great deal of current without overheating. failure, (fire) in every case I've seen comes where the solid copper ends and a connection. Imagine a mobile home circuit, wire is as good as any home, outlets are cheap. A pre assembled "daisy chain" branch circuit is run before the outside sheathing is installed. Duplex receptacles are of the backwired variety, a hole in the plastic outlet passes a wire into a spring terminal gripping the wire. A wiring diagram would look like a ladder, the uprights formed by the cable interrupted at each three prong by these back wire terminals.The first receptacle's spring terminals carry the load of the full circuit. UL tests these things new not old. As time passes heated winter air leaks through these outlets to where it is cold. in cooling air relative humidity rises, as it exceeds 100% condensation begins. Net result: all winter these terminals are moist. Corrosion or at least oxidation (tarnish). Oxidized surface isn't as good a conductor. It resists the flow of electricity. As current flows through a resistor heat is produced. As heat is produced oxidation proceeds, more resistance, more heat. These spring terminals loose their temper, gripping the wire weakly, more resistance, more heat. Circuit voltage falls off at this high resistance connection. some voltage is used here in the wall converting electrical energy to heat energy. Less is available for the lamp 60 feet away. When enough heat builds up, a fire ensues.
Bad connections not wire cause most catastrophic failures in electrical circuits. I've never tried it but I bet #14 wire could conduct 40 amps before becoming dangerously hot, nonetheless a weak connection at a wire nut might cause fire at 10 amps. Why do you need to heavily load a circuit.
I take exception to using only #12 circuits, lighting load is predictable, with energy efficient lighting it is possible to light 130 11 watt LED bulbs with a 15 amp circuit. You wouldn't want every light in a house on one circuit. Trip that circuit, it's dark! # 14 or even smaller in future code editions are a good tool to reduce overcrowding in switch boxes. Do lighting only in 14, Use 12 for outlet circuits, it is hard to predict what the occupant will plug in
GFCI outlets sense balance in the "hot and neutral" conductors by passing both through a coil of wire called a current transformer. As the hot flows out creating a magnetic field, the neutral flows in creating a opposing magnetic field. The C.T. senses no field. If power flows from the hot conductor through you to the ground provided by a puddle, less will flow through the neutral conductor, The hot field, stronger than opposing neutral field tells the GFCI to shut off, interrupting power soon enough to prevent electrocution.
