I believe the NEC allows 12/3 cable to be used to wire two branch circuits in a kitchen, where half of the outlets use the red wire for hot and the other half use the black. Usually ever other outlet gets a red and vice versa for black. Every outlet uses the same 20AWG white wire for neutral. At the circuit panel the red is wired to one 20A breaker and the black wire to a separate 20A breaker. The white is connected to the neutral bar like any other 120V branch.
Given this wiring, the 12AWG white wire could be carring as much as ~40A if a both circuits are just under maximum loading. The circuit breakers would not trip if each branch was using, say, 19A. 19A would be going through the red wire, 19A through the black wire. These would be safe. However, the white wire would be carrying 38A, which is way above its capacity of 20A. Why does the NEC allow this? I know this is an unlikely occurance(such high loading), but it is possible, especially in an kitchen. It seems like an inconsistancy in the usually very conservative code.
GFCI's would not help in this situation either. As I understand, a GFCI trips if current is detected going throuh the ground pin instead of the neutral. This is usually caused by a short to ground in an appliance, say water gets in there and shorts power to the chassis of the device. In my example, the appliances are operating normally, there is just a lot of high power devices.
To any of you who may think that because the neutral wire is at 0V, there is no current flowing throuh it, that is not true. That would only be true if there was no load in the circuit. The current(Amps) in a circuit depends on the voltage supplied and the size of the load. And, in home wiring, the current flows through the hot wire, through the load(appliance or light, etc), then through the neutral wire back to the panel. Current ALWAYS flows in a loop in this manner.
If there is anyone out there who knows why the code would allow this, please respond. I think this is not a safe wiring scheme and I would always use two separate cables for anything like this.