A circuit breaker does not have a wire fuse in it.
Yes, you install a GFCI on a 2 wire circuit.
No. In a 20 amp circuit all wire has to be 12 AWG or larger.
No. The wire size is dependent on the circuit protection. If the circuit uses a 20 amp breaker you need to run 12 AWG wire on all devices connected to that circuit.
You never, ever mix wire sizes within a circuit. However you can wire a 30 amp circuit using AWG # 8/3 wire, although that is overkill and AWG #10 wire is what you would normally use on a 30 amp circuit.
A GFCI can not be used on a three wire branch circuit. It has to be on a single two wire circuit.
The wire comes from the installer or manufacturer of the circuit.
A circuit breaker does not have a wire fuse in it.
The wire in a circuit helps to pass power to the electrical appliances.
Yes, you install a GFCI on a 2 wire circuit.
If you have 100 amp wire, you can use it for a 60 amp circuit, or for any circuit of 100 amps or less. But if you have a 60 amp circuit, 60 amp wire is thinner and cheaper than 100 amp wire.
The smaller the wire size used in a circuit limits the amount of current that is allow to be flow through that circuit.
A parallel circuit is :)
One
The live wire
Close circuit is a complete circuit of battery wire bulb and a switch
No current will flow though a circuit which is broken.