8 AWG copper.
You use the correct size breaker depending on the size wire in the circuit. If the circuit is wired with AWG #12 wire use a 20 amp breaker. If it is wired with AWG #14 wire then use a 15 amp breaker.
NEC states that all GFCI's require a 20 amp circuit, the wire size for a 20 amp circuit is 12 gauge.
A 15 amp circuit breaker is used to protect a # 14 copper wire which is the minimum size wire that can be connected to a 15 amp receptacle.
AWG 10 guage.
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.
AWG 2/0 copper wire.
Depends on the size of the circuit which you did not list.
You use the correct size breaker depending on the size wire in the circuit. If the circuit is wired with AWG #12 wire use a 20 amp breaker. If it is wired with AWG #14 wire then use a 15 amp breaker.
NEC states that all GFCI's require a 20 amp circuit, the wire size for a 20 amp circuit is 12 gauge.
AWG #4 copper.
Use 8 gauge wire.
A 15 amp circuit breaker is used to protect a # 14 copper wire which is the minimum size wire that can be connected to a 15 amp receptacle.
AWG 10 guage.
# 3 gauge
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.
Depends on how many amps it pulls and the size of the wire in the circuit.
The size breaker you use is determined by the size wire used in the circuit. If you use AWG #12/2 wire then use a 20 amp breaker. If you use AWG # 14/2 then use a 15 amp breaker.