AWG 12 will suffice.
A 100 amp residential service requires a size #8 copper wire, it should be insulated in green.
2 copper
Service entrance wire should be AWG #3 copper.
You need to look at the regulations that apply in your country. If in doubt, use a neutral wire of the same size as the live wire or wires.
A 100 amp service requires that you use AWG 4 copper wire or AWG 2 aluminum wire.
10 guage
A 100 amp residential service requires a size #8 copper wire, it should be insulated in green.
This can't be answered without knowing the voltage, and ground is not the same as neutral, in AC circuits, which I'm assuming this is.
# 6 copper wire.
The wire size depends on the amperage, not the voltage. The fact that the welder runs on 440 volts does not affect the wire size! You need to ask this question instead: What size wire do you need for a three phase welder on an eight foot run if the nameplate amps are 45A?
Service wire required is AWG # 3/0 copper.
Need to know the wire size.
2 copper
Service entrance wire should be AWG #3 copper.
six awg
Service wire for 100 amps requires AWG #3 copper.
You need to look at the regulations that apply in your country. If in doubt, use a neutral wire of the same size as the live wire or wires.