4 AWG copper or 2 AWG aluminum.
No !
12 AWG in most circumstances.
AWG # 10 wire on 30 amp circuit.
The question can't be answered - it has no meaning :)
what kind of circuit? if it's a subfeed from one panel to another same size as hot wires. if it's a straight 240 volt circuit #8. depends on what kind of circuit.
# 3 gauge
Can you supply three 220 -240 volt 16.6 amp infrared heaters with one circuit?
No !
Use 8 gauge wire.
12 AWG in most circumstances.
Depends on the size of the circuit which you did not list.
AWG # 10 wire on 30 amp circuit.
10000 watts / 220 volts = 45.4545 amperes
The voltage cannot just be increased in a circuit because there is a risk of damage, blown circuit breakers and/or fire. However an appliance desgined to run on 220 v will use 6/11ths of the current used by an identical appliance designed for 120 v.
Most residential service in USA has both. In the USA 110 to 120 volts is a given and it would be very unusual not to have 220 to 240 volts. It can be easily tested at main panel with a volt meter. Or as an alternative call your power company.
The question can't be answered - it has no meaning :)
what kind of circuit? if it's a subfeed from one panel to another same size as hot wires. if it's a straight 240 volt circuit #8. depends on what kind of circuit.