I would use AWG # 4 copper.
AWG # 10 wire on 30 amp circuit.
Answer for USA, Canada and countries running a 60 Hz supply service.240 volt two wire circuits loads do not need a neutral to operate. A three wire 240 volt circuit that needs a neutral, will have a neutral the same size as the current carrying conductors. In this case a #10.
The ground wire in a two or three conductor #12 cable is a #14 bare ground wire.
No, not a good idea. You have to use a 347 volt ballast.
The size of the wire is set by the maximum current it has to carry. The voltage sets the size of the insulation. In the UK a 230 v (nominal) ring-circuit supplying a set of power sockets is rated at 30 amps and uses a ring of 2.5 sq-mm cable.
AWG # 10 wire on 30 amp circuit.
The wire size depends on how much current it will conduct.
Use 8 gauge wire.
# 3 gauge
If it is a 110 volt light it can safely run on a 20 amp circuit with AWG # 12 wire.
Depends on the size of the circuit which you did not list.
Wire size is governed by amperage not voltage. Voltage is an insulation factor when talking about wire. Add up the amperage of fixtures you want in the circuit. Once that is found then the size of the wire can be calculated.
Answer for USA, Canada and countries running a 60 Hz supply service.240 volt two wire circuits loads do not need a neutral to operate. A three wire 240 volt circuit that needs a neutral, will have a neutral the same size as the current carrying conductors. In this case a #10.
The ground wire in a two or three conductor #12 cable is a #14 bare ground wire.
No, not a good idea. You have to use a 347 volt ballast.
A 500 MCM copper conductor will limit the voltage drop to 3% or less when supplying 350 amps for 150 feet on a 208 volt system.
The size of the wire is set by the maximum current it has to carry. The voltage sets the size of the insulation. In the UK a 230 v (nominal) ring-circuit supplying a set of power sockets is rated at 30 amps and uses a ring of 2.5 sq-mm cable.