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1) Lights are never wired in series, they are always wired in parallel to one another.
2) Many metal halides have a 120, 208, 240 277-volt quad tap ballast. 2 blacks suggest that it can only be wired as 220 volts. If the fixture does not contain a white or a gray neutral wire and only 2 blacks, then that's a 220 volt fixture. Double check if this fixture allows for various voltage inputs on an internal tap transformer. The wire taps originating from the transformer should be labeled indicating the desired voltage.
The spec sheet I am reviewing shows that it can be wired for either 120, 208, 240 or 277 voltages
If wiring to 220 volts, a 220 volt double pole breaker will be required and cannot be mixed into an existing 120 volt lighting branch circuit.
3) Will it be connected to another 120 volt light fixture or a 220 voltage?
4) The green wire is used as the grounding wire only.