^^
I agree. Do at least 2 circuits in case one is having a problem. That's a great design idea.

In any case, if this run to any of the MH lights is over 200' I would suggest using #10 THWN on a 20A circuit breaker. Just because 240 volts is twice the voltage of 120 doesn't necessarily mean that there'll be less voltage drop. It's all dependant upon the size and length of the conductor and there is more resistance in a #12 wire than a #10.