Hey bro, word to the wise, don't use a hood mounted ballast.
I'm guessing you're going the budget route and using a parking lot light, you can do it, but you need to rig you ballast so it's remote, like the traditional hps/mh grow systems.
The bulb itself puts off so much heat, controling just the light heat is tricky, but if you have the ballast on top of the hood, forget it. It can't be done in an enclosure.
You need a cool tube, or some type of air cooled reflector and a high power fan doing the work, and that's just for the light.