do i need a separate breaker for a 400w hps or can that be run along with my 3 flourescent fixtures and 2 small fans
You will be fine on a 15A breaker if that's all you're going to do. Go with a 20A if you want to run more. Just look at the requirements of your ballast and lights, etc. Just be sure you aren't pulling off of a circuit that has a dryer, washer, washing machine, or any other high voltage appliance attached to it. So just map your wiring, see what the draw is, find out the service you have, and go from there. I run a 400W MH/HPS system on a 15A breaker and its fine. I also run a fan, dehumidifier, etc on the same circuit. All plugged into a surge protected power strip, which is plugged into a GFCI 120V AC outlet. It isnt a dedicated circuit either, there is a 40 gal hot water heater and a jacuzzi tub on it. ~Experimentalist