Howdy-
Please, do yourself a favor. Before DOING anything- read your ballasts/boxes. Some ballasts can do 240 or 120V, some only do one or the other. I have yet to come accross a 240V T5 system, not that I am looking for them either. Same for 400W MH or HPS, haven't come across one that is both 240V and 120 (got about 5 of them sitting around too, just went over them). 600W and 1000W yep, quite a few you can do either 240 or 120.
Now, Add up the total Watts and Amps of your ballasts- which would be slightly different if you run 240 your amps drop.
From what you are describing- running both 120v and maybe 240v, you may want both lines in the room.
240v= 10/2 romex wire- 30 Amp cuircuit
120v=12/2 romex wire- 20 Amp circuit
I would personally recommend running 10/3 ------ BUT only if you know what you are doing- which I can see you are having a hard time grasping this- the 10/3 on a dual pole 30 Amp circuit would allow to run both 240v and 120v circuits from the single breaker...... giving you roughly 20-24 amps for lights, and 5 amps for other stuff. If the existing circuit is in the same room just use that.
If you only have one breaker available on your panel box- This is a Red FLAG- You need to look at your master breaker at the top.... it should say either 200 or 400. Then you need to look at each breaker and hopefully they are labelled and find out where they go and how much power you are constantly using. If you have a 100 amp coming in, and you have a constant rough draw of say 60 amps (fridge, ac, oven etc) then you should not add another breaker, instead- you need to update your panel and maybe add more power coming in (through the power company). Otherwise you will trip breakers constantly and risk a fire.
I just added (x2) 20 Amp circuits AND 1 30 Amp 240 Circuit (to a dedicated light controller box, closed circuit). I used ALL 10/2 wire because of the constant load I am putting on it. So I can run bigger ballasts and more of them on the 240 line, and smaller ballasts or just 120 only ballasts, t5s and fans etc on the 20A 120 circuits. BUT- I had space for 10 breakers, and only had a constant draw of 35A on 200A master breaker- so adding my 70A is ok.
You need to remember that these lights dont turn on and off like your fridge does, or Air conditioner, they are a constant draw. When you are running a constant draw that is more than 2/3 of your master breakers max, you risk throwing breakers when your other appliances trigger on, if they trigger on at the same time could cause breakers to trip or a fire if there is a fault in the wiring somewheres.
You need to know what you have and what you need before you start messing with anything. Sure installing it is all fairly simple until you start splitting a 120 off of a 240V and pigtailing into other circuits etc (which is against code, it works, but don't ever do it.)
Make a list of all equipment, power source options for it, and its usage.... then decide what you need and where you need it so you can put your lines in....