should i plug into the 120 volt or 240 volt???

so i bought my new 400 watt HPS setup, and for a ballast i got the Sun system 10, and its has the 120 volt & 240 volt inputs for the powercord, ive already had it on and runnin with 20v, but my question is, should i run my 400watt light on 120v or 240? if its the 240 && should i use the powercord that came with it, cus i think it is an MVP cord (cus it says its a 600volt cord) on it. any ideas????

Where do you live?
If your in the US use the 120 setting.