I'm *guessing* we're talking USA here, 120volts. Most all residential has minimum 15amp circuits by FHA guidelines (some things have more, like your A/C and Dryer). The formula for amps is AMPS=WATTS/VOLTS, in this case volts=120 and watts=400 so amps=3.33. You should be able to run 15-amps off of a single circuit, and standard wiring (white stuff) is good to 15-amps. The yellow-wrapped stuff is 20-amp plus. I know this because on my aquarium I did have to add a 20-amp circuit, but I was pushing multiple MH lights, water cooler, heater, pumps out the yin-yang, etc.
The thing about not putting more than one MH light on the 15-amp circuit is because at startup a 400-watt ballast will draw much more than the 3.3 amps we calculated earlier, but only momentarily. Staggering their turn-on (even by 1-minute) will cure that.
Keep in mind, that 15-amp circuit might be connected to MULTIPLE outlets, lights, etc. Sooooo, just find out what all is on that circuit and figure all the wattages out, calculate your amperage, and if you're under 15 you're good. Actually, I would stay under 12 or so just to prevent possibly tripping a breaker if something gets turned on. Still, 12 amps is 1440 watts which is a lot of juice.