3x250 is 750, which is less than half of what a 15 amp circuit is rated for (1800 max, 1200 continous I think). There
may be a somewhat higher draw on start up but not necessarily, depends on your ballasts, bulbs etc.
While you're out picking up the wire, why not grab an EM100 energy monitor. Looks a lot like a large timer and will show any number of paramaters of what you have plugged into it, current, voltage, wattage, Kwh, even cost if you enter your hydro rate into the thing. When I checked my 2 MH 250s with this device, it actually showed they were drawing somewhat less than the rated 250 watts. I'm currently using cheapo Plusrite 14000K lamps and the two of them draw just over 400 amps once fully fired up. If I remember correctly when I monitored the wattage with the EM100 on start up, they drew even less until fully warmed up. Instead of a surge at startup as with most other components, particularly motors, my MH set up actually builds up wattage slowly to less than max rated. 10000K XM lamps which I had installed before, drew about 485 watts when fully warmed up on my Advance M58 magnetic ballasts. I'd say if the 3x250 MH are all you're going to run on a single 15 amp circuit, you should be fine.
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