Most of you guys looking for extra 240V power in average house when facing inadequate power and potential danger of circuit overload relax, there is an easy solution applicable in most cases most "experts" and "electricians" on this thread are missing.
If your grow house has any of following: Central AC, Electric water heater, Electric kitchen range, electric dryer, AND any of those are not used, or INFREQUENTLY used you have a potential of plenty 240V power WITHOUT ANY rewiring, subpanels and other nonsense suggested.
Modern digital HPS ballasts run on 120/240 automatically switchable just by applying proper cord with either 120V or 240V plug.
Here is your solution:
1.
Central A/C (I assume not used in winter) average 10KW (mine is 5 ton AC draws 20KW).
Even if yours is 3 ton central A/C you can run 8 1000W lights out of this circuit when AC is not used obviously.
2 Water heater (240V) 4500W to 5500W . Easy 4 1000 hps lights at 4500W.
3 Kitchen range (10KW) easy 8 1000W Hps lights.
4 Electric dryer (3kW +) 3 1000w HPS lights
All it takes is proper extension cord, ASSUMING you are not using those appliances at the time.
So even if you live in the house you are growing how often you use Clothes dryer?
When you are flowering you can run lights at night. How often you are using kitchen range at night?
Or even better, put your water heater on timer (off when you are flowering) and you have easy 4 1000W HPS here.
How often you are using AC in winter, spring or fall depending where you live? Central AC is the biggest power hog in the house, so it is a biggest potential for those hungry 1000W HPS lights.
My house is not the most modern house, built in 1978, only 150 amps panel, but I could run easy 10 KW + of lights in summer or 30 KW in winter if I wanted to. THATS 10 - 30 fricken 1000 watters !!!
Summing up if you run your lights on 240V you can free all 120V circuits to run fans, controllers, pumps which run on 120V, but that should be no issue since those devices draw a fraction of power the big HPS lights require.