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Well my home has 100amp service at 240volt. If I chose to use 120 volt for everything in the entire house, I would effectively have 200 amps.
So if I run 50 amps @ 240volt, then I effectively have 100 potential amps if it was all converted to 120volt. I chose to run my ballasts on 240 volt which took 20 of my 50 amps. That leaves me with 30 amps @ 240 volt which I used to run 60 amps @ 120 volt for utilities.
All these numbers don't account for things like continuous load, voltage drops, etc.
When in doubt, hire an electrician. I didn't do this all myself, my homie is a journeyman and I contracted him to help out. I'm sure I'd be dead or severely injured if I tried all this myself, a couple Gs in the name of personal and property safety is never a waste in my book.
I'm horrible with electric and my electrician sucks at putting it into words I can understand.
So let me get this straight.. 50 amp 240 subpanel. Im using 20amps 240 for ballasts so I'm left with only 30 amps 120?