I have seen it done many times. The trick is it has to be a magnetic ballast (they draw less). It has always been on older gear...? I don't know why. It can be sketchy, the breaker can pop at any second!!!! As long as you have time to re-patch and wait for a hot re-strike!
Another idea is that Arri is making "ALF" ballasts that are more efficient that may draw less than 20amps. I don't know, but they might!
Have a good shoot!
Rich
Another thought, you said fresnell. I am assuming you want a sharp beam, try to get an "Arri single ended 1200 fresnell" It will be as bright as a standard 2500 frez (double ended). I own 2 of the Arris for that reason! If you need soft light go 1200 par, it's brighter.
Cheers,
Rich