As Jordan said, if you're getting a 30 gallon pot, chances are you wont be able to do 5 gallon batches ... maybe.
We have a 30 gallon pot and the elements sit about 3 or 4 inches off the bottom of the pot, I'd assume right around the 3 or 4 gallon line ... i'd think you'd at least want a couple inches of liquid on top of the elements too for that you don't scorch the wort. Diameter of the pots will play a role in this decision too. The skinnier/taller the pot, the smaller batches you'll be able to do (asdsuming you're putting elements at the 3 or 4 inch off bottom mark), but if short and stubby you'll have hard time doing 5 gallon batches.