I installed one in my s&b, will not do it again.
You must flow water at a certain rate (on my 9200 watt model it's .9 gpm) or the unit doesn't turn on. And if the water flows too fast, it doesn't get sufficiently heated.
For your theoretical 30 amp use, this model is an example,
Aqua power unit3000 watts @ 120 VAC and will provide water at about 90-100 F from a 55 degree supply at a flow rate of .59gpm.
Trying to fill a soaking tub will take a very long time and will cool before you can get it filled.
PDF showing flow rate and temp rise for various models
operating guide
