SecondAttempt
Thinks s/he gets paid by the post
Exactly what make/model are you looking at? You must be speaking of a "tankless" model or a tank model in the 80gal range that pulls some serious wattage for the heating elements? I have been speaking to you WRT electric water heaters only - nothing gas related and nothing "tankless".
Whatever you are talking about will not draw 80A from (2) 40a DP breakers. If it did, you would constantly trip the breakers. By NEC code, breakers for specific needs are 'oversized' so as to provide the required max load but also provide some overhead to mitigate breaker tripping and circuit overheating. By code, breakers must be sized at ~125% of the intended max load. So that is to say, at full load on the circuit, the breaker will handle ~25% more electrical load than the water heater requires. So if you are being told you need (2) 40a DP breakers, at max load, the heater will pull ~60a. Codes vary - but this is the general rule.
If you are maxed ~ 100a on your panel, I'd discourage you from installing this device into your mix. Remember, your panel @100a is also intended to provide that same ~25% overhead window - meaning that you really should have no more than ~75a total in-use in your home at any one time. If you drop this 60a max load device into your house, that gives you ~15a for everything else. That's not gonna happen and in actuality, invites IMO, an electrical fire at some point.
All of the info posted is based on my experience & understanding of the NEC circa 2008 - with some changes thru the latest edition. I have read/studied the majority of the ~3" binder. I also spent most of my life in a small town with homes ~ 70-150yrs old where multiple homes burned every Winter because of overloading their 100a (and smaller) panels. Hope this helps.
I appreciate the comments and information. I understand there is overhead so my heater may only draw 0.75*2*40 amps =60 amps and maybe less. But stove plus over, even with derating puts it too close to the main. Maybe not unsafe but approaching the annoying category.