I have a 200 amp panel for an apartment with separate service from the rest of the house.
I want to add an electric tankless water heater.

Here's the one I'm looking at buying:
Stiebel Eltron Electric Tankless Water Heater - 28/21.6 kw - 120 Amp - 240 Volt - Tempra 29 Plus

Just to give you an idea, on that panel I already have an electric furnace, heat pump, and
Range with two breakers each, (100 30 30), totaling 320, and misc that add up to another
90 amps (20 20 20 30).

I know I need to calculate the load, but I don't see how to tell the load coming from
This new heater.

Is this too close to tell and I just need to call a pro before I order it?

For my background, I generally run my own 120 and so forth. I can do a three-way
Switch. But I don't do things I don't understand. And I don't understand 240, so
I let the electricians do those. 8:-)

Thanks,
Brad.

You state: "but I don't see how to tell the load coming from
this new heater."

But you tell us the load of the new heater is 28.8 KW and is 120 amps at 240 volts.

I am confused.

You added up the ratings of the circuit breakers to arrive at a total of 320 amps plus 90 amps.

That information is irrelevant, means absolutely nothing.

To calculate connected load, you need to list the load in amps or watts of each appliance, furnace AC unit, and provide the square footage of livable space in the apartment.

Because of the electric furnace I assure you the 200 amp service will not have the capacity to serve a 120 amp load.

I would absolutely call a pro before you order that thing.

No offense, but you really don't know what you're doing, and this is no small project to get yourself involved in, with quite the monetary outlay.
I would not chance it by trying this yourself.


Oh, and I also agree that adding this monster to a 200A service with an electric furnace and other things will not work.