This is 110v. 3 Circuits each on it' own DP 40 amp breaker.
The specs (see attached) on the Stiebel Eltron Tempra 29 Trend and 29 Plus are pretty clear--at least if familiar with electrical wiring. It's quite similar to the Ecosmart Ecos 27 in that it requires pretty much the same electrical wiring and breakers, etc.
Yes, each Model 29 requires THREE dual pole breakers, 40A. A dual pole breaker is by very definition a 208/220/240V setup, not 110V (in a typical residential setting). So you are bringing in two "hots"
which together make the higher voltage. The physical wiring requirement is 8/2 copper, which itself is pretty hefty. 8/2 wire will typically have a white and a black insulated conductor (along with a bare ground) and my first response to you was ensure you blacken the white insulation on this wire with a Sharpie or black electrical tape so that the wire is designated in both the service panel AND at the water heater as a
hot conductor. This is just common sense and is also code if you are using a white insulated conductor in a hot application. White generally means neutral and you have no neutrals in this wiring.
Since this is a replacement you're doing, but for the benefit of any watchers here, this is a pretty large load on most residential service panels, be sure you have at least a 200A panel. You need six open positions for three dual pole breakers and enough room for those three 8/2 cables. And, (this is a joke) ensure the meter on your home is well mounted, you don't want it flying off the home! LOL 21-28 kW isn't chump change to run.
The wiring diagram for the Tempra 29 shows that there is
one grounding point inside the unit, on the block with 3 x L1 and L2 combination points. What I would do is tie all three of the ground wires together in the unit; tie that to the ground point inside the unit, and tie the three individual ground wires to the ground bus bar inside the service panel. What they actually show in the Stiebel manual is a 2 conductor plus ground for one of the cables, and 2 conductor without ground for the other two. However, If I see a ground wire, I expect it to be grounded. If you use the same 8/2 + Ground cable for all three, best to ensure all the grounding conductors are actually grounded.
Code and smart protocol and procedures. Mark the hots as hots, ground the bare wires.
