No, you cannot use the existing line to supply the new tankless heater. The existing cable contains 10 AWG conductors, which are only good for supplying 30 ampere loads. The ecosmart 27 requires three 40 ampere 240 volt circuits, which requires at least 8 AWG copper conductors.
The nameplate on the unit says that it draws a maximum load of 112.5 amperes. If you wanted to supply the unit from a second panel, you'd need to feed that panel with at least 2 AWG copper conductors (or 1/0 aluminum). Feeding the heater from a second panel would be done as follows:
- Install a 125 ampere double pole breaker in the main panel
- Run two 2 AWG copper conductors (and a properly sized ground) between the two panels.
- Install the three 40 ampere double pole breakers for the heater in the second panel.
- Run six 8 AWG copper (6 AWG aluminum) conductors (plus properly sized grounding conductor(s)) from the second panel to the heater.