Read the installation manual handed to you along with the purchase of the heating unit to get the type of wires recommended for the wiring of the heater. Purchase the outlined cable types from an electrical hardware store near you.
Inspect all plumbing in the system, including the conduits for the cables, and make sure they are suitably laid according to the shower heater's specifications in the installation manual. Check for leaks dents or cracks along the pipe works of the system.
Turn off the power to the whole house from the main circuit breaker panel. Isolate a circuit breaker on the panel solely for the shower heater installation to avoid overloading any other circuit in the house. Mount the heating unit according to the instructions on the installation manual and connect both the inlet and outlet pipes to it. The inlet pipe leads water into the heater and the outlet pipe leads water from the heater to the shower head.
Run two cables, one red and the other white, through the conduits from the circuit breaker panel to the shower heater. Connect the red wire to the hot wire terminals on the circuit breaker marked as L. Join the white wire to the terminals marked N on both the circuit breaker and the shower heater. The white wire is the neutral while the red one is the hot wire.
Find a suitable location for the switch, preferably somewhere the cables run through near the shower booth. Make sure the position you choose is out of reach of water since wet switches can cause electrocutions. Connect the hot wire to the lower terminal of the switch and another wire of the same color to the other terminal and route it through the conduit up to the heating unit and link it to the terminal marked L.
Ground any metallic pipes in the system. Run a green and yellow cable through the conduit to the circuit breaker panel and join it to the ground terminal. On its other end, connect it to the metallic pipe. You may solder or simply use an electrical tape to attach the naked end of the wire to the pipe.