Connection to the wiring in the sauna will be via a three-phase circuit breaker and a magnetic starter. The first device will protect the heater from short circuit and overvoltage. A magnetic starter is needed for automatic heating control. Automation is represented by a temperature and humidity sensor, which sends a signal to the control unit to switch on / off the heater. 
For you to see the correct connection of the wires, we provide the connection diagram of the electric heater for 380 V: 
. If the power of the heater is small, we recommend connecting it to a single-phase network. To do this, all three phases( L1, L2, L3 or A, B, C in the diagram) from the furnace must be combined and connected to the terminal block XT1, and from this one wire to 220 volts is output. We draw your attention to the fact that when using a single phase, the conductor cross section should be larger, otherwise it will simply not withstand current loads. Before connecting, you must calculate the cable cross-section in terms of power and current in order to select the most suitable core diameter. Specialists recommend using a copper wire with a cross section of at least 4 mm2, better than 6.
We also recommend you to watch the video tutorials that demonstrate the connection of the heater to the 220 and 380 volt networks:
That's the whole scheme for connecting an electric heaterin sauna. As you can see, the installation in a three-phase and single-phase network is not too complicated, which allows you to cope with the work of even an electrician newcomer!
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