I am not an engineer nor an electrician but I just installed an Atmor 5.5kw water heater with a 12mm wire and a 30amp breaker at 220 voltage.
I install it separately with plastic molding outside my house from my bathroom at the attic because there is no outlet for the water heater down to my main panel.
This is a single phase only but I observe that the wire is getting warm whenever I turn on the heater.
Is this nothing to worry? Can the wire tolerate the resistance of my water heater up to it's highest point?