A 5KW rated single phase heater draws between 18 and 22 amps depending on supply voltage at that location.
I work on them.
240 volt 5KW=20.83 amps
240 volt 10KW=41.67 amps
240 volt 15KW=62.5 amps
Of course, very few places have exactly 240 volts. And the heaters resistance will change slightly with the temp of the heater as it varies with air flow. So the amp draw will change also.
Yes,you are completely correct. I apologize for my shortness in my prior post. 20A per 5kw, not 10 kw. And I did mean W1 and W2. Long day at work with some very ignorant customers. Not an excuse however for my rudeness.