it is first necessary to recognise whether the application is single or three-phase. For single phase (1PH) you can use the calculation: total load (in watts) over the load voltage (L to N or L1 to L2) to give the current value (I = P/V).
For example, load is 12kW and the voltage (L to N) is 240V giving a current of 50A. We normally add a safety margin of 15-20% to this nominal current in selecting a thyristor power controller to ensure we allow for any fluctuations in voltage supply or temperature, etc. This means we are not switching at the unit’s maximum which results in a longer thyristor life.
Three phase (3PH) systems are normally used for larger power consuming loads. The voltage between any two legs of a three system is a sinusoidal AC waveform but the voltage waveform between each successive pair of legs will be displaced in time by 120 electrical degrees from the other two. This corresponds to 6.67 milliseconds for a 50Hz system. Assuming the load is 12kW but connected to 3 phase we would have 12000 watts over the voltage, times by the square root of 3 or 1.73 for convenience. So the current would be 12000 / 415 X 1.73 = 16.8 amps.