current - Phase voltage vs main voltage in three phase system

My question is from my book : A frequency converter is connected to three-phase network (400 V, 50 Hz). How much phase current it pulls from the grid during normal operation (785 W).

the correct answer is : I=875/√3/400=1,26 A .

How I know if I should use the main voltage or the phase voltage to get the correct answer?

How you know that 400 V is a phase voltage and we have to use the main voltage in our equation?

Thanks

Answer

You can use either but you have to remember that the phase to phase voltage is \$ \sqrt{3} \$ times the phase to neutral voltage.

Figure 1. The phasor 3-phase and neutral diagram.

I find the simplest way to remember is to use the phase-neutral voltage if you have it:

$$ P = V_{P-N} I $$

If you have the phase-phase voltage then you need to divide:

$$ P = \frac {1}{\sqrt{3}}{V_{P-N} I} $$

The \$ \sqrt{3} \$ term just comes from the trigonometric relationship between the voltages in Figure 1.