This calculation is done based on the general formula PF=P/S, where S=V×I.
A 3ph circuit requires an explanation. Generally, we have to add up "P" and "S" values from all phases, where Sph=Vph×Iph. What complicates matters is that Wye configuration is usually referred to by its line-to-line voltage, which is VL-L= √3×Vph. Conversely, for delta you usually know line current, which is ILINE=√3×Iph. Fortunately, for balanced loads in both cases the total apparent power can be described by the same equation: S=3×Vph×Iph=√3×VL-L×IL. Therefore to avoid confusion and to use a common approach, for 3ph configuration our widget uses line voltages and currents- that's what you need to enter into respective fields for volts and amps. For sizing of power factor correction capacitors, see our PFC caps widget.