What is percent reactance?
Percent reactance is the ratio of the voltage drop across the reactor (when the rated current of the circuit at rated frequency is flowing through the reactor) to the voltage between line and neutral on three-phase circuits, or the voltage between the lines on single-phase circuits.
For example, in a single phase circuit, a 5% reactor in a 6600 volt circuit means that the reactor will have a drop of 5%, or 330 volts, when rated current is flowing.
In a three-phase, 25 kV (line-to-line) circuit with 5% reactors in each phase, the voltage drop across each reactor will be:25,000/√3 x 0.05 = 721 volts when normal current is flowing.