Reactance is the opposition to alternating current caused by inductance or capacitance. In cable calculations, reactance forms the imaginary part of impedance and is important for voltage drop, fault current and complex power calculations.
For the wider impedance context, see Cable Impedance. For the underlying cable inductance calculation, see Cable Inductance.
Inductive reactance
The inductive reactance of a conductor is:
| X | Reactance, ohm/m |
| f | Frequency, Hz |
| L | Inductance, H/m |
Inductive reactance increases with frequency and inductance. For power-frequency cable systems, it is normally the main reactive component used in voltage drop and short-circuit calculations.
Capacitive reactance
The capacitive reactance of a conductor is:
| X | Reactance, ohm/m |
| f | Frequency, Hz |
| C | Capacitance, F/m |
Capacitive reactance decreases as frequency or capacitance increases. In many low-voltage cable sizing calculations it is less prominent than resistance and inductive reactance, but it becomes relevant in longer or higher-voltage cable systems.
Use in cable calculations
Reactance combines with resistance to form cable impedance. The impedance can be written as Z = R + jX, where R is resistance and X is reactance. This form is used directly in Voltage Drop, Complex Power and fault-current calculations.