AC foundations
Capacitive reactance
- where
- capacitive reactance (Ω)
- frequency (Hz)
- capacitance (F)
Explain this equation
Works out how strongly a capacitor opposes alternating current at a given frequency. Used for power factor capacitors, filters and suppression.
X꜀ is the capacitive reactance in ohms. f is the frequency and C is the capacitance in farads. 2πf is the angular frequency.
A capacitor passes current by charging and discharging. At higher frequency, or with more capacitance, more charge moves each second, so reactance falls. That is why f and C sit underneath.
Sense check: 100 µF at 50 Hz is about 31.8 Ω. Double the frequency and X꜀ halves. At DC it is effectively infinite.
Watch out: work out 2πfC first, then divide 1 by it.
A capacitor’s reactance falls as frequency or capacitance increases. Calculate the entire denominator before taking its reciprocal.
This is a positive magnitude. In complex impedance, the capacitor contributes a negative imaginary term.
Enter your values to see the result.
Common mistake
Dividing by 2π then multiplying by fC.
Limits
Sinusoidal steady state, ideal capacitor.
Sources
The tools calculate the displayed equation. They do not automatically choose cable sizes, approve installations, supply current regulatory tables, certify equipment or replace inspection and testing.