Reactance is the opposition to AC due to capacitance and inductance, measured in ohms, which temporarily stores and releases energy rather than dissipating it as heat like standard resistance. When you apply alternating current to a coil or a capacitor, the component fights the change in current or voltage, creating a frequency-dependent bottleneck that shifts the phase relationship between volts and amps.

Bench Rule of Thumb: You cannot measure reactance with the DC resistance (Ω) setting on your standard multimeter. A healthy inductor will read near 0Ω DC, and a healthy capacitor will read infinite (OL) DC, even though both heavily restrict AC current flow. You must calculate it, measure it with an LCR meter, or derive it from AC voltage and current readings.

What Reactance Actually Changes in an AC Circuit

Unlike a standard carbon film resistor, which limits current equally at 0 Hz (DC) and 1 MHz, reactance is entirely dependent on frequency. This happens because inductors and capacitors do not burn off energy as heat; they store it in magnetic or electric fields and push it back into the circuit.

Inductive Reactance ($X_L$) occurs in coils, transformers, and motor windings. An inductor resists changes in current. When AC voltage rises, the inductor generates a back-EMF (electromotive force) that fights the incoming current. This causes the current waveform to lag behind the voltage waveform by exactly 90 degrees in a purely inductive circuit. As frequency increases, the inductor has less time to build its magnetic field before the current reverses, making it fight harder. Therefore, inductive reactance increases with frequency.

Capacitive Reactance ($X_C$) occurs in parallel plates, motor run/start capacitors, and cable parasitic capacitance. A capacitor resists changes in voltage. It draws a massive rush of current to charge its plates before the voltage can actually rise across it. This causes the current waveform to lead the voltage waveform by 90 degrees. As frequency increases, the capacitor charges and discharges more rapidly, allowing more current to flow. Therefore, capacitive reactance decreases with frequency.

The Core Formulas:
Inductive: $X_L = 2 \pi f L$ (where $f$ is Hz, $L$ is Henries)
Capacitive: $X_C = \frac{1}{2 \pi f C}$ (where $f$ is Hz, $C$ is Farads)

This phase shifting is what creates 'reactive power' (measured in VARs). While reactive power does no actual mechanical work, it forces your wires, breakers, and transformers to carry extra current, which generates $I^2R$ heat losses. According to Fluke's power factor guidelines, utilities actively penalize industrial facilities with low power factors because this reactive current wastes grid capacity.

Inductive vs. Capacitive Reactance Reference Chart

Before wiring filters or sizing motor components, you need to know how these two forces behave under different conditions. Here is a direct comparison of their operational characteristics.

Property Inductive Reactance ($X_L$) Capacitive Reactance ($X_C$)
Primary Component Coils, Chokes, Motor Windings, Transformers Film Capacitors, Electrolytics, Ceramic Discs
Symbol & Unit $X_L$, measured in Ohms (Ω) $X_C$, measured in Ohms (Ω)
Phase Relationship Voltage LEADS Current by 90° (ELI) Current LEADS Voltage by 90° (ICE)
Behavior at DC (0 Hz) Zero reactance (acts as a short circuit) Infinite reactance (acts as an open circuit)
High-Frequency Behavior Reactance approaches infinity (blocks highs) Reactance approaches zero (passes highs)
Common Failure Mode Insulation breakdown, shorted turns (lowers L) Dielectric drying, ESR increase, shorting
Energy Storage Medium Magnetic Field Electric Field

Worked Numeric Example: 45µF Motor Run Capacitor at 60Hz

Let’s move from theory to the jobsite. You are troubleshooting a single-phase 240V HVAC compressor that is humming but failing to start. The schematic calls for a 45µF motor run capacitor. You need to know what the expected AC current through this capacitor should be to verify if it is degraded.

Step 1: Calculate the expected Capacitive Reactance ($X_C$)

  • Capacitance ($C$) = 45µF = 0.000045 Farads
  • Frequency ($f$) = 60 Hz (North American grid)
  • $X_C = \frac{1}{2 \times \pi \times 60 \times 0.000045}$
  • $X_C = \frac{1}{0.0169646}$
  • $X_C = 58.94 \Omega$

Step 2: Calculate the Expected Current

  • Voltage ($V$) = 240V RMS
  • Using Ohm's Law for AC ($I = \frac{V}{X_C}$):
  • $I = \frac{240}{58.94}$
  • $I = 4.07$ Amps
Troubleshooting Insight: If you clamp your meter around the capacitor wire and read 4.0A, the capacitor is likely healthy. If you read only 1.8A, the internal dielectric has degraded, dropping the actual capacitance to roughly 20µF. This raises the reactance to ~132Ω, starving the motor's start winding of the phase-shifted current it needs to generate starting torque. Replace the capacitor with a high-quality Cornell Dubilier (CDE) or AmRad unit rated for at least 370VAC (440VAC preferred for longevity).

Regional Variant (50Hz Grids): If you are in the UK, EU, or Australia running a 50Hz system, that exact same 45µF capacitor yields an $X_C$ of 70.7Ω, resulting in a lower current of 3.39A at 240V. This is why you cannot blindly swap 60Hz motor capacitors into 50Hz equipment without recalculating the required microfarads to maintain the same phase-shift current.

Where You Meet This in Practice and Common Confusions

Reactance is not just a textbook concept; it dictates how we design, filter, and protect modern electrical systems. Here is where you will actively deal with the opposition to AC due to capacitance and inductance.

1. Power Factor Correction (PFC) Banks
Industrial facilities are heavily inductive due to hundreds of AC induction motors. This inductive reactance causes massive current lag. To fix this, electricians install capacitor banks. The capacitive reactance perfectly cancels the inductive reactance ($X_L = X_C$), bringing the phase angle back to zero. This makes the utility grid see a purely resistive load, eliminating VAR penalties and reducing feeder wire heating.

2. Audio Crossover Networks
If you build custom speakers, you use reactance as a frequency filter. A series inductor (low-pass filter) has low reactance at bass frequencies but high reactance at treble frequencies, blocking highs from reaching the woofer. A series capacitor (high-pass filter) blocks bass but passes treble to the tweeter. The crossover frequency is precisely the point where the component's reactance equals the speaker's nominal impedance (usually 8Ω).

3. Variable Frequency Drive (VFD) Output ChokesVFDs output high-frequency PWM (Pulse Width Modulation) waveforms to control motor speed. Because motor windings are inductive, their reactance ($X_L = 2\pi f L$) spikes at the high PWM switching frequencies, causing severe voltage reflections and 'dV/dt' spikes that can destroy motor insulation. Installing an output dV/dt choke (a specifically tuned inductor) smooths this opposition and protects the motor.

What People Commonly Confuse Reactance With

The most frequent mistake DIYers and junior technicians make is confusing Reactance ($X$) with Impedance ($Z$).

Reactance is strictly the opposition caused by energy storage (inductors and capacitors). Impedance is the total, combined opposition of both resistance and reactance in a real-world circuit. Because resistance and reactance are 90 degrees out of phase, you cannot simply add them together ($R + X$). You must use vector addition:

Impedance Formula: $Z = \sqrt{R^2 + X^2}$

For example, a real-world copper inductor has both inductive reactance ($X_L$) and the DC resistance of the copper wire ($R$). If you are sizing a breaker for a motor circuit, you must calculate the total impedance ($Z$) to determine the true current draw, not just the reactance. Furthermore, as noted in Electronics Tutorials' guide on AC reactance, ignoring the Equivalent Series Resistance (ESR) of a capacitor in high-frequency switching power supplies will lead to catastrophic thermal failure, as the ESR dissipates real heat while the capacitive reactance does not.

Understanding the opposition to AC due to capacitance and inductance is the dividing line between someone who can only wire a DC battery circuit and someone who can design, troubleshoot, and optimize complex AC power systems and high-frequency electronics.