Power Systems FundamentalsPreliminary engineering
Power Factor Correction Capacitor Sizing
Required capacitor kVAR and per-phase capacitance to raise power factor to a target value.
About this calculator
Correcting power factor means adding capacitive kVAR to cancel part of an inductive load's reactive power demand, reducing the current the source must supply for the same real power. The required capacitor size is the difference between the reactive power at the existing power factor and at the target power factor: Q_c = P(tanθ₁ − tanθ₂), where θ = cos⁻¹(pf). This calculator computes that required kVAR, the resulting drop in line current, and the equivalent per-phase capacitance for a wye- or delta-connected bank at the system voltage and frequency.
Use it to size a capacitor bank for a facility with a utility power-factor penalty, or to check a vendor-quoted bank size against a hand calculation. The current reduction figure is useful for confirming whether correction will meaningfully relieve loading on an upstream transformer or feeder.
Common pitfall: over-correcting past unity power factor (target pf too close to 1.0, or a bank sized for light-load conditions) can cause leading power factor and overvoltage at light load, since the capacitor's kVAR stays fixed while the load's kVAR demand drops. Utilities often want a target in the 0.95–0.98 range rather than exactly 1.00, and any bank should be checked against the load's minimum expected demand, not just its peak.
Assumptions
- Balanced three-phase load and capacitor bank.
- Bank is sized for the stated real power and existing power factor at that operating point — verify against the load's minimum demand, not only its peak.
- Capacitor bank connection (wye or delta) only affects the per-phase capacitance value, not the required kVAR.
When to use this calculator
Appropriate for
- Estimating the capacitor kVAR needed to raise a balanced load's power factor to a target value
- Gauging the demand-charge or released-capacity benefit of correction at a steady operating point
- Early sizing before selecting a specific capacitor bank or detuned/harmonic-filter design
Not suitable for
- Systems with significant harmonics, where plain capacitors can resonate with the source and a detuned reactor or active filter is required
- Loads whose reactive demand varies widely, which need automatic staged banks and a controls study rather than a single kVAR figure
- Final bank specification, protection, and switching-transient evaluation for a real installation
What this calculator does not cover
- Corrects displacement power factor only — capacitors do not correct harmonic distortion, and with nonlinear loads they can resonate with source inductance; a harmonic study may be required.
- No capacitor switching transient, inrush, detuning reactor, or overvoltage evaluation.
- Sizes fixed compensation at a single operating point — plants with varying load typically need staged or automatically switched banks.
- As with every calculator on this site: results are preliminary and educational, are not verified for any specific installation, and must be reviewed against the applicable code edition and stamped by a licensed Professional Engineer before real-world use.
Frequently asked questions
How is required capacitor kVAR calculated?
Q_c = P × (tanθ₁ − tanθ₂), where θ₁ and θ₂ are the power factor angles before and after correction (θ = cos⁻¹(pf)). This is the reduction in reactive power needed to move from the existing to the target power factor.
Can a capacitor bank be too large?
Yes. If the bank's fixed kVAR exceeds the load's reactive demand at light load, the power factor becomes leading and the bus voltage can rise. Size correction banks against minimum expected load, or use switched/automatic banks that track demand.
Does the wye/delta connection change how much kVAR I need?
No — the required kVAR depends only on the real power and the power factor change. The connection only changes the per-phase capacitance value needed to deliver that kVAR, since a delta-connected capacitor sees full line voltage while a wye-connected one sees phase voltage.
References
- IEEE Std 1036, IEEE Guide for the Application of Shunt Power Capacitors
- IEEE Std 141 (Red Book), Ch. 8 — Power Factor Capacitors
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