Power Factor Correction Calculator
Enter the load, voltage and existing and target power factor to size the capacitor bank.
Step-by-step
- Enter valid values to begin.
Delta-connected capacitors assumed for the microfarad value; a wye bank needs three times the capacitance per phase for the same kVAR at the same line voltage. Resonance order is an estimate for the transformer only (utility source neglected). Verify with a harmonic study if drives or rectifiers make up more than about 20% of the load.
Power Factor Correction: Sizing Capacitors Without Creating a Resonance Problem
Core Engineering Principles
Induction motors and transformers draw two kinds of current. One does work. The other builds the magnetic field and goes back to the source every half cycle. That second one is reactive current, and it makes your meter read lower power factor than you want. A capacitor does the opposite of an inductor: it supplies reactive current locally, so the motor swaps it with the capacitor and the cable and transformer never see it. The real kW is unchanged. What drops is the kVA, and with it the current in everything upstream.
To size the capacitor, work with the power triangle. At the existing power factor the angle θ1 gives a reactive power of P × tan(θ1). At the target it’s P × tan(θ2). The difference is the kVAR you have to add. To get microfarads per phase you divide by 3 × 2πf × V² for a delta-connected bank. People forget one thing, though: a capacitor and the transformer inductance form a resonant circuit. If that resonance lands near a harmonic your drives produce, say the 5th or 7th, the bank amplifies it, and the capacitor fuses start blowing.
Resonance order h = √(kVAsc / kVARcap)
NEC & Standard References
NEC Article 460 covers capacitors: 460.8 sets conductor ampacity at 135% of the rated capacitor current, 460.8(B) covers overcurrent protection, and 460.28 requires a means to discharge stored energy within one minute (75 V or less for 600 V and below). IEEE 519 limits harmonic distortion at the point of common coupling and is the standard you cite when sizing detuned reactors. IEC 60831 covers low-voltage power capacitors. When harmonic loads are more than about 20% of the bus, specify a detuned bank, usually 5.67% or 7% reactor in series, so the resonance sits below the 5th harmonic.1. tan(cos−1 0.78) = 0.802; tan(cos−1 0.95) = 0.329.
2. Qc = 200 × (0.802 − 0.329) = 94.7 kVAR. Next standard bank: 100 kVAR.
3. C = 94,700 / (3 × 377 × 480²) = 363 µF per phase (delta).
4. Fault level = 500 / 0.0575 = 8,696 kVA, so h = √(8,696 / 94.7) = 9.6, comfortably away from the 5th and 7th.
- Never overcorrect. Leading PF can raise bus voltage and cause generator instability. Stay at 0.95 to 0.98.
- Switch banks in steps. A fixed bank on a lightly loaded plant overcorrects at night.
- Wait for discharge. A capacitor holds charge after you open the disconnect. Verify zero volts before touching.
- Check resonance if you have drives. Capacitor failures and nuisance trips on a plant with VFDs are almost always harmonic resonance, not bad capacitors.
- Don’t put a bank on the load side of a VFD. Correct on the line side only.