Power Factor Correction — Sizing a Capacitor Bank

Utilities penalise low power factor. A capacitor bank pays for itself quickly — but only if it is sized and stepped correctly.

The correction formula

To move from an existing power factor cos φ₁ to a target cos φ₂ at constant real power P:

Worked example

Plant load 300 kW at 0.75 PF (φ₁ = 41.4°, tan = 0.882), target 0.95 PF (φ₂ = 18.2°, tan = 0.329):

Select a standard bank: 3 × 50 kVAR + 1 × 25 kVAR = 175 kVAR, or an automatic bank of 25 kVAR steps.

Fixed vs automatic

• Fixed banks suit steady loads (transformers, constant motors). Size to no more than 90–95% of the transformer’s no-load kVAR to avoid over-correction at light load.
• Automatic banks with a PF controller switch steps in and out as load varies — needed where load swings widely.

Two things that go wrong

• Over-correction: leading power factor at night can raise voltage and still draw a utility penalty. Keep the target at 0.95, not unity.
• Harmonic resonance: capacitors and transformer inductance form a resonant circuit. On sites with variable-speed drives or large rectifier load, specify detuned reactors (typically 7% or 14%) in series with the capacitors.

What goes in the BOQ

Capacitor modules, contactors or thyristor switches, detuned reactors if required, the PF controller, an enclosure with ventilation, and protective fuses or breakers per step.