What is power factor and why does it need correction?▼
Power factor (PF, or cos φ) is the ratio of real power (kW, the useful work you pay for) to apparent power (kVA, what the grid sees). A low PF means your equipment draws reactive power (kVAR) back and forth that never does useful work, raising supply current, overheating cables and transformers, and often triggering utility penalties. Correction adds capacitors that supply reactive power locally instead of importing it from the grid.
How do I calculate capacitor bank size for power factor correction?▼
Measure your total real power P in kW, measure or estimate your current power factor PF₁, pick a target PF₂ (0.95 is standard, 0.98 is aggressive), then apply: kvar needed = P × (tanφ₁ − tanφ₂), where tanφ = √(1/PF² − 1). Example: 100 kW at PF 0.7 → PF 0.95: tan0.7 ≈ 1.02, tan0.95 ≈ 0.329, difference = 0.691, so kvar = 69.1. Enter the same four numbers in the calculator above — it returns kvar, µF per phase, rated current and an inrush estimate.
How do I convert kvar to microfarads (µF)?▼
First convert kvar to var (× 1000). The reactive power Q = ω × C × V² relates var, frequency ω = 2πf, capacitance C in farads and voltage V. Rearranged for three-phase Y connection: C_Y = Q / (ω × V²) in farads, × 10⁶ for µF. For Δ connection (each phase sees line voltage): C_Δ_per_phase = Q / (3 × ω × V²). Single-phase simplifies to C = Q / (ω × V²). The calculator above handles all three configurations.
Why do Y and Δ connections give different µF values?▼
In Y (star) connection each capacitor sees the phase voltage V/√3, so to produce the same total kvar it needs more microfarads (roughly 3× the per-phase µF of a Δ bank for the same line voltage). In Δ (delta) each capacitor sees the full line voltage. Y banks are common for medium-voltage systems with 230/400 V supplies because 400 V capacitors can be used on the phase voltage of ~230 V. Δ banks are common for 400/690 V supplies where each phase sits at full 400 V.
What is capacitor bank inrush current?▼
When the first step of capacitors switches in, the bank is uncharged and acts almost like a short — the inrush current is limited only by the supply impedance, the transformer and the capacitor's own ESL. Typical inrush is 5–20× rated current, which can damage switchgear contacts and create transients that trip protective relays. Industrial banks use soft-start circuits (series resistor that shortouts after charge) or detuned reactors (L-C series) to limit inrush and suppress harmonics.
What is a detuned reactor and do I need one?▼
A detuned reactor adds an inductor in series with each step capacitor, forming a tuned L-C pair. Typical detuning frequencies are 135 Hz (for 50 Hz systems, detuned below the 3rd harmonic at 150 Hz) and 180 Hz (for 60 Hz systems). The reactor prevents anti-resonance with upstream capacitor banks and limits inrush. If your site already has a power factor correction bank from the utility or a neighbouring facility, anti-resonance is a real risk that must be simulated — do not copy a bank design blindly.
How many steps should my capacitor bank have?▼
A stepped capacitor bank (APFC — Automatic Power Factor Correction) splits the total kvar into 4–12 binary-weighted steps (e.g. 12.5, 25, 50, 100 kvar) so the controller can approximate any required kvar. The finer the steps, the less PF oscillates around the target. Rule of thumb: the largest step should be ≤ 25% of total kvar, giving at least 4 steps. Low-power sites (≤ 50 kvar) often use a single fixed step.
Why won't the calculator let me enter PF₂ lower than PF₁?▼
Because capacitors can only add reactive power — they raise PF toward unity, never lower it. If PF₂ < PF₁ you are asking the capacitors to undo something, which they cannot. If you actually need PF₂ < PF₁ your real problem is over-correction from a previous bank or site capacitors, and you should disconnect some of them rather than add more.
