How to size and specify a capacitor bank for power factor correction: kVAr calculation, step sizing, C/K settings, switching contactors and detuning.
Power factor (PF) is the ratio of working power (kW) to apparent power (kVA) drawn from the supply. A motor running at half load might draw 100 kVA to deliver 60 kW — a PF of 0.60. The utility still has to size transformers, cables and switchgear for the full 100 kVA, which is why so many industrial tariffs bill reactive energy (kVArh) or penalise PF below a threshold, typically 0.90 or 0.95.
For panel builders and distributors the argument is simpler: correcting PF at the switchboard frees transformer capacity, cuts cable losses and removes penalty line items from the customer's bill. A bank that lifts PF from 0.72 to 0.97 can free 20–30% of the apparent power the installation was consuming — capacity that is often cheaper than new switchgear.
Fixed banks suit steady, predictable loads — a single large compressor, or a production line that runs at constant output. They are the cheapest per kVAr and the easiest to commission, but they cannot follow load changes: at light load the same bank pushes PF leading and raises the voltage across the capacitors.
Automatic banks switch capacitor steps in and out as the load varies. Almost every mixed installation — machine shops, plastics processing, buildings with HVAC — needs an automatic bank. The extra cost is the power factor correction controller plus capacitor switching contactors, and that is where most specification mistakes happen.
The reactive power the bank must supply is:
Qc = P × (tan φ1 − tan φ2)
where P is the measured active power demand in kW, φ1 the present power factor angle and φ2 the target angle. Worked example: a workshop draws 250 kW at PF 0.72 (tan = 0.964) and the target is 0.97 (tan = 0.251). The bank must supply 250 × (0.964 − 0.251) ≈ 178 kVAr.
Two practical rules. Never compensate to unity — leave a small inductive margin (PF 0.95–0.98) or the controller will hunt. And size on the measured demand, not the connected load: a week of logging with a clamp meter or the site's power analyzer beats any nameplate sum.
An automatic bank divides its kVAr into steps — typically 5, 12.5, 25 or 50 kVAr each. Steps should get finer as they get more numerous: a coarse first step hunts, a fine last step polishes. A common layout for a 300 kVAr bank is 4 × 75 kVAr coarse plus 2 × 25 kVAr and 1 × 50 kVAr fine.
The C/K ratio — first-step kVAr against the current transformer primary current — sets controller sensitivity:
C/K = Qstep ÷ (Un in kV × ICT in A)
A 50 kVAr first step on a 400 V system with a 1000/5 A CT gives C/K = 50 ÷ (0.4 × 1000) = 0.125. Set it too low and the controller over-switches; too high and it never quite catches up.
The controller measures PF from the CT and voltage inputs, compares it with the target cos φ and connects or disconnects steps to close the gap. The settings that matter:
Switching a capacitor step is not switching a motor. At the instant of contact closure the capacitor is discharged and the supply is near peak — inrush current can reach roughly 100× rated. Standard contactor main contacts weld under that stress.
Capacitor duty contactors solve it with early-make auxiliary contacts in series with damping resistors: the auxiliary contacts close first and pre-charge the step, then the main contacts close with almost no inrush. On opening, the main contacts break first so the resistors take the arc. The same contactor is not suitable for motor switching, and a standard contactor is never an acceptable substitute — one welded set of contacts takes the whole bank offline.
Variable frequency drives, LED drivers, UPS and rectifiers inject harmonic currents. Capacitors are low-impedance at harmonic frequencies, so an undetuned bank amplifies those currents, overheats and fails early. The symptoms — blowing fuses, failing contactors, bulging capacitors — are routinely misdiagnosed as “bad capacitors”.
The standard fix is a detuned reactor bank: a series reactor (typically 7% at line frequency) tunes the bank-capacitor circuit below the dominant harmonic, so the bank looks inductive where it matters and stops amplifying. In plants dominated by VFDs and rectifiers, specify detuned banks as the default, not as a costed option.
Measure the active power demand P (kW) and the present power factor. The bank must supply Qc = P × (tan φ1 − tan φ2), where φ1 is the present angle and φ2 the target angle (usually 0.95–0.97). Example: 250 kW at PF 0.72 to PF 0.97 needs about 178 kVAr.
No. Closing onto a discharged capacitor draws an inrush current up to roughly 100× rated, which welds standard main contacts. Use capacitor duty contactors with early-make auxiliary contacts and damping resistors that pre-charge each step before the main contacts close.
C/K = first-step kVAr ÷ (system voltage in kV × CT primary current). A 50 kVAr first step on a 400 V (0.4 kV) system with a 1000/5 A CT gives C/K = 50 ÷ (0.4 × 1000) = 0.125. Too low and the controller hunts; too high and it responds sluggishly.
Harmonics from drives and rectifiers. Capacitors are low-impedance at harmonic frequencies, so they amplify harmonic current, overheat and bulge. Specify a detuned reactor bank (typically 7% series reactor) as the default in harmonic-rich installations, not as an option.
Automatic, unless one load dominates. A fixed bank sized for peak demand over-compensates at light load, driving PF leading and raising capacitor voltage. An automatic bank with a coarse-plus-fine step mix follows the load and keeps PF in the 0.95–0.98 band.
Send the ratings, dimensions and quantities you are working to and we will confirm the closest match from the ELEHPD catalogue, the compliance documents available for it and the lead time. OEM branding, private model numbering and buyer-specific cartons are available on qualified volumes.