How to size a power factor correction capacitor bank: kVAr calculation, motor vs shunt capacitors, standard vs detuned, and what to specify on your RFQ.
A power factor correction (PFC) capacitor is the cheapest reactive-power device in a low-voltage panel. Instead of drawing the full apparent power from the supply, the capacitor supplies the magnetizing vars that motors, transformers and ballasts need locally — so the utility sees a smaller current for the same real work. Size it right and the board runs cooler, the cables are smaller and the power-factor penalty on the bill disappears. Size it wrong and you over-compensate into a leading power factor, excite resonances, or fit a capacitor that cannot survive the harmonics on the bus. This guide covers what a buyer or panel builder should actually specify.
Inductive loads — motors, welders, fluorescent and LED drivers, transformers — draw a current that lags the voltage. The ratio of real power (kW) to apparent power (kVA) is the power factor, and the gap between them is reactive power measured in kilovar (kVAr). A capacitor bank injects leading vars that cancel part of that lag, pulling the power factor toward 1.0. For a site billed on kVA or penalized below a contract pf (often 0.9), a capacitor is usually the fastest payback upgrade available.
The target is a corrected pf, typically 0.95, not 1.0 — fully cancelling the lag risks a leading pf and over-voltage. Estimate the required kVAr from the load kW and the starting and target pf, or use the rule of thumb that a 0.7 to 0.95 correction needs roughly 0.55 kVAr per kW of load. For a single motor the capacitor is sized to its magnetizing requirement, not its full rating, so it does not over-excite the machine when it coasts down.
| Motor / load (kW) | kVAr at 0.7 to 0.95 | Typical unit |
|---|---|---|
| 1.5 | 0.8 to 1.0 | single motor-run cap |
| 7.5 | 4 to 5 | shunt capacitor step |
| 22 | 11 to 13 | automatic PF controller step |
| 75 | 38 to 45 | multi-step capacitor bank |
| 160 | 80 to 95 | detuned bank, harmonic site |
These are planning figures; confirm against the actual metered pf and the utility contract, because a fixed bank sized to peak load will over-correct at light load.
Do not confuse the two. A motor-run (CBB / AC motor) capacitor stays permanently connected across one winding to boost that motor starting torque and running pf — it is part of the machine. A shunt (BSMJ self-healing) capacitor is a panel-mounted bank switched by a contactor or PF controller to correct the whole installation. Putting a motor-run cap on the bus does nothing useful; putting a shunt bank across a single motor can over-excite it on decel. Match the type to the job.
If you cannot measure the harmonics, assume the worst and specify detuned — a standard bank on a harmonic-heavy bus is the most common cause of capacitor failure we see.
Rate the capacitor above the system voltage, not at it: a 400 V three-phase bank should use 415/440 V rated units so switching transients do not age them. Confirm 50/60 Hz, the connection (delta for three-phase correction, single for a motor), and the enclosure — IP20 in a clean panel, higher IP where it is exposed. Also check the discharge resistor: a PF capacitor must self-discharge to a safe voltage within a minute of de-energizing, by code.
ELEHPD stocks BSMJ self-healing shunt capacitors, CBB motor-run capacitors and matching contactors and PF controllers for fixed and automatic correction stages, supplied to IEC practice and ready to assemble into a low-voltage panel.
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.