Practical guide to power capacitor selection: shunt, motor-run and metallized types, voltage/kvar ratings, detuning reactors, and safe panel installation.
A power capacitor stores and releases reactive energy to counteract the inductive load created by motors, transformers and ballasts. In a low-voltage panel this is the cheapest way to raise the power factor, cut line losses and free up capacity on the supply transformer. Specified correctly, a capacitor bank can pay for itself in months through lower demand charges.
Two numbers define the job: the voltage rating (it must equal or exceed the system voltage) and the reactive power in kvar (the amount of correction delivered). Everything else—dielectric type, terminal layout, protection—follows from those two.
In a typical low-voltage switchboard the capacitor bank lives beside the main incomer, fed through a contactor and protected by a fuse. Because it is permanently connected to the bus, its rating must survive the full supply voltage and the switching transients of the panel—not just the nominal load.
| Type | Typical use | Voltage range | Notes |
|---|---|---|---|
| Self-healing shunt (BSMJ) | Power-factor correction banks | 230–690 V | Metallized film, compact, automatic cut-out on fault |
| Motor-run (CBB60/61/65, CL11) | Single-phase motor start/run | 250–450 V AC | Long life, low dissipation; size by motor kW |
| Metallized pulse (MSMJ) | High-pulse discharge circuits | Up to 1000 V | Very low ESR, for ignition and trigger use |
| Capacitor + PFC controller | Stepped automatic correction | 400/415 V | Contactor-switched stages; detune in dirty networks |
For most panel builders the shunt capacitor is the workhorse; motor-run types are selected per motor, not per bank.
The correction needed is the difference between the present and the target reactive power:
Q (kvar) = P (kW) × (tan φ₁ − tan φ₂)
Example: a 75 kW load at cos φ₁ = 0.75 (φ₁ ≈ 41.4°) raised to cos φ₂ = 0.95 (φ₂ ≈ 18.2°) needs Q = 75 × (0.882 − 0.325) ≈ 41.8 kvar. Round to standard stages (for example 12 + 12 + 18 kvar) and let the controller step them.
In networks with variable-speed drives, LED ballasts or welding plant, harmonics push capacitors into resonance and premature failure. A detuning reactor in series shifts the bank's natural frequency below the dominant harmonic (typically tuned to 189 Hz on 50 Hz or 210 Hz on 60 Hz grids), so the capacitor no longer amplifies the 5th and 7th harmonics. Use a detuned bank wherever the total harmonic distortion of current (THDi) exceeds about 15%. Detuned banks are slightly larger and carry a small extra loss, but they protect the capacitors and the rest of the installation from harmonic heating, so on mixed industrial sites they are the default choice.
A capacitor draws a large inrush current the instant it is energised—often 50 to 100 times its rated current for a few milliseconds. A standard AC contactor will weld its contacts within weeks if used directly. Capacitor-duty contactors solve this with pre-charge resistors or damping modules that limit the first half-cycle. For automatic banks, fit one contactor per stage and let the PFC controller decide how many stages to engage from the live power factor. Never switch a large bank with a plain motor contactor.
Reputable low-voltage capacitors are built to IEC 60831 (or UL 810 in North America). Ask the supplier for the type-test report, not just the nameplate. A genuine self-healing film unit states its end-of-life mode, temperature class and expected lifetime at rated conditions—typically 100,000 hours. Cheap units that omit this data usually cut corners on the dielectric and fail early in hot panels.
Choosing the right power capacitor is mostly discipline: size by kvar, match the voltage and frequency, and detune when the network is dirty. Get those three right and the bank will run for its full rated lifetime.
A shunt (self-healing) capacitor is built for fixed power-factor correction across a whole panel or feeder, sized in kvar. A motor-run capacitor is selected per motor to create a phase shift for starting or running a single-phase machine, sized in µF and volts. If you are correcting a whole installation, use shunt stages; if you are fixing one motor, use a motor-run type.
The usual causes are heat, harmonics and inrush. Poor ventilation raises the core temperature and ages the film; a dirty network without a detuning reactor drives resonance; and switching without capacitor-rated contactors welds the contacts. Check spacing, add a detuning reactor when THDi > 15%, and use pre-charge contactors.
Only in harmonic-rich networks—those with lots of VSDs, LED drivers or welders. A detuning reactor shifts the bank's natural frequency below the 5th harmonic so it cannot resonate. On a clean supply with THDi under about 15% a standard tuned bank is fine and cheaper.
No. Oversizing over-corrects, pushing the power factor leading and raising the bus voltage, which stresses other equipment and can breach the utility limit. Size to the calculated Q and let a controller step stages to track the load.
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.