An initial APFC capacitor-bank rating can be estimated from the measured active power and the change required between the existing and target power factor. The estimate is only a starting point. Final kVAr, step arrangement, switching method, reactor requirement, protection and ventilation should be confirmed from the load profile, system voltage and harmonic conditions.
The basic capacitor-bank sizing formula
For a three-phase system, the required reactive compensation can be estimated as:
Qc = P x (tan phi1 – tan phi2)
- Qc: required capacitor-bank output in kVAr
- P: measured active power in kW
- phi1: phase angle corresponding to the existing power factor
- phi2: phase angle corresponding to the target power factor
Use measured operating data where possible. The transformer or incomer rating alone does not show how much reactive power the connected load actually requires.
Illustrative calculation
The following example demonstrates the calculation method only; it is not a recommended project rating.
| Input | Illustrative value |
|---|---|
| Measured active power | 500 kW |
| Existing power factor | 0.75 |
| Target power factor | 0.95 |
| tan phi1 | Approximately 0.882 |
| tan phi2 | Approximately 0.329 |
| Estimated Qc | 500 x (0.882 – 0.329) = approximately 277 kVAr |
The calculated result would then be checked against actual minimum and maximum demand, available standard step sizes, the required control resolution and the risk of overcompensation at light load.
Inputs required before selecting the final kVAr
| Input | What to provide | Why it matters |
|---|---|---|
| System data | Voltage, frequency and earthing arrangement | Defines the electrical basis for capacitors, protection and insulation |
| Measured load | kW, kVA, current and power factor at representative operating points | Shows the actual compensation demand |
| Load profile | Minimum, normal and peak demand plus major load changes | Supports total kVAr and step-size selection |
| Target power factor | Utility requirement or project target | Defines the intended correction range |
| Nonlinear loads | VFD, UPS, rectifier and welding-load information | Identifies the need for harmonic assessment |
| Existing correction | Installed capacitor banks, filters and operating condition | Prevents duplicated or conflicting compensation |
| Installation conditions | Ambient temperature, altitude, indoor or outdoor location and ventilation | Affects enclosure and thermal design |
How to choose the number and size of steps
An automatic bank should follow changes in reactive demand without repeatedly overcorrecting or switching too frequently. Smaller first steps provide finer control, while larger later steps can cover major load changes. The correct sequence depends on the minimum operating load, demand variation, controller logic and switching duty; a fixed universal step sequence should not be assumed.
Contactor switching or thyristor switching?
Capacitor-duty contactors are commonly considered where load changes are moderate and normal switching response is acceptable. Thyristor switching may be reviewed for rapidly changing loads where faster, transient-reduced switching is required. The choice should follow the actual process duty, switching frequency and project specification.
When detuned reactors need review
Capacitors interact with the network impedance. Where VFDs, UPS systems or other nonlinear loads are significant, harmonic measurements or a power-quality study should be reviewed before selecting a conventional capacitor bank. A detuned-reactor arrangement must be based on the system conditions and specified tuning basis rather than added as a generic option.
Protection, temperature and enclosure checks
- Confirm incomer and step protection against the proposed capacitor and switching arrangement.
- Allow for capacitor tolerances, operating voltage and current conditions in the engineering calculation.
- Coordinate discharge devices, interlocking and safe access requirements.
- Check heat dissipation, ventilation, ambient temperature and enclosure protection.
- Define alarms, controller signals and communication points required by the project.
The completed assembly scope should also be reviewed against the applicable project specification and routine-verification requirements. See the IEC 61439 procurement checklist and our quality and standards basis.
APFC panel RFQ checklist
- System voltage and frequency
- Measured kW, kVA and existing power factor
- Target power factor and required total kVAr, if already specified
- Minimum, normal and peak load information
- VFD, UPS and other nonlinear-load details
- Available harmonic measurements or power-quality report
- Preferred switching method and controller requirements
- Indoor or outdoor installation, IP rating and ambient conditions
- Component preferences, quantity and destination country
When the APFC panel is part of a wider distribution package, include the SLD and BOQ. The SLD and BOQ preparation guide explains the document set used for a project quotation.
Frequently asked questions
Can an APFC bank be sized from transformer kVA alone?
No. Transformer capacity does not show the actual active load, existing power factor or reactive-power variation. Representative measured load data is the better basis.
Why should the target power factor not simply be set to 1.00?
The target should follow the utility and project requirements while avoiding leading power factor and unstable switching at light load. The practical control range must be checked against the load profile.
How many capacitor steps are required?
The number and size of steps depend on total kVAr, minimum load, demand variation and acceptable control resolution. There is no single sequence suitable for every installation.
Are detuned reactors required for every APFC panel?
No. Their use should follow the harmonic conditions, network data and project specification. Significant nonlinear loads should trigger a technical assessment.
What should be measured before requesting a quotation?
Provide representative kW, kVA, current and power-factor readings across operating conditions, plus any available harmonic data and a list of major nonlinear loads.
Request an engineered APFC proposal
Review the APFC capacitor bank panel product page, then send the measured load data, SLD, project requirements, quantity and destination for engineering review.