Calculator Inputs
Enter measured real power and power factors. Add voltage and frequency to estimate current and capacitance.
Example Data Table
This example illustrates a typical correction scenario for a site feeder.
| Active Power (kW) | Existing PF | Target PF | Voltage (V) | Phases | Result (Approx kVAr) |
|---|---|---|---|---|---|
| 120 | 0.78 | 0.95 | 400 | 3 | ~75 |
| 55 | 0.70 | 0.92 | 230 | 1 | ~41 |
| 200 | 0.82 | 0.98 | 415 | 3 | ~72 |
Example values are approximate and depend on rounding and margin.
Formula Used
Power factor correction is based on the power triangle. Real power stays constant while reactive power is reduced by capacitors.
- S = P / PF (apparent power, kVA)
- φ = arccos(PF) (power angle)
- Q = P × tan(φ) (reactive power, kVAr)
- Qc = P × (tan(φ1) − tan(φ2)) (required correction, kVAr)
- Single-phase: Q = V² × ω × C
- Three-phase delta: Q = 3 × V_L² × ω × C
- Three-phase star: Q = V_L² × ω × C
- Where ω = 2πf and C is in Farads
Always confirm final capacitor selection with manufacturer data and site standards.
How to Use This Calculator
- Measure active power and the existing power factor at steady load.
- Choose a realistic target power factor based on utility requirements.
- Enter system voltage, frequency, and phase configuration.
- Select capacitor connection used in your panels or banks.
- Add a design margin for variation, then press Calculate.
- Review kVAr, kVA reduction, and current reduction above the form.
- Download CSV or PDF for approvals, procurement, and records.
Practical Considerations
- Use staged capacitor steps to match varying site loads.
- Consider detuned reactors when harmonics are present.
- Check switching duty, ventilation, and temperature ratings.
- Coordinate protective devices and verify capacitor discharge time.
Professional Article
Power factor (PF) improvement is a practical way to reduce electrical losses and free up capacity on construction sites where temporary and permanent loads run together. Induction motors, welders, hoists, compressors, and HVAC equipment draw reactive power, which increases current without producing useful work. Utilities often apply penalties when PF stays low, and low PF can also cause nuisance trips, hot cables, and voltage drop at long feeder runs.
In the field, the goal is not to “hit 1.00,” but to reach a target that balances savings, stability, and equipment limits. The calculator estimates the reactive power correction (kVAR) needed to raise PF from an existing value to a desired value at a given real power level. It also estimates the line current before and after correction so you can see how cable loading and transformer utilization improve.
The underlying relationship uses apparent power S = P/PF, reactive power Q = √(S² − P²), and required capacitor kVAR as Qc = Q1 − Q2. When PF rises, S falls, which reduces current: I ≈ (P×1000)/(√3×V×PF) for three‑phase systems. Lower current means lower I²R heating, better voltage regulation, and headroom for additional tools or future expansion.
Capacitor banks are usually installed at the main distribution board or near large motor groups. For variable site loading, staged steps (for example 5, 10, 15 kVAR) controlled by an automatic regulator track PF without over‑compensating. Over‑correction can push PF leading, creating voltage rise and resonance risk, so switching strategy and minimum step size matter.
Example: a site consumes 120 kW at PF 0.78 on a 400 V, three‑phase feeder. Raising PF to 0.95 typically requires about 75 kVAR of capacitance, and the feeder current drops noticeably. That reduction can allow smaller voltage drop, cooler terminations, and improved generator performance if the site is operating on temporary power.
Commissioning should include verifying voltage, step switching, and PF readings under several load levels, then logging results for a week to confirm stability. If a utility meter reports kvarh and kWh, compare bills before and after correction to quantify savings. On generator-backed sites, monitor kVA and frequency response; improved PF often reduces kVA demand and improves transient performance.
Before procurement, verify harmonics from VFDs and non‑linear loads; detuned reactors or harmonic filters may be required. Always coordinate protection, confirm discharge resistors, and follow local electrical codes and manufacturer data. With sound sizing and commissioning, PF improvement becomes a low‑maintenance upgrade that supports reliable, efficient site operations.
FAQs
1) What does this calculator output?
It estimates required capacitor kVAR to raise power factor from the existing value to the target, plus the line current before and after correction for one- or three-phase systems.
2) What inputs should I use for real power?
Use measured kW from a reliable meter under typical operating load. If you only have kVA and PF, convert using kW = kVA × PF, then enter the resulting kW.
3) Where should capacitor banks be installed on a site?
Common locations are the main distribution board, a motor control center, or near a large motor group. Placing correction close to the load reduces upstream current and voltage drop.
4) Do harmonics affect capacitor sizing?
Yes. Drives, welders, and other non-linear loads can amplify harmonic currents and stress capacitors. In those cases, use detuned reactors or harmonic filters and follow manufacturer recommendations.
5) Can improving PF cause problems?
Over-correction can make PF leading, raising voltage and increasing resonance risk. Use staged steps, proper control settings, and avoid adding excessive kVAR compared with the typical reactive demand.
6) How do I confirm savings after installation?
Log kW, kVAR, and PF before and after commissioning, then compare utility bills for penalty changes. Also check feeder temperatures and voltage drop to verify the electrical benefits.
7) Does this replace an electrical design review?
No. It provides a planning estimate. Final selection should consider switching duty, ambient temperature, protection coordination, discharge time, and compliance with local codes and manufacturer data.
Accurate corrections cut losses, improve capacity, and reliability overall.