Capacitor Sizing Calculator

Example Data Table

Formula Used

How to Use This Calculator

1. Choose system type and, for three-phase, select star or delta.
2. Enter frequency and voltage based on your supply.
3. Provide real power and measured initial power factor.
4. Set a realistic target power factor, commonly 0.95 to 0.99.
5. Add a margin for future growth and a derating if needed.
6. Select steps for staged correction, then calculate results.
7. Use CSV or PDF downloads to attach the sizing summary.

Professional Notes on Capacitor Bank Sizing

1) Why power factor correction matters on sites

Construction facilities often run induction motors, welders, compressors, and temporary distribution boards. These loads draw reactive power that increases line current, warms cables, and reduces usable transformer capacity. Improving power factor typically reduces kVA demand for the same kW, supporting smaller feeders and steadier voltage.

2) Measuring the baseline accurately

Measure real power and power factor during representative operation, not during light-load idle periods. Clamp meters, portable power analyzers, or panel meters should capture average PF, kW, and voltage for at least 15–30 minutes. If loads cycle, record several operating states to avoid overcorrection.

3) Understanding kVAr requirements

The calculator uses the standard relationship between power factor angle and reactive power: required kVAr equals kW multiplied by the difference of tan(acos(PF)). This links your measured PF to the target PF. Typical targets range from 0.95 to 0.99, depending on utility rules and equipment limits.

4) Selecting voltage, frequency, and connection

Use line-to-line voltage for three-phase banks and confirm the site frequency (50 or 60 Hz). Delta banks place each capacitor at line voltage and often deliver higher kVAr per microfarad. Star banks place capacitors at lower phase voltage, which can simplify insulation coordination in some layouts.

5) Margin, derating, and real-world tolerances

A practical margin helps when additional tools or motors are added during later project phases. Derating can represent high ambient temperatures, capacitor tolerance, or conservative engineering practice. If harmonics are present, detuned reactors may be needed; that choice can change effective kVAr and heating.

6) Step control for variable loads

Automatic banks with multiple steps reduce the risk of leading power factor at light load. Smaller steps track changing demand more smoothly, while fewer steps reduce switching complexity. Many panels use a PF controller that adds or removes steps based on measured PF and line current.

7) Installation and protection considerations

Provide fusing or breakers sized for capacitor inrush and continuous current, plus ventilation for heat. Verify discharge resistors for safe touch voltage after shutdown. Keep cable runs short, use solid earthing, and label isolation points for site safety.

8) Commissioning checks before energizing

Confirm voltage rating, step sequencing, and contactor condition, then measure current per step at steady load. Check that PF improves without overvoltage or nuisance trips. Record the final settings and keep the CSV or PDF summary in the project documentation for inspections and maintenance planning.

FAQs

1) What target power factor should I use?

Many sites aim for 0.95 to 0.99. Choose a target that meets utility requirements without pushing equipment into leading power factor when loads are light. Variable loads usually benefit from stepped banks.

2) Why does the required kVAr change with load?

Reactive demand generally scales with motor loading and the mix of inductive equipment. When kW changes, the correction kVAr changes too, so a fixed capacitor can overcorrect at low load and undercorrect at high load.

3) Should I use star or delta connection?

Delta is common for low-voltage three-phase banks and gives more kVAr per microfarad at the same line voltage. Star may be preferred for specific insulation, protection, or switching strategies. Follow your design standard.

4) What margin is reasonable for construction projects?

A 5–15% margin is often used when future tools, pumps, or temporary services may be added. If the load profile is stable and well measured, you can reduce the margin to avoid oversizing.

5) Do harmonics affect capacitor sizing?

Yes. Harmonics can increase capacitor current and heating, sometimes requiring detuned reactors or higher current-rated capacitors. If you have VFDs, UPS systems, or significant non-linear loads, verify harmonic levels before final selection.

6) Can capacitors reduce my cable and transformer losses?

Improving power factor reduces current for the same kW, which can lower I²R losses and voltage drop. The reduction depends on starting PF, target PF, and how long the load operates at that level.

7) What checks should I do after installation?

Measure voltage and current on each step, confirm step sequencing, and ensure PF improves under normal load. Check for overheating, unusual noise, or nuisance tripping. Record readings for maintenance and future resizing decisions.