Power Factor Correction: engineer's guide
Correcting power factor reduces demand kVA, cuts utility penalty, and improves voltage regulation. The required kVAR is a function of active power and the two power factor angles.
Open the Power Factor Correction →
What this calculator does
The calculator returns the kVAR of capacitor bank needed to move from the existing power factor to a target power factor at a specific active power load.
Formulas used
- Required kVAR:
Qc = P × (tanφ1 - tanφ2) - kVA reduction:
S2 = P / cosφ2
Variables
- P = active power, kW
- cosφ1 = existing power factor
- cosφ2 = target power factor
Worked example
Scenario: 400 kW plant at cosφ 0.75 lagging, target 0.95 lagging.
- tanφ1 = tan(acos 0.75) ≈ 0.882
- tanφ2 = tan(acos 0.95) ≈ 0.329
- Qc = 400 × (0.882 - 0.329) ≈ 221 kVAR
- Choose an automatic bank in six 40 kVAR steps totalling 240 kVAR, with detuned reactors if THD-i exceeds 15%.
Common mistakes to avoid
- Overcorrecting into leading power factor at light load
- Ignoring resonance risk when capacitors interact with transformer inductance and harmonic loads
- Fixed capacitor banks on variable loads — use automatic step control
- Placing capacitors upstream of protective devices sized for the un-corrected current
Frequently asked questions
Detuned or standard capacitor bank?
Detuned when harmonic distortion at the connection point exceeds around 5% THD-v. Standard banks amplify harmonics and fail early.
Target power factor?
Utility contract minimum plus 2% margin. 0.95 to 0.98 is typical for commercial buildings.
Fixed or automatic?
Fixed for a stable base load, automatic for a fluctuating load with variable reactive demand.
Standards and references
IEC 61921, IEEE 18, IEEE 1036
Related tools on this site
Open the calculator that pairs with this guide: Power Factor Correction.
Verification note
This guide is planning support only. Final design must be verified against the applicable project specification, authority requirements, manufacturer data, and the standards cited above. Consult a licensed engineer for construction-issue calculations.