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Power factor correction (PFC) reduces reactive power demand on the electrical system, lowering utility demand charges (which are often assessed on kVA, not just kW) and reducing conductor current for the same real power load. This directly reduces I²R losses in conductors and transformers and frees up system capacity.
The capacitor sizing calculation is straightforward: the required kVAR of capacitance equals the real power (kW) multiplied by the difference between the tangent of the initial power factor angle and the tangent of the target power factor angle. Algebraically: kVAR = kW × (tan(arccos(PF_initial)) − tan(arccos(PF_target))). For example, to correct a 500 kW load from 0.75 to 0.95 PF: kVAR = 500 × (tan(41.4°) − tan(18.2°)) = 500 × (0.882 − 0.329) = 276 kVAR.
Capacitors should generally be located as close to the lagging load as possible to maximize conductor current reduction. Centrally-applied capacitors at the main switchboard correct the utility billing PF but do not reduce branch circuit conductor current. Motor-applied capacitors correct at the source of reactive demand and reduce feeder current.
NEC 460.9 requires that motor capacitors used for PFC be sized so the no-load motor voltage rise does not exceed 10% of nameplate voltage. This limits motor-applied capacitor size to approximately 90% of the motor's no-load reactive demand — exceeding this limit can cause overvoltage and self-excitation when the motor is de-energized.
Utilities typically impose PF penalties when facility PF falls below 0.85 to 0.90. Calculate the exact penalty structure from your utility tariff before sizing PFC banks. Harmonic loads (VFDs, UPS, arc furnaces) require detuning reactors to prevent capacitor resonance with transformer inductance.
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kVAR = kW × (tan(arccos(initial PF)) − tan(arccos(target PF))). For a 500 kW load at 0.75 PF corrected to 0.95 PF: kVAR = 500 × (tan(41.4°) − tan(18.2°)) ≈ 276 kVAR of capacitance.
NEC 460.9 limits motor-applied PFC capacitors so the resulting no-load terminal voltage rise does not exceed 10% of nameplate voltage. This limits capacitor size to approximately 90% of the motor's no-load reactive demand to prevent self-excitation.
Yes. VFDs, UPS systems, arc furnaces, and other harmonic-generating loads can cause capacitor resonance with the system inductance, leading to harmonic amplification and potential capacitor failure. Detuning reactors (typically 5-7% impedance) are required for PFC banks in facilities with significant harmonic loads.
For maximum benefit, install capacitors as close to the lagging load as possible. Motor-applied capacitors reduce feeder and conductor current. A central capacitor bank at the main switchboard corrects utility billing PF but does not reduce branch circuit conductor loading.
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