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Leading PF vs Lagging PF

Leading vs lagging power factor: capacitive vs inductive loads, phase angle relationships, reactive power effects, and power factor correction methods.

Enginist Team
Published: November 10, 2025
Updated: November 13, 2025

Table of Contents

Leading vs Lagging Power Factor: Complete Comparison Guide

Quick AnswerWhat is the difference between leading and lagging power factor?
Lagging PF means current lags voltage (inductive loads like motors)—consumes reactive power. Leading PF means current leads voltage (capacitive loads)—supplies reactive power. Most loads are inductive (lagging). Power factor correction adds capacitors to counteract lagging loads. Unity PF (1.0) is optimal; both leading and lagging reduce efficiency.

Quick Verdict

Power factor direction indicates how current and voltage waveforms relate in time. Understanding leading vs lagging is essential for power factor correction and avoiding utility penalties.

Bottom Line: Lagging PF is far more common (motors, transformers) and typically penalized by utilities. Leading PF occurs with capacitive loads and can cause overvoltage. The goal is unity (1.0) or slightly lagging (0.95-0.98) to maximize efficiency without overcorrection risks.

At-a-Glance Comparison Table

FeatureLeading PFLagging PFNotes
Current vs VoltageCurrent leadsCurrent lagsPhase relationship
Load TypeCapacitiveInductiveCommon examples
Phase Angle (φ)NegativePositiveConvention
Reactive Power (Q)Negative (supplies)Positive (consumes)VAR direction
Common CausesCapacitors, cablesMotors, transformersTypical sources
PrevalenceLess commonVery commonIndustrial reality
Utility ConcernOvervoltagePenalty chargesDifferent issues
CorrectionAdd inductanceAdd capacitanceOpposite methods

Understanding Phase Relationships

Voltage and Current Waveforms

In AC circuits, voltage and current are sinusoidal:

v(t)=Vpeaksin⁡(ωt)v(t) = V_{peak} \sin(\omega t) i(t)=Ipeaksin⁡(ωt−ϕ)i(t) = I_{peak} \sin(\omega t - \phi)

Where φ (phi) is the phase angle between them.

Phase Angle Convention

Phase Angle (φ)MeaningPower Factor
φ = 0°In phaseUnity (1.0)
φ > 0°Current lagsLagging
φ < 0°Current leadsLeading
φ = 90°Pure reactiveZero

Power Factor from Phase Angle

PF=cos⁡(ϕ)PF = \cos(\phi)

Note: PF value is always positive (0 to 1). The leading/lagging designation indicates the direction.

Lagging Power Factor (Inductive)

How Inductors Cause Lag

Inductors resist changes in current:

vL=Ldidtv_L = L \frac{di}{dt}

Current builds up gradually after voltage is applied, causing current to lag voltage by up to 90°.

Common Lagging Loads

Load TypeTypical PFNotes
Induction motors0.80-0.90Most common industrial load
Transformers (no load)0.10-0.20Magnetizing current
Transformers (loaded)0.85-0.95Improves with load
Fluorescent ballasts0.50-0.60Magnetic type
Induction heaters0.70-0.85Industrial heating
Arc welders0.40-0.60Highly inductive

Lagging PF Power Triangle

For lagging power factor:

         kVA (Apparent)
        /|
       / |
      /  | kVAR (Reactive, +)
     /   |
    /φ___|
   kW (Real)
  • Reactive power (Q) is positive
  • Phase angle (φ) is positive
  • Load consumes VARs from system

Leading Power Factor (Capacitive)

How Capacitors Cause Lead

Capacitors resist changes in voltage:

iC=Cdvdti_C = C \frac{dv}{dt}

Current flows immediately when voltage changes, causing current to lead voltage by up to 90°.

Common Leading Loads

Load TypeTypical PFNotes
Capacitor banks0 (pure reactive)Used for PF correction
Synchronous motorsAdjustableOver-excited operation
Long cables (unloaded)0.90-0.95 leadingCapacitance dominates
Electronic power supplies0.95+ leadingSome designs
Underground cablesVariableHigh capacitance

Leading PF Power Triangle

For leading power factor:

   kW (Real)
    \φ___
     \   |
      \  | kVAR (Reactive, -)
       \ |
        \|
         kVA (Apparent)
  • Reactive power (Q) is negative
  • Phase angle (φ) is negative
  • Load supplies VARs to system

Power Triangle Mathematics

Apparent Power

S=P2+Q2S = \sqrt{P^2 + Q^2}

Where:

  • S = Apparent power (kVA)
  • P = Real power (kW)
  • Q = Reactive power (kVAR)

Power Factor Calculation

PF=PS=cos⁡(ϕ)PF = \frac{P}{S} = \cos(\phi)

Reactive Power Sign Convention

ConditionQ SignPF Type
Inductive load+QLagging
Capacitive load-QLeading
Resistive loadQ = 0Unity

Example: 80 kW load with 60 kVAR reactive power (inductive)

S=802+602=6400+3600=100S = \sqrt{80^2 + 60^2} = \sqrt{6400 + 3600} = 100 kVA

PF=80100=0.80PF = \frac{80}{100} = 0.80 lagging (because Q is positive/inductive)

ϕ=cos⁡−1(0.80)=36.87°\phi = \cos^{-1}(0.80) = 36.87° (current lags voltage by 36.87°)

Identifying Leading vs Lagging

Using Power Meters

Meter ReadingMeaning
PF = 0.85 lagInductive, current behind voltage
PF = 0.95 leadCapacitive, current ahead of voltage
PF = 1.00Unity, in phase

Using Oscilloscope

  1. Display voltage and current waveforms together
  2. Compare zero-crossings or peaks
  3. If current crosses zero after voltage: Lagging
  4. If current crosses zero before voltage: Leading

Using Phase Angle

From power analyzer readings:

Phase AnglePower Factor
+30°0.866 lagging
0°1.00 (unity)
-30°0.866 leading
+45°0.707 lagging
-45°0.707 leading

Effects on Electrical Systems

Lagging PF Effects

EffectImpact
Higher currentFor same kW, lower PF = higher I
Utility penaltiesFees for PF < 0.85-0.95
Increased lossesI2RI^2R losses in cables
Voltage dropMore drop per ampere
Reduced capacityTransformers, cables derated

Leading PF Effects

EffectImpact
Voltage riseCapacitive VARs raise voltage
Self-excitationRisk with generators
ResonanceCan amplify harmonics
Utility concernsSome penalize leading PF
Generator stabilityMay affect AVR operation

Power Factor Correction

Correcting Lagging PF

Add capacitors to supply reactive power locally:

Qcapacitor=P×(tan⁡ϕ1−tan⁡ϕ2)Q_{capacitor} = P \times (\tan\phi_1 - \tan\phi_2)

Where:

  • φ₁ = Original phase angle
  • φ₂ = Target phase angle

Example: Correct 150 kW load from PF 0.75 to 0.95 lagging

ϕ1=cos⁡−1(0.75)=41.4°\phi_1 = \cos^{-1}(0.75) = 41.4° ϕ2=cos⁡−1(0.95)=18.2°\phi_2 = \cos^{-1}(0.95) = 18.2°

Qcap=150×(tan⁡41.4°−tan⁡18.2°)Q_{cap} = 150 \times (\tan 41.4° - \tan 18.2°) Qcap=150×(0.882−0.329)Q_{cap} = 150 \times (0.882 - 0.329) Qcap=150×0.553=83Q_{cap} = 150 \times 0.553 = 83 kVAR capacitors needed

Correcting Leading PF

MethodApplication
Remove capacitorsIf over-corrected
Add reactorsAbsorb excess VARs
Synchronous motorsUnder-excite to absorb VARs
Static VAR compensatorDynamic correction

Automatic PFC Systems

FeatureBenefit
Real-time monitoringTracks changing loads
Staged switchingMatches correction to load
Anti-huntingPrevents oscillation
Harmonic filteringDetuned reactors

Utility Billing Implications

Typical Penalty Structure

Power FactorBilling Impact
0.95-1.00No penalty (optimal)
0.90-0.95Minor penalty or warning
0.85-0.90Moderate penalty
< 0.85Significant penalty
LeadingSome utilities penalize

kVA Demand Billing

Many utilities bill on kVA demand:

kVA=kWPFkVA = \frac{kW}{PF}

Example: 200 kW load

PFkVA DemandExtra Capacity Billed
1.00200 kVA0%
0.90222 kVA+11%
0.80250 kVA+25%
0.70286 kVA+43%

Measurement and Monitoring

Power Quality Analyzers

Modern analyzers provide:

  • Real-time PF with lead/lag indication
  • Phase angle display
  • Harmonic content
  • kW, kVAR, kVA readings
  • Logging for trend analysis

Key Parameters to Monitor

ParameterTargetAction if Out of Range
PF0.95-1.00Add/remove correction
Phase angleUnder 18°Same as PF
THDUnder 5%Add harmonic filters
Voltage±5%Check for leading PF

Related Tools

Key Takeaways

  • Lagging PF: Inductive loads, current after voltage, consumes VARs
  • Leading PF: Capacitive loads, current before voltage, supplies VARs
  • Most loads are inductive (lagging)—motors, transformers
  • Correction: Add capacitors for lagging, remove for leading
  • Target PF: 0.95-0.98 lagging (not exactly unity)
  • Utility penalties: Typically for PF < 0.85-0.95

Further Reading

References & Standards

  • IEEE 1459: Power definitions for non-sinusoidal systems
  • IEEE 519: Harmonic limits and PF correction
  • IEC 61000-3-2: Harmonic current emissions
  • NEC Article 460: Capacitor installations

Frequently Asked Questions