Power factor describes how effectively a load turns the current it draws into useful work. A low power factor means the system carries more current than needed, which wastes capacity and can cost money on a utility bill.
The power triangle
- kW (real power) does the work.
- kVAR (reactive power) builds magnetic fields in motors and transformers.
- kVA (apparent power) is the combination: kVA = √(kW² + kVAR²).
Power factor = kW ÷ kVA.
Worked example
A 480 V three-phase plant draws 100 kW at a power factor of 0.80. Apparent power is 100 ÷ 0.80 = 125 kVA, and line current is 125,000 ÷ (1.732 × 480) = 150 A. To raise power factor to 0.95, the required capacitor is kVAR = 100 × (tan(36.9°) − tan(18.2°)) = 100 × (0.750 − 0.329) = about 42 kVAR. After correction apparent power is 105.3 kVA and current drops to about 127 A, a 16% reduction.
Why it matters
- Utilities may bill a penalty for low power factor on commercial and industrial accounts.
- Lower current means lower conductor losses and less voltage drop.
- Transformers and generators can carry more real load.
Safety and design notes
NEC Article 460 covers capacitors, including conductor sizing, disconnects and discharge. On systems with variable frequency drives or other nonlinear loads, harmonics can interact with capacitors and create resonance, so detuned reactors or an engineering study may be needed. Avoid over-correcting to a leading power factor at light loads.
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