Power Factor Correction Calculator (Required kvar)

Why correct the power factor?

Inductive loads such as motors, transformers and ballasts draw reactive power in addition to active power. Reactive power does no work but still flows as current: it loads cables, transformers and switchgear and raises voltage drop and losses. A capacitor next to the load supplies that reactive power locally, so the current drawn from the supply falls.

How is the capacitor rating calculated?

Q_c = P × (tan φ1 − tan φ2)

P is the active power of the installation, φ1 the present phase angle and φ2 the target one. From a power factor, tan φ = √(1 − cos² φ) / cos φ. The result is in kvar.

Example

A plant draws 120 kW at a power factor of 0.6 and the target is 0.8. tan φ1 = 1.333 and tan φ2 = 0.75, so the capacitor rating is 120 × (1.333 − 0.75) = 70 kvar. Apparent power falls from 200 kVA to 150 kVA and line current drops by 25 percent.

How is capacitance found?

For a three phase delta connected bank each branch needs C = Q_c / (3 × 2π × f × V²). For a star connected bank C = Q_c / (2π × f × V²), where V is the line to line voltage. The second formula also applies to single phase.

What this tool does not do

It does not choose the number or size of steps and gives no controller settings. Where harmonics are present a plain capacitor bank can resonate and be damaged: such sites need detuned banks sized from measurements. With a varying load a fixed capacitor can over correct. Take the target power factor from the applicable tariff and specification.