Capacitor Bank Size Calculator

Related tools: Power Factor Calculator, Reactive Power Calculator, and Buck-Boost Transformer Calculator.

A capacitor bank size calculator determines the reactive power compensation required to raise an electrical system from its existing power factor to a selected target power factor. The primary result is the required correction in kVAR, which is the starting point for evaluating a fixed or switched power-factor-correction capacitor bank.

Poor power factor increases current for a given kW load. On a feeder, service, transformer secondary, or motor-heavy distribution system, that added current can affect conductor ampacity review, feeder loading, voltage-drop evaluation, transformer utilization, and utility billing exposure. Capacitor banks supply leading reactive power locally, reducing the lagging reactive power drawn from the source.

The calculator also estimates capacitance per phase from the selected capacitor connection, system voltage, and frequency. That capacitance result is an arithmetic design value, not a capacitor-bank equipment selection.

Power-Factor Correction kVAR

The calculator uses these inputs:

Input	Electrical use
Real power (kW)	The active load being evaluated. This is the load that performs useful work, such as motor shaft output, heating, lighting, or process power.
Current power factor (PF)	The existing displacement or planning power factor of the load.
Target power factor (PF)	The corrected power factor selected for the system review.
System voltage (V)	The voltage used to estimate capacitor capacitance per phase.
Frequency (Hz)	The system frequency used in the capacitance calculation.
Capacitor connection	The arithmetic connection model used for capacitance per phase, such as three-phase delta.

For a given real-power load, the reactive component is determined from the power triangle:

\(\displaystyle Q = P \tan\left(\cos^{-1}(PF)\right)\)

Where:

\(\displaystyle Q = \text{reactive power in kVAR}\)

\(\displaystyle P = \text{real power in kW}\)

\(\displaystyle PF = \text{power factor}\)

The calculator determines the existing reactive power and the target reactive power, then subtracts them:

\(\displaystyle \text{Required correction} = P \left[ \tan\left(\cos^{-1}(PF_{\text{current}})\right) – \tan\left(\cos^{-1}(PF_{\text{target}})\right) \right]\)

The result is the capacitor-bank correction requirement in kVAR:

\(\displaystyle Q_C = Q_{\text{current}} - Q_{\text{target}}\)

A positive result represents the leading kVAR that the capacitor bank must supply to offset part of the system’s lagging reactive demand.

Calculation Example

For a 480 V, 60 Hz three-phase system with a 120 kW load, current power factor of 0.80, and target power factor of 0.95:

Result	Value
Current reactive power	90 kVAR
Target reactive power	39.4421 kVAR
Required correction	50.5579 kVAR
Capacitance per phase	194.0235 uF
Connection divisor	3 x
Frequency used	60 Hz

The current reactive power is:

\(\displaystyle Q_{\text{current}} = 120 \times \tan\left(\cos^{-1}(0.80)\right) = 90 \text{ kVAR}\)

The target reactive power is:

\(\displaystyle Q_{\text{target}} = 120 \times \tan\left(\cos^{-1}(0.95)\right) = 39.4421 \text{ kVAR}\)

The required capacitor correction is:

\(\displaystyle Q_C = 90 - 39.4421 = 50.5579 \text{ kVAR}\)

For early equipment planning, the system would therefore be evaluated around a 50.6 kVAR capacitor-bank requirement, subject to available standard bank sizes, load variation, switching requirements, harmonics, voltage tolerance, and the utility’s power-factor requirements.

Capacitance Per Phase

The capacitor-bank kVAR result establishes the reactive-power requirement. The Capacitance per phase output converts that requirement into an estimated capacitance value for the chosen connection model.

Capacitive reactive power is based on voltage, frequency, and capacitance:

\(\displaystyle Q = 2 \pi f C V^2\)

For a three-phase capacitor-bank estimate, the calculator applies the selected Connection divisor. With a three-phase delta connection, the total bank kVAR is divided across three capacitor phases:

\(\displaystyle C_{\text{phase}} = \frac{Q_C \times 1{,}000} {3 \times 2\pi f V^2}\)

Where:

\(\displaystyle C_{\text{phase}} = \text{capacitance per phase in farads}\)

\(\displaystyle Q_C = \text{required correction in kVAR}\)

\(\displaystyle f = \text{frequency in Hz}\)

\(\displaystyle V = \text{system voltage used for the capacitor estimate}\)

The final value is displayed in microfarads:

\(\displaystyle 1 \text{ F} = 1{,}000{,}000 \text{ uF}\)

For the example, 50.5579 kVAR at 480 V and 60 Hz in a three-phase delta arrangement produces an estimated 194.0235 uF per phase.

A delta capacitor bank applies each capacitor across line-to-line voltage. A different connection model changes the voltage relationship and the capacitance required per phase even when the total kVAR correction remains the same.

Use in Electrical Design Review

The required correction kVAR can be used during preliminary electrical design and existing-system assessment to identify whether power-factor correction may reduce source-side reactive demand. Typical review points include:

• Service and feeder loading where poor power factor increases line current for the same kW load.
• Transformer loading where kVA capacity is consumed by both real and reactive power.
• Motor distribution systems with substantial inductive load, especially where large motors operate for extended periods.
• Voltage-drop review where reduced upstream current may improve feeder voltage performance.
• Utility billing review where a utility applies demand, kVA, or low-power-factor charges.
• Equipment planning for fixed capacitor banks, automatically switched capacitor banks, or staged correction systems.

Power-factor correction does not reduce the actual kW consumed by a motor or other load. It reduces the reactive kVAR that must be supplied through upstream conductors and equipment. The resulting reduction in current must be evaluated at the portion of the system upstream of the capacitor bank location.

For example, a capacitor bank installed at a motor control center may reduce current on the feeder supplying that MCC, but it does not automatically change branch-circuit conductor requirements for every motor downstream. Branch-circuit ampacity, motor circuit conductor sizing, overcurrent protection, disconnect ratings, terminal ratings, and equipment listing remain separate electrical design decisions.

Field Verification

The calculator provides a capacitor-bank estimate only. It does not create a capacitor step schedule, determine switching gear, perform harmonic or resonance analysis, verify utility approval, select listed equipment, establish equipment ratings, or determine NEC compliance.

Confirm the following before selecting or installing a capacitor bank:

• Actual load profile, including minimum load, maximum load, and changing operating conditions.
• Whether a fixed bank can cause leading power factor during light-load operation.
• Harmonic-producing equipment such as variable frequency drives, rectifiers, UPS systems, welders, or nonlinear electronic loads.
• System impedance and potential resonance between capacitors and the transformer or distribution system.
• Capacitor voltage rating, kVAR rating at the operating voltage, fuse protection, switching duty, enclosure conditions, and available fault current.
• Equipment manufacturer requirements, utility requirements, project specifications, and AHJ acceptance.

Use the Required correction output as the power-factor-correction basis, then select and engineer the actual capacitor-bank assembly for the electrical system where it will be connected.