Capacitor Bank Sizing for Power Factor Correction

kVAR Required by Motor Horsepower

The following table provides recommended capacitor kVAR ratings for individual motor power factor correction. These values are based on typical NEMA Design B motors operating at full load with a target power factor of approximately 0.95 at the motor terminals.

Three-Phase Motors — 460V / 480V

Values based on NEMA Design B, 4-pole, TEFC motors at full load. Actual kVAR may vary by motor manufacturer and design. Always verify with motor manufacturer data when available.

Three-Phase Motors — 200V / 230V

Single-Phase Motors — 230V

Single-phase motors typically have lower power factors than three-phase motors. Capacitor sizing is more critical to avoid self-excitation — do not exceed 90% of the motor's no-load magnetizing current.

Individual Motor Correction

Individual motor correction places a capacitor directly at each motor's terminals, typically between the motor starter and the motor. This is the most effective method for large motors (typically 10 HP and above).

Advantages

• Reduces current in the motor feeder, starter contacts, overload relay, and all upstream distribution equipment
• Minimizes losses in the motor branch circuit conductors
• Improves voltage at the motor terminals
• Reduces kVA demand from the utility
• Each motor is corrected independently — no overcorrection risk for the overall system

Sizing Rules for Individual Motor Correction

When sizing capacitors for individual motor correction, follow these critical rules:

1. Do not exceed motor no-load magnetizing kVAR. The capacitor kVAR must be ≤ 90% of the motor's no-load magnetizing current (I₀). Exceeding this value can cause self-excitation — the motor continues to rotate at near-synchronous speed after disconnection, generating voltage that can damage equipment and create a safety hazard.
2. Use the kVAR table above as a starting point, then verify against motor manufacturer data.
3. For VFD-driven motors, do not install capacitors on the motor side of the VFD. Place correction on the line side if needed.

Connection Diagram

The capacitor is connected between the motor starter's load side contacts and the motor terminals. It is energized and de-energized with the motor. The overload relay must be set based on the reduced current (motor FLA minus capacitor current), or the capacitor can be connected ahead of the overload relay if the relay is already sized for the motor's corrected current.

Group Capacitor Correction

Group correction places a single capacitor bank at the main switchboard or distribution panel to correct the combined power factor of multiple loads. This approach is common in facilities with many small motors where individual correction would be impractical or uneconomical.

Sizing Group Correction Banks

To size a group correction bank:

1. Measure or calculate the total real power (kW) and existing power factor at the point of correction (e.g., main switchboard metering)
2. Apply the power factor correction formula:

Where P_avg is the average measured real power, PF₁ is the measured power factor, and PF₂ is the target power factor (typically 0.95).

Example: Group Correction

A distribution panel supplies multiple motors with a total average demand of 300 kW measured at 0.78 power factor. Target is 0.95.

Select a 150 kVAR capacitor bank or an automatic switched bank with multiple steps.

Automatic vs. Fixed Group Correction

Hybrid Correction Approach

The most cost-effective strategy for many facilities combines individual correction for large motors (typically 50 HP and above) with group correction at the main switchboard for the remaining load.

Benefits of the hybrid approach:

• Large motors (highest current, longest feeder runs) get maximum loss reduction from individual correction
• The group bank corrects the remaining reactive power from smaller motors and other inductive loads
• The group bank can be sized smaller because the large motors are already corrected
• Total cost is typically 20–30% less than full individual correction, with 90% of the benefit

Capacitor Bank Sizing Formula

The general formula for calculating required capacitor kVAR for power factor correction is:

For three-phase systems, you can also calculate the capacitor current to verify conductor sizing:

Where V_LL is the line-to-line voltage. For a 150 kVAR bank at 480V:

Conductor Sizing

Per NEC 460.8, conductors for capacitor circuits must be sized for at least 135% of the capacitor's rated current. For the 150 kVAR example:

Voltage Considerations for Capacitor Banks

Capacitor kVAR output varies with the square of the applied voltage. A capacitor rated 100 kVAR at 480V will produce different kVAR at other voltages:

Detuned Capacitor Banks for Harmonic Environments

In facilities with significant non-linear loads (VFDs, rectifiers, UPS systems, LED lighting), standard capacitor banks can resonate with the system inductance at harmonic frequencies, causing:

• Amplification of harmonic voltages and currents
• Capacitor overcurrent and premature failure
• Malfunction of sensitive electronic equipment
• Overheating of transformers and conductors
• Nuisance tripping of protective devices

Detuned Bank Solution

Detuned banks add a series reactor (typically 5.67%, 7%, or 14% impedance) in series with each capacitor stage. The reactor shifts the natural resonant frequency of the capacitor-inductor combination below the lowest significant harmonic.

Installation Best Practices

Proper installation is critical for capacitor bank reliability, safety, and performance:

Ventilation and Cooling

Capacitor banks generate heat — typically 0.5 to 1.5 watts per kVAR. For a 300 kVAR bank, this means 150–450W of heat dissipation in the enclosure. Provide adequate ventilation or air conditioning. Capacitor life decreases by approximately 50% for every 10°C above rated operating temperature. NEMA CP-1 recommends a maximum ambient temperature of 46°C (115°F).

Fusing and Protection

Each capacitor element or group should have individual fusing rated at 135–165% of rated current. Fuses must be rated for the capacitor's inrush current (which can be 200–400 times rated current on energization). Use time-delay fuses or purpose-designed capacitor fuses. Provide a disconnecting means that is lockable in the open position per NEC requirements.

Discharge Resistors

Per NEC 460.6, capacitors must be equipped with internal discharge resistors that reduce the residual voltage to 50V or less within 1 minute for capacitors rated 600V or less, and within 5 minutes for higher voltages. Most commercial capacitor banks include built-in discharge resistors. Verify this specification before installation.

Mounting and Clearance

Install capacitor banks with adequate clearance for ventilation and maintenance access. NEC 110.26 requires minimum working clearances: 3 feet (914 mm) for equipment rated 0–150V to ground, and 3.5 feet (1067 mm) for 151–600V. Mount banks in a vertical position with terminals accessible for inspection and fuse replacement.

NEC Article 460 Requirements

NEC Article 460 specifically covers Capacitors. Key requirements include:

• 460.6 — Discharge: Automatic discharge device to reduce voltage to ≤50V within 1 min (≤600V) or 5 min (>600V)
• 460.8 — Conductors: Minimum 135% of rated capacitor current for conductor sizing
• 460.8(a) — Fusing: Individual fusing for each capacitor unit or section
• 460.8(b) — Disconnecting means: Integral with capacitor or separately mounted, lockable in open position
• 460.8(c) — Group switching: Where multiple capacitors are grouped, a single disconnecting means is acceptable if it disconnects all capacitors simultaneously
• 460.9 — Over 600V: Additional requirements for high-voltage capacitors including fenced enclosures and warning signs
• 460.10 — Grounding: Capacitor cases and frames must be grounded per NEC Article 250