Capacitor Bank Design and Sizing Guide: PFC, DC Link, and Industrial Applications
Last Updated: February 2026 | Reading Time: 16 minutes
A single capacitor has limits. Its voltage rating caps out at what the dielectric can withstand. Its capacitance is constrained by physical size. Its current handling is bounded by ESR and thermal dissipation. When an application demands more voltage, more capacitance, or more current than a single unit can provide, you build a capacitor bank.
Capacitor banks are fundamental to industrial power systems. They correct power factor in factories, smooth DC bus voltage in motor drives, store energy in renewable inverters, and provide reactive power support on electrical grids. A poorly designed capacitor bank doesn't just underperform — it can fail catastrophically through thermal runaway, resonance-driven overvoltage, or cascading fuse failures.
This guide covers the electrical design, sizing calculations, protection requirements, and maintenance practices for capacitor banks in the most common industrial applications.
A capacitor bank is a group of capacitors connected in series, parallel, or series-parallel combinations to achieve a voltage rating, capacitance, or current handling capability that no single capacitor can provide. The term applies equally to three 450V electrolytic capacitors paralleled on a VFD DC bus and to a warehouse-sized installation of film capacitors correcting power factor for an entire manufacturing plant.
In some cases you can. But capacitor banks exist because:
- Voltage requirements exceed single-unit ratings. No single aluminum electrolytic capacitor is rated above approximately 600V. A 900V DC bus requires at least two capacitors in series.
- Capacitance requirements exceed single-unit ratings. The largest screw-terminal electrolytics top out around 100,000 µF. High-energy applications may need millions of microfarads.
- Ripple current requirements exceed single-unit ratings. Paralleling capacitors divides the ripple current, reducing heating in each unit.
- Redundancy is needed. In critical systems, individual capacitors can be fused and replaced without shutting down the entire bank.
- Physical constraints. Multiple smaller capacitors may fit where one large unit cannot.
Capacitors in parallel add their capacitance values directly:
C_total = C1 + C2 + C3 + ... + Cn
For n identical capacitors of value C:
C_total = n * C
Ripple current sharing: The total ripple current rating scales with the number of parallel capacitors, but only if they are identical and have matched ESR values. The ripple current divides inversely proportional to ESR — a unit with lower ESR carries more current and runs hotter. In practice, use the same manufacturer, part number, and production lot for all paralleled capacitors.
Layout considerations: Bus bars or PCB traces connecting parallel capacitors must have equal impedance to each unit. Unequal trace lengths cause unequal current sharing, concentrating stress on the closest capacitors. Symmetrical star or balanced bus bar layouts are essential.
Capacitors in series add their voltage ratings (approximately) while reducing total capacitance:
1/C_total = 1/C1 + 1/C2 + ... + 1/Cn
For n identical capacitors of value C:
C_total = C / n
The voltage sharing problem: In theory, identical series capacitors share voltage equally. In practice, manufacturing tolerances in capacitance, leakage current, and dielectric absorption cause unequal voltage distribution. The capacitor with the lowest capacitance or highest leakage sees the highest voltage — and fails first.
Passive balancing resistors: Connect a resistor across each series capacitor. The resistors form a voltage divider that forces equal voltage sharing regardless of capacitor parameter variations.
Sizing rule: The balancing resistor current should be at least 10 times the maximum leakage current of the capacitors. For aluminum electrolytic capacitors with 1 mA leakage, the resistor should carry at least 10 mA. At 450V per capacitor: R = 450V / 10 mA = 45 kohm. Power dissipation: P = 450² / 45,000 = 4.5W per resistor. Use 10W wirewound resistors with adequate heat sinking.
Active balancing: For high-power applications where passive balancing losses are unacceptable, active circuits using op-amps, MOSFETs, or dedicated balancing ICs redistribute charge between series capacitors. This is common in supercapacitor banks but less common in traditional capacitor banks.
Most large capacitor banks combine series and parallel connections. The standard approach: first determine how many series capacitors are needed to meet the voltage requirement, then determine how many parallel strings are needed to meet the capacitance and ripple current requirements.
Example: 900V DC bus requiring 10,000 µF and 50A ripple current
Step 1: Voltage — 900V / 450V per cap = 2 capacitors in series
Step 2: Series capacitance — Two 10,000 µF caps in series = 5,000 µF per string
Step 3: Total capacitance — 10,000 µF / 5,000 µF per string = 2 parallel strings
Step 4: Ripple current — Check that each string handles 25A ripple (50A / 2 strings)
Step 5: Total count — 2 series × 2 parallel = 4 capacitors
Industrial loads — especially induction motors, transformers, and fluorescent lighting — draw reactive current that oscillates between the source and load without doing useful work. This reactive current increases line losses, reduces transformer capacity, causes voltage drops, and triggers utility penalties when power factor falls below 0.90 to 0.95 (depending on the utility).
Capacitor banks supply reactive power locally, reducing the reactive current that must be supplied by the utility. This is power factor correction (PFC).
The required reactive power compensation in kVAR:
kVAR = kW * (tan(φ1) - tan(φ2))
Where:
kW = real power of the load
φ1 = angle corresponding to current power factor (cos φ1 = PF_current)
φ2 = angle corresponding to target power factor (cos φ2 = PF_target)
Example: A 500 kW facility with power factor 0.75, targeting 0.95:
φ1 = arccos(0.75) = 41.4° → tan(41.4°) = 0.882
φ2 = arccos(0.95) = 18.2° → tan(18.2°) = 0.329
kVAR = 500 * (0.882 - 0.329) = 500 * 0.553 = 276.5 kVAR
This facility needs approximately 275 kVAR of capacitor bank compensation.
Fixed banks provide a constant kVAR output. Suitable when the load is consistent — a motor that runs continuously at constant load, for example. Simple and inexpensive.
Automatic (switched) banks use a power factor controller that monitors the power factor in real time and switches capacitor stages in and out using contactors. Each stage provides a fixed kVAR increment. The controller engages enough stages to bring the power factor to the target value as the load varies throughout the day.
Typical automatic PFC bank configuration:
| Stage | kVAR | Cumulative kVAR |
|---|
| 1 | 25 | 25 |
| 2 | 25 | 50 |
| 3 | 50 | 100 |
| 4 | 50 | 150 |
| 5 | 50 | 200 |
| 6 | 75 | 275 |
This six-stage bank provides 275 kVAR in 25 kVAR steps, with finer resolution at low compensation levels where small load changes have larger PF impact.
PFC capacitors are almost exclusively metallized polypropylene film type, rated for continuous AC duty:
| Parameter | Typical Specification |
|---|
| Voltage rating | 440 VAC, 480 VAC, or 525 VAC (three-phase systems) |
| Capacitance | Specified in kVAR rather than µF |
| Frequency | 50 or 60 Hz rated |
| Dielectric | Metallized polypropylene film |
| Self-healing | Yes — essential for long-term reliability |
| Discharge resistors | Integrated or external; must reduce voltage to < 50V within 60 seconds |
| Life expectancy | 100,000 to 200,000 hours at rated conditions |
| Manufacturer examples | Vishay/ESTA PhMKP series, TDK/EPCOS MKK/MKV series, ABB CLMD series, Eaton/Cooper CPC series |
Modern facilities with variable frequency drives (VFDs), rectifiers, and switching power supplies generate harmonic currents — particularly the 5th (250/300 Hz), 7th (350/420 Hz), and 11th (550/660 Hz) harmonics. PFC capacitors connected to a bus with harmonic pollution can experience resonance if the capacitor bank's natural frequency coincides with a dominant harmonic:
f_resonant = f_fundamental * sqrt(kVA_sc / kVAR_bank)
Where:
kVA_sc = system short-circuit capacity
kVAR_bank = capacitor bank reactive power rating
If the resonant frequency falls near the 5th or 7th harmonic, current amplification can destroy the capacitor bank within hours. The solution is detuned reactors — series inductors tuned to shift the resonant frequency below the lowest significant harmonic (typically tuned to 189 Hz or 4.7th harmonic for 60 Hz systems). Every PFC installation in a facility with more than 20% non-linear load should use detuned reactors.
Variable frequency drives use large DC bus capacitors to smooth rectified AC power and provide energy storage for dynamic braking. The DC bus voltage is typically 325V (single-phase 230V input), 650V (three-phase 480V input), or 930V (three-phase 690V input).
DC bus capacitor requirements:
- Capacitance: Sufficient to limit bus voltage ripple to 3-5% at full load
- Ripple current: Must handle full rectifier output ripple plus inverter-reflected current ripple
- Voltage rating: At minimum 15% above nominal bus voltage; 450V caps for 325V bus, 500V or series 450V for 650V bus
Common capacitors for drive DC links include Nichicon LGU/LGN series, TDK/EPCOS B43456/B43458 series, and Kemet ALS30/ALS31 series — all large can, screw-terminal aluminum electrolytic types.
Solar inverters and wind turbine converters use similar DC link banks, typically at 600-1,500V DC. Film capacitors (polypropylene) are increasingly preferred over electrolytic for solar applications due to longer calendar life and lack of electrolyte dry-out.
Every parallel branch in a capacitor bank should be individually fused. If one capacitor fails short, the fuse isolates it before the fault current damages neighboring capacitors or the bus.
PFC bank fusing: Use HRC (High Rupture Capacity) fuses rated at 1.5 to 1.65 times the capacitor rated current to allow for harmonic overcurrents without nuisance blowing. IEC 60269-series and UL 248-series fuses are appropriate.
DC link fusing: Use DC-rated fuses with adequate voltage rating and interrupting capacity. Standard AC fuses cannot safely interrupt DC fault currents.
Capacitor banks store dangerous energy. Discharge resistors are legally required in most jurisdictions and are essential for personnel safety:
- PFC banks: IEC 60831 and NEC 460.6 require discharge to below 50V within 60 seconds of disconnection. Resistors are sized accordingly: R = t / (C * ln(V_initial / 50)).
- DC link banks: Internal or external bleed resistors should discharge the bus to safe levels (< 50V DC) within 5 minutes for maintenance safety. Many VFDs include built-in discharge circuits.
Uncharged capacitors initially appear as short circuits. Energizing a large capacitor bank without current limiting causes massive inrush currents that can weld contactor tips, trip protective devices, and damage capacitors through dielectric stress.
PFC banks: Use pre-insertion resistors (bypassed by a contactor after charging), or specify contactors with built-in pre-charge resistors. Alternatively, use thyristor-switched stages that energize at the voltage zero-crossing to minimize inrush.
DC link banks: Pre-charge circuits using resistors in series with the rectifier input are standard. The pre-charge resistor limits current during initial charging, then a bypass contactor shorts around it once the bus voltage reaches approximately 80% of nominal.
Capacitor banks in PFC applications can experience overcurrent from harmonic amplification, overvoltage, or system resonance. Overcurrent relays or electronic protection in the PFC controller should trip the bank if current exceeds 1.3 times rated current (per IEC 60831-1).
Capacitor bank temperature directly affects life and reliability:
- Aluminum electrolytic capacitors: Life halves for every 10°C above rated temperature
- Film capacitors: Degradation accelerates above 70-85°C depending on the dielectric
Design practices:
- Provide at least 10mm spacing between capacitors for air circulation
- Position capacitor banks where ambient temperature is minimized — not adjacent to VFD heat sinks or in unventilated enclosures
- Use forced air cooling for high-power-density installations
- Monitor capacitor surface temperature with thermocouples or IR thermography during commissioning
Maintenance and Monitoring#
| Parameter | Method | Frequency | Warning Threshold |
|---|
| Capacitance | LCR meter or capacitance bridge | Annual | < 80% of rated value |
| ESR | ESR meter at rated frequency | Annual | > 200% of initial value |
| Leakage current | Insulation resistance tester | Annual | Trending upward |
| Temperature | IR thermography | Quarterly | > 20°C above ambient in PFC; > 10°C hot spots |
| Visual inspection | Physical inspection | Quarterly | Bulging, leaking, discoloration |
| Power factor (PFC) | Power analyzer | Monthly | Not reaching target |
Replace capacitors when:
- Capacitance has dropped below 80% of rated value
- ESR has risen above 200% of initial value
- Visual signs of distress: bulging, leaking, cracking, or discoloration
- The capacitor is beyond its rated calendar life (typically 7-15 years for electrolytic, 15-20+ years for film)
When replacing capacitors in a bank, it is best practice to replace all capacitors in a series string simultaneously. Mixing old and new capacitors with different parameters causes unequal voltage sharing and accelerates failure of the remaining old units.
Calculate the required kVAR using the formula: kVAR = kW * (tan(arccos(PF_current)) - tan(arccos(PF_target))). Measure the facility's real power (kW) and current power factor from the utility meter or a power analyzer. Choose a target power factor of 0.95 to 0.98 — going to unity (1.0) is unnecessary and risks overcorrection (leading power factor) during low-load periods. Select a bank with total kVAR equal to or slightly above the calculated requirement, with automatic staging if the load varies throughout the day.
For parallel banks, it is strongly recommended to use the same manufacturer, part number, and ideally the same production date code for all capacitors. Different brands have different ESR, capacitance tolerance, and leakage current characteristics that cause unequal current sharing and voltage distribution. For series banks, matching is even more critical — use identical units with matched capacitance values (within 5%) and balancing resistors across each capacitor regardless.
Catastrophic capacitor bank failures typically result from one of four causes: (1) harmonic resonance amplifying currents far beyond rated values, causing rapid overheating; (2) a single capacitor failing short without adequate fusing, leading to cascading failures; (3) incorrect voltage rating or missing voltage balancing in series connections; (4) re-energizing a capacitor bank before it has fully discharged, causing transient overvoltage up to twice the rated voltage. Proper fusing, detuned reactors, discharge resistors, and time-delayed re-closing prevent these failure modes.
A fixed capacitor bank provides constant reactive power compensation regardless of load conditions. An automatic bank uses a microprocessor-based controller that monitors the system power factor in real time and switches capacitor stages on and off to match the changing reactive power demand. Fixed banks are appropriate when the load is constant (single large motor, continuous process). Automatic banks are necessary when the load varies (mixed facility with motors starting and stopping, shift changes, variable production schedules). Most industrial installations above 100 kVAR use automatic banks.
Visual inspection and IR thermography should be performed quarterly. Capacitance and ESR measurements should be taken annually. Power factor should be monitored monthly or continuously with the PFC controller's built-in logging. PFC capacitors in harmonic-rich environments (facilities with many VFDs) should be inspected more frequently, as harmonic overcurrents accelerate aging. Many modern PFC controllers include capacitance monitoring that detects degradation automatically and generates alarms.
