Corpus sync: neutrality remakes, note hygiene, datasheet relocation, transformer-split cross-refs
---
title: Power-Factor Correction Capacitors
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## Where the assembly is factory-built and listed, it shall be furnished as a single listed assembly under UL 508A rather than as field-assembled components.
−## A harmonic-impact assessment shall be performed before any capacitor bank is sized or installed on a bus that serves non-linear loads. {note}
+## A harmonic-impact assessment shall be performed before any capacitor bank is sized or installed on a bus that serves non-linear loads.
## Adding capacitance to a distribution system shifts the system's resonant frequency. When that frequency lands near a harmonic produced by VFDs, rectifiers, or UPS loads, the capacitor reactance and the upstream source inductance form a parallel resonant circuit that amplifies harmonic current and voltage. This is the single most common and most expensive failure mode for capacitor banks, so the assessment governs whether detuning reactors are required and is treated as a precondition, not an optional study. {note}
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## Factory-built automatic and thyristor-switched assemblies shall be listed under UL 508A as industrial control panels.
−## The manufacturer shall be regularly engaged in the production of power capacitors and capacitor banks of the type and rating specified. {note}
+## The manufacturer shall be regularly engaged in the production of power capacitors and capacitor banks of the type and rating specified.
## Capacitor banks are not interchangeable commodities; the dielectric system, discharge design, and detuning-reactor coordination determine field reliability. Requiring an established manufacturer of this specific equipment class screens out repackaged assemblies that have not been engineered for the harmonic and switching environment of the application. {note}
−## The available fault current used to establish the assembly SCCR shall be taken from the project short-circuit and arc-flash study, not from a generic assumption. {note}
+## The available fault current used to establish the assembly SCCR shall be taken from the project short-circuit and arc-flash study, not from a generic assumption.
## An assembly whose SCCR is below the available fault current at its terminals is unlisted for that location and is an arc-flash hazard. Tying the SCCR back to the project study, rather than a catalog default, is what makes the listing valid at the actual point of installation. {note}
# Environmental and Service Conditions {toc}
−## The enclosure type shall be selected for the installation environment. {note}
+## The enclosure type shall be selected for the installation environment.
## Capacitor banks are frequently placed in pump rooms, process areas, and outdoor yards where moisture, dust, and corrosives are present. A NEMA 1 assembly installed in a wet or corrosive location fails prematurely, so the enclosure rating is coordinated with the room and area conditions defined for the serving equipment in [[sync/low-voltage-switchgear]]. {note}
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## System Voltage Class {toc}
−### The capacitor bank voltage class and nominal system voltage shall be established before any other selection. {note}
+### The capacitor bank voltage class and nominal system voltage shall be established before any other selection.
### Low-voltage banks (208 to 600 V) and medium-voltage banks (2.4 to 35 kV) differ fundamentally in construction, protection, and discharge timing. The voltage class drives the protection scheme, the discharge requirement, and whether unbalance and neutral-current relaying applies, so it is selected first. {note}
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## Correction Strategy {toc}
−### The correction location shall be selected from a load survey of the bus to be corrected. {note}
+### The correction location shall be selected from a load survey of the bus to be corrected.
### Centralized bus correction is the most economical for facilities with diverse loads and is the default. Group correction follows the load when a cluster of motors or feeders has a distinct duty cycle, and individual motor-terminal correction is reserved for steady, continuously running motors. The choice changes the controller location and the protection it coordinates with. {note}
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```
−### The bank shall not over-correct the system into a leading power factor at any load condition. {note}
+### The bank shall not over-correct the system into a leading power factor at any load condition.
### Over-sizing produces voltage rise, nuisance protective-relay operation, motor overspeed on disconnection, and, on many tariffs, a leading-power-factor penalty that mirrors the lagging penalty the bank was meant to avoid. Targeting 0.95 to 0.97 lagging rather than unity leaves margin against the lightest-load condition where over-correction is worst. {note}
## Bank Type and Switching {toc}
−### The bank type shall be selected from the variability of the load and the transient sensitivity of the served equipment. {note}
+### The bank type shall be selected from the variability of the load and the transient sensitivity of the served equipment.
### A fixed bank suits a constant reactive load and is the simplest and least costly. An automatic, step-controlled bank follows a varying load by switching contactor stages. A thyristor-switched bank switches at the voltage zero crossing for transient-free, millisecond response and is reserved for loads that cannot tolerate switching transients. The load profile and the sensitivity of nearby equipment, not preference, drive this selection. {note}
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```
−### For automatic banks, the step size shall be coordinated with the load profile to prevent hunting. {note}
+### For automatic banks, the step size shall be coordinated with the load profile to prevent hunting.
### A step that is large relative to the smallest variable load causes the controller to switch a stage in and out repeatedly (hunting), which rapidly destroys contactors and capacitors. As a working rule the smallest step is kept below one-third of the smallest variable load, so the step size is matched to the load, not to a convenient round number. {note}
−```datasheet
−label: Automatic bank step size
−type: range
−unit: kVAR
−min: 25
−max: 100
−step: 25
−default: 50
−```
+### The number of switched capacitor steps shall be specified, sized so the smallest step does not exceed one-third of the smallest variable load served.
```datasheet
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```
+```datasheet
+label: Automatic bank step size
+type: range
+unit: kVAR
+min: 25
+max: 100
+step: 25
+default: 50
+```
+
## Total Bank Rating {toc}
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# Harmonic Detuning {toc}
−## Detuning reactors shall be required wherever the harmonic-impact assessment shows the bus is at risk of resonance or harmonic amplification. {note}
+## Detuning reactors shall be required wherever the harmonic-impact assessment shows the bus is at risk of resonance or harmonic amplification.
## For any facility with meaningful VFD, rectifier, or UPS load, detuned banks are essentially standard practice. The default position of this standard is that a harmonic survey is performed and detuning is provided unless the survey demonstrates that the current total harmonic distortion is below the IEEE Std 519 screening threshold. Placing a series reactor ahead of the capacitor lowers the bank's resonant frequency below the lowest significant harmonic so the bank cannot form a parallel resonance with the source. {note}
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```
−### The detuning reactor impedance shall be selected to tune the bank below the lowest significant harmonic present on the system. {note}
+### The detuning reactor impedance shall be selected to tune the bank below the lowest significant harmonic present on the system.
−### A 5 to 7% reactor tunes the bank to about 189 Hz, below the 5th harmonic at 300 Hz on a 60 Hz system, and is the common choice where 5th-and-higher harmonics dominate (the VFD case). A 14% reactor tunes to about 160 Hz, below the 3rd harmonic, and is used in high-distortion environments where triplen harmonics are significant. The tuning frequency is chosen from the harmonic spectrum found in the survey, not assumed. {note}
−
```datasheet
label: Detuning reactor impedance
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```
−### Where a detuning reactor is present, the capacitor voltage rating shall account for the fundamental voltage rise across the reactor. {note}
+### A 5 to 7% reactor tunes the bank to about 189 Hz, below the 5th harmonic at 300 Hz on a 60 Hz system, and is the common choice where 5th-and-higher harmonics dominate (the VFD case). A 14% reactor tunes to about 160 Hz, below the 3rd harmonic, and is used in high-distortion environments where triplen harmonics are significant. The tuning frequency is chosen from the harmonic spectrum found in the survey, not assumed. {note}
+### Where a detuning reactor is present, the capacitor voltage rating shall account for the fundamental voltage rise across the reactor.
+
### A series reactor raises the fundamental voltage impressed on the capacitor above the system voltage, typically by a factor of about 1.06 to 1.10 depending on the reactor impedance. Specifying the capacitor at only the system voltage in a detuned bank guarantees overvoltage failure, so the capacitor rating in a detuned bank is uprated to cover the reactor's voltage drop. {note}
# Capacitor Unit Ratings {toc}
−## The capacitor voltage rating shall provide continuous-operation headroom above the system nominal voltage. {note}
+## The capacitor voltage rating shall provide continuous-operation headroom above the system nominal voltage.
## IEEE Std 18 permits continuous operation at 110% of rated voltage and requires the unit to withstand that level indefinitely. A capacitor rated exactly at the system voltage has no headroom for normal voltage rise and tap variation; for a 480 V system the unit is specified at 525 V or 600 V so that ordinary system voltage swings stay within the capacitor's continuous rating. {note}
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### Capacitor units shall be capable of continuous operation at 135% of nominal rms current per IEEE Std 18.
−## The capacitor dielectric technology shall be selected for the application. {note}
+## The capacitor dielectric technology shall be selected for the application.
## Metallized polypropylene film is self-healing and is the default for low-voltage banks. All-film and oil-impregnated film-foil constructions offer higher current density and are typically applied at medium voltage or in high-duty switching environments. The dielectric choice trades self-healing behavior against current-handling capacity. {note}
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### A disconnecting means shall be provided for the capacitor per NEC 460.8.
−## A discharge means shall be provided to reduce the residual voltage to a safe level after disconnection. {note}
+## A discharge means shall be provided to reduce the residual voltage to a safe level after disconnection.
## A disconnected capacitor stores a lethal charge. NEC 460.6 requires the residual terminal voltage to fall to 50 V or less within 1 minute for low-voltage equipment and within 5 minutes for equipment above 600 V. Discharge resistors are factory-installed and their resistance value is verified against the required discharge time, because an undersized or open discharge resistor leaves the bank energized after isolation. {note}
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## Capacitor cases and the assembly enclosure shall be grounded per NEC 460.12.
−## For medium-voltage banks, unbalance protection shall be provided to detect the loss of individual capacitor elements. {note}
+## For medium-voltage banks, unbalance protection shall be provided to detect the loss of individual capacitor elements.
## Medium-voltage protection differs from low-voltage practice. IEEE Std 1036 addresses fuse coordination, unbalance relaying, and neutral-current protection in detail, because the failure of internal elements in a MV bank shifts the neutral and overstresses the remaining units. Unbalance or neutral-current relaying detects this degradation before a cascading failure, and it has no low-voltage equivalent. {note}
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# Controls {toc}
−## The capacitor controller type shall be selected for the switching basis required by the application. {note}
+## The capacitor controller type shall be selected for the switching basis required by the application.
## A power-factor-sensing (kVAR-based) controller is the default for automatic correction because it switches steps directly on the measured reactive demand. Voltage-based control is used where the objective is voltage support, and time-based control suits a predictable daily load cycle. The controller type sets what the bank responds to. {note}
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```
−## The controller current-sensing input shall be taken from a point that measures total load, not the load side of the capacitor bank. {note}
+## The controller current-sensing input shall be taken from a point that measures total load, not the load side of the capacitor bank.
## A current transformer placed on the load side of the bank cannot see the reactive demand the bank is correcting, so the controller either hunts or fails to bring the power factor up. The sensing point and the metering point referenced by [[sync/electrical-power-monitoring]] must be the same node, which is why the controller CT location is specified explicitly rather than left to the installer. {note}
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### The power-factor improvement shall be verified at the service meter against the design target.
−### Field acceptance testing shall be coordinated with the overall electrical acceptance testing program for the project. {note}
+### Field acceptance testing shall be coordinated with the overall electrical acceptance testing program for the project.
### Capacitor field tests share instruments, an energized bus, and a sequence with the broader commissioning effort. Folding them into the project acceptance-testing plan, rather than running them as a standalone activity, keeps the switching sequence and lockout coordinated with the rest of the distribution equipment. {note}
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## The location, mounting arrangement, and routing of the capacitor bank shall be as shown on the drawings. [[drawing: capacitor bank location and feeder routing]]
−## Utility coordination shall be completed before the bank is energized where the serving utility requires prior approval. {note}
+## Utility coordination shall be completed before the bank is energized where the serving utility requires prior approval.
## Many utilities require advance approval for capacitor banks above a size threshold (often around 100 kVAR on low-voltage systems) because the added capacitance raises voltage on their distribution system. Confirming the interconnection requirement early, rather than at energization, avoids a held commissioning and a possible forced removal. {note}
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- Control and capacitor fuses (10% of installed, minimum one set)
```