NOTE This standard covers fixed and automatic shunt capacitor banks furnished for power-factor correction on AC distribution systems rated 600 V and below (low voltage) and above 600 V through 35 kV (medium voltage). (1.1)

NOTE The equipment serves industrial manufacturing plants, water and wastewater treatment facilities, data centers, hospitals, large commercial and institutional buildings, and utility-interface upgrades driven by power-factor tariff penalties. It includes individual capacitor units, detuned bank assemblies (capacitor units in series with detuning reactors), thyristor-switched banks, capacitor controllers, and the overcurrent-protection and switching devices furnished as part of the integrated assembly, for both indoor and outdoor, bus-mounted and freestanding installations. (1.2)

NOTE A harmonic-impact assessment shall be performed before any capacitor bank is sized or installed on a bus that serves non-linear loads. (1.5)

NOTE 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. (1.6)

NOTE The following are outside the scope of this standard and are governed elsewhere. (1.8)

NOTE The manufacturer shall be regularly engaged in the production of power capacitors and capacitor banks of the type and rating specified. (4.3)

NOTE 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. (4.4)

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. (4.5)

NOTE 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. (4.6)

NOTE The enclosure type shall be selected for the installation environment. (5.1)

NOTE 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 Low Voltage Switchgear. (5.2)

NOTE The capacitor bank voltage class and nominal system voltage shall be established before any other selection. (6.1.1)

NOTE 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. (6.1.2)

NOTE The correction location shall be selected from a load survey of the bus to be corrected. (6.2.1)

NOTE 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. (6.2.2)

NOTE Individual motor-terminal capacitors shall not exceed the no-load reactive requirement of the motor as limited by NEMA MG 1 Section 14. (6.2.4)

NOTE A capacitor larger than the motor's no-load magnetizing reactive demand causes self-excitation: when the motor is disconnected while still rotating, the capacitor drives it as a self-excited generator and produces dangerous overvoltage. The MG 1 Section 14 limit is the boundary that prevents this, which is why motor-terminal correction is sized to the motor, not to a round kVAR target. (6.2.5)

NOTE The bank shall not over-correct the system into a leading power factor at any load condition. (6.2.7)

NOTE 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. (6.2.8)

NOTE The bank type shall be selected from the variability of the load and the transient sensitivity of the served equipment. (6.3.1)

NOTE 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. (6.3.2)

NOTE For automatic banks, the step size shall be coordinated with the load profile to prevent hunting. (6.3.6)

NOTE 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. (6.3.7)

NOTE Detuning reactors shall be required wherever the harmonic-impact assessment shows the bus is at risk of resonance or harmonic amplification. (7.1)

NOTE 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. (7.2)

NOTE The detuning reactor impedance shall be selected to tune the bank below the lowest significant harmonic present on the system. (7.2.2)

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. (7.2.3)

NOTE Where a detuning reactor is present, the capacitor voltage rating shall account for the fundamental voltage rise across the reactor. (7.2.4)

NOTE 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. (7.2.5)

NOTE The capacitor voltage rating shall provide continuous-operation headroom above the system nominal voltage. (8.1)

NOTE 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. (8.2)

NOTE The capacitor dielectric technology shall be selected for the application. (8.3)

NOTE 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. (8.4)

NOTE Overcurrent protection shall be provided for the capacitor circuit and coordinated with the upstream system per NEC Article 460 and Article 240. (9.1)

NOTE NEC 460.8 sizes the circuit conductors at 135% of the rated capacitor current, and 460.9 requires the overcurrent device to be set as low as practicable while still allowing the capacitor's energizing inrush. The protection both protects the conductors and clears a faulted unit before it cascades, so it is coordinated with the upstream device rather than set in isolation. (9.2)

NOTE A discharge means shall be provided to reduce the residual voltage to a safe level after disconnection. (9.3)

NOTE 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. (9.4)

NOTE For medium-voltage banks, unbalance protection shall be provided to detect the loss of individual capacitor elements. (9.6)

NOTE 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. (9.7)

NOTE The capacitor controller type shall be selected for the switching basis required by the application. (10.1)

NOTE 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. (10.2)

NOTE The controller current-sensing input shall be taken from a point that measures total load, not the load side of the capacitor bank. (10.3)

NOTE 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 Electrical Power Monitoring must be the same node, which is why the controller CT location is specified explicitly rather than left to the installer. (10.4)

NOTE Field acceptance testing shall be coordinated with the overall electrical acceptance testing program for the project. (11.2.4)

NOTE 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. (11.2.5)

NOTE Utility coordination shall be completed before the bank is energized where the serving utility requires prior approval. (12.3)

NOTE 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. (12.4)

NOTE Conductor entry, terminations, and clearances shall maintain the working space and arc-flash boundaries established by the project electrical study. (12.6)

NOTE A capacitor bank is part of an energized assembly with the same shock and arc-flash exposure as the switchgear that feeds it. Preserving the working-space clearances and labeling from the project study at the bank, as coordinated with Low Voltage Switchgear and Medium Voltage Cables, keeps the installed equipment within the boundaries the study assumed. (12.7)