ScaleSet

Power Factor Correction Calculator: kVAR, True Power Factor and Detuned Banks

Power factor correction (PFC) calculator for Australian electrical engineers, designers, electricians and estimators. Enter the load in kW, kVA or line amps, the measured displacement power factor (cos φ) and a target, and it returns the capacitive reactive power required, Qc = kW × (tanφ₁ − tanφ₂) in kVAR, rounds it up to the next standard bank size and works the power triangle: apparent power, reactive power and the kVA and current saved. It sizes the installed bank itself: capacitance per phase in microfarads and the bank current Ic, and the 135% design current that AS/NZS 3000:2018 Clause 4.15.2.3 requires its conductors to carry, from which it auto-sizes the bank feeder cable and circuit breaker to AS/NZS 3008.1.1:2025. Switch on harmonics for a board with drives, LED lighting or computers and it estimates the current THD from that load, or takes a power-quality meter's THD or true PF, and works true power factor (λ = P ÷ S, the IEEE 1459 power factor, also called total power factor): before and after the bank, and the most a capacitor bank alone can reach. With harmonics on, the bank gets reactors, a 7% detuned (189 Hz) or 14% detuned (134 Hz) bank, specified the way it is bought: reactor in mH per phase and capacitor nameplate kVAR at 440, 480 and 525 V. A harmonic limit (Victoria's Table 5, a THDv allocation or amps per harmonic) sizes an active harmonic filter. A single line diagram, a power triangle and a PDF report with the full working come with it. Single-phase 230 V and three-phase 400 V at 50 Hz.

Power factor correction, true power factor and detuned banks

Correcting power factor cuts the kVA a site draws for the same real work, which is what reduces demand charges and frees up consumer mains capacity. Enter the load in kW, kVA or line amps, the measured displacement power factor (cos φ) and the target, and this calculator returns the capacitive reactive power required from Qc = kW × (tanφ₁ − tanφ₂) in kVAR, rounds it up to the next standard bank size, and sizes the bank, its feeder cable and breaker. Switch on harmonics for a board with drives, LED lighting or computers, say how much of that load there is (or enter a meter reading), and it estimates the true power factor, gives the bank reactors (a detuned bank) and sizes an active filter to a harmonic limit.

Power factor correction at a glance

  • Required capacitor size is kVAR = kW × (tan φ1 − tan φ2) to move from the existing to the target displacement power factor.
  • Australian demand tariffs typically bill kVA, so improving power factor directly reduces the billed demand.
  • A target of 0.95 to 0.98 lagging is common; overcorrection risks a leading power factor at light load.
  • A capacitor bank corrects displacement power factor (cos φ) only. True power factor is λ = cos φ ÷ √(1 + THDi²) (IEEE 1459), and no bank lifts it past P ÷ √(P² + D_I²): 100 kW at 40% THDi tops out at 0.88 however large the bank.
  • Correcting cos φ raises the measured THDi: the harmonic amps stay while the fundamental current falls.
  • Harmonic compliance is judged on voltage distortion and harmonic current, not true power factor. In Victoria the Distribution Code holds a site's harmonic current to its Table 5 and to IEEE 519-2014.
  • With no harmonic load the bank is standard capacitors. With drives, LED lighting or computers on the board each step gets a reactor in series, tuned below the lowest harmonic (a detuned bank), so it cannot resonate with the supply.
  • A detuned bank's capacitors run above the network voltage, U ÷ (1 − p): on 400 V with 7% detuning they see 430 V, are rated 480 V, and need about 100 kVAR of nameplate to deliver 75 kVAR at the busbar.

Who this page is for

Electrical engineers, designers, electricians, estimators and facilities managers sizing fixed and automatic capacitor banks to improve power factor, reduce kVA demand charges and free up consumer mains capacity on commercial and industrial installations across Australia and New Zealand, including sites with variable speed drives, UPS and other harmonic load.

Relevant standards

  • AS/NZS 3000:2018 Clause 4.15.2.3 (conductors to a capacitor controlled by a circuit-breaker: not less than 135% of the capacitor rated current, or the breaker setting)
  • AS/NZS 3000:2018 Clauses 4.15.2.2 and 4.15.3.1 (switching suitable for capacitors, such as AC-6b contactors; discharge to 50 V within 1 minute)
  • AS/NZS 3008.1.1:2025 (Cable Selection: used to size the capacitor bank feeder cable and protective device)
  • IEEE 1459 (power definitions: power factor, displacement power factor and current distortion power)
  • IEC 60076-5 Table 1 (minimum transformer short-circuit impedance, used when the nameplate %Z is not entered)
  • AS 60038 (Standard Voltages: 230 V / 400 V nominal)
  • IEC 60831-1 (shunt power capacitors for networks up to 1000 V)

What this tool helps with

  • Calculates the required capacitive reactive power Qc = kW × (tanφ₁ − tanφ₂) in kVAR to move from the present displacement power factor to a target, with the load entered in kW, kVA or line amps.
  • Rounds the required kVAR up to the next standard capacitor bank size from the common 1 to 600 kVAR ladder so the recommendation matches real switchable bank steps.
  • Sizes the installed bank: capacitance per phase in microfarads, the bank current Ic and the 135% design current of AS/NZS 3000 Clause 4.15.2.3, and auto-sizes the bank feeder cable and breaker to AS/NZS 3008.1.1:2025 (copper, X-90 XLPE, unenclosed and spaced, 20 m, 3% maximum voltage drop).
  • Works true power factor from a measured true PF or current THD: λ = cos φ ÷ √(1 + THDi²) before and after the bank, the ceiling a capacitor bank alone cannot pass, and the active harmonic filter current needed to meet a harmonic limit (Victoria's Distribution Code Table 5, a THDv limit or amps per harmonic) or a true PF target.
  • Chooses a standard, heavy-duty, 7% detuned (189 Hz) or 14% detuned (134 Hz) bank by the industry's selection rule (harmonic-generating load Gh against transformer rating Sn), and detunes wherever a standard bank would resonate with the transformer, h = √(Ssc ÷ Qc), at the 5th, 7th, 11th or 13th harmonic.
  • Specifies a detuned bank the way it is bought: capacitor voltage U ÷ (1 − p), the series reactor in mH per phase, and the capacitor nameplate Qrated = Qbus × (1 − p) × (Urated ÷ U)² at 440, 480 and 525 V.
  • Draws the single line diagram (transformer, the bank with its reactor, any active filter, the loads as motors, drives or electronic loads) and the power triangle, and exports a PDF report with the full working.

How to size a power factor correction bank

  1. Describe the board: Under The board, enter the load in kW, or switch the unit to kVA or line amps and type the meter reading, pick 1-phase 230 V or 3-phase 400 V, and enter the power factor now (cos φ) from the meter or the load schedule.
  2. Correct the power factor: Leave Correct the power factor on and type the target, any value up to 1.00; 0.95 to 0.98 is usual. Switch it off where only harmonics are to be dealt with.
  3. Switch on harmonics (optional): For a board with drives, LED lighting, computers or UPS, switch Harmonics on, pick what the load mostly is and enter how much of it there is. The current THD is estimated from that load, or entered from a power quality meter (THD or true PF) for an existing site.
  4. Choose a harmonic limit (optional): Pick the limit the site is held to (Victoria's Table 5, a THDv limit or amps per harmonic) and enter the distributor's fault level to size an active filter.
  5. Read what to install: The result gives the required kVAR and the bank size, the capacitors to buy (rated voltage and nameplate kVAR, at 440, 480 and 525 V for a detuned bank), the reactors, any active filter, the 135% design current of AS/NZS 3000 Clause 4.15.2.3 and the bank feeder cable and breaker, with why the bank is standard or has reactors and the true power factor before and after.
  6. Export the report: Export the PDF report with the single line diagram, the power triangle and the full working.

Power factor correction, true power factor and detuned capacitor banks for Australian installations

Why power factor correction matters

A low power factor (cosφ below 0.90) means the installation draws more apparent power (kVA) than real power (kW), increasing the current in the consumer mains, the I²R losses, the voltage drop and the kVA capacity charged by the network operator. Distributors set a minimum: in Victoria the Electricity Distribution Code of Practice (Table 4) asks low-voltage customers for 0.8 up to 2 MVA and 0.85 above, and a tariff that bills kVA demand rewards going higher.

Power factor correction adds capacitive reactive power (kVAR) to offset the inductive kVAR drawn by motors, transformers and other reactive loads, bringing cosφ closer to unity. Correctly sized PFC reduces the kVA demand and the line current for the same real power output.

The PFC sizing formula

The required capacitive reactive power is Qc = P × (tanφ₁ − tanφ₂) where P is the real power in kW, φ₁ is the present displacement power factor angle and φ₂ the target. For P = 100 kW, original cosφ = 0.75 (tanφ = 0.882), target cosφ = 0.95 (tanφ = 0.329), Qc = 100 × (0.882 − 0.329) = 55.3 kVAR.

The load can be entered in kW, or as the kVA or line current a meter reads: P = S × λ, or P = √3 × V × I × λ for three-phase, with λ the true power factor where one is entered, because a meter's kVA and amps include the harmonics. The calculator works the power triangle (initial and target apparent power and reactive power, the correction Qc, and the kVA and current saved) and rounds Qc up to the next standard bank size from the common ladder (1, 2.5, 5, 7.5, 10 … 600 kVAR).

Displacement power factor and true power factor

A capacitor bank corrects displacement power factor: cos φ, the phase shift between voltage and current at the 50 Hz fundamental. The bank is always sized on it, so enter the displacement power factor (cos φ or DPF on a power-quality meter) as the present and target PF. True power factor is real power over total apparent power with harmonics included, λ = P ÷ S, which IEEE 1459 and IEC call simply the power factor and which is also called the total power factor. On a site with non-linear loads (variable speed drives, rectifiers, UPS, LED drivers, switch-mode supplies) it is lower than cos φ.

With a sinusoidal supply voltage the apparent power splits as S² = P² + Q² + D_I², where D_I = S₁ × THDi is the current distortion power of IEEE 1459, carried by the harmonic current. True power factor is then λ = cos φ ÷ √(1 + THDi²). The bank removes Q and leaves D_I, so the true power factor after correction is P ÷ √(S₂² + D_I²), and the most any bank can reach, at cos φ = 1, is P ÷ √(P² + D_I²). For 100 kW at cos φ 0.75 and 40% THDi, true PF is 0.70 today, 0.87 after correcting cos φ to 0.98, and no bank passes 0.88. The harmonic current is taken as unchanged by the bank.

Correcting cos φ also raises the THDi a meter reports, because the harmonic amps stay while the fundamental falls: 40% becomes 52% in the example. Supply compliance is judged on harmonic distortion, not on true power factor, so the calculator sizes the filter to the harmonic limit the site is held to. It is worked at the entered load; rate the filter on the worst load the board carries.

Active harmonic filters: sized to the harmonic limit

In Australia harmonic compliance is set on voltage distortion and harmonic current, not on true power factor. The National Electricity Rules (S5.1a.6) hold network voltage distortion to the compatibility levels of IEC/TR 61000.3.6, and distributors allocate each site a share: a voltage distortion (THDv) limit, or amps per harmonic order. In Victoria the Electricity Distribution Code of Practice (cl 20.6.3, Table 5) holds a customer's harmonic current to a percentage of its maximum demand current, set by the ratio of the short-circuit current to that current, and to IEEE 519-2014 at the point of supply.

The calculator spreads the measured harmonic current over the orders as a typical spectrum for the load gives them (six-pulse drives with or without a choke, 12-pulse drives, single-phase rectifiers), then sizes the filter to bring each order, or the voltage distortion, within the limit. Harmonic voltage is estimated as V_h = k × h × X₁ × I_h on the network short-circuit reactance, with k = 2 as the usual allowance. The filter carries the rms of the current it removes.

Filter makers ask for a line reactor or DC choke of at least 3% on every drive, no un-isolated capacitors downstream of the filter's CTs, and a feeder breaker of at least 125% of the unit rating. The harmonic check stops at the highest order the spectrum gives (the 19th for drives), and the fault level is the distributor's.

A capacitor bank and an active filter together

An active filter can correct displacement power factor as well as remove harmonics. Doing both, it carries √(Ih² + Iq²): where the reactive current is no more than the harmonic current, that adds at most 41% to the filter, and the calculator recommends the filter alone, with no capacitors, no switching and no resonance. A 400 kW drive load at cos φ 0.98 is that case.

Where the reactive need is large against the harmonics, such as a site at cos φ 0.75 with modest distortion, taking it on the filter multiplies the filter's rating, and a detuned bank is the cheaper source of kVAR. The bank then sits where it is not downstream of the filter's CTs, which filter makers ask for: the calculator puts those CTs on the load side, past the bank. The page offers either choice.

Standard or detuned: the selection rule and resonance

The bank type follows the industry's published selection rule. Gh, the kVA of the harmonic-generating load on the board (drives, rectifiers, UPS; a device quoted in kW taken at PF 0.7), is set against Sn, the rating of the transformers feeding it: up to 10% of Sn, standard capacitors; 10 to 20%, heavy-duty capacitors or capacitors rated 10% above the network; 20 to 50%, capacitors rated 20% above with a detuned reactor; above 50%, harmonic filtering as well. Above 2 MVA the same rule is applied against the short-circuit power, at Ssc/120, Ssc/70 and Ssc/30.

A standard capacitor bank and the supply transformer form a parallel resonant circuit at h = √(Ssc ÷ Qc), where Ssc is the short-circuit power (the transformer kVA divided by its per-unit impedance, the IEC 60076-5 minimum for the rating where the nameplate %Z is not known) and Qc the bank kVAR. A 400 kVAR bank on a 1000 kVA, 5% transformer resonates near the 7th harmonic. An automatic bank switching its stages in sweeps that order down from high to its full-bank value, so every characteristic harmonic above it (5th, 7th, 11th, 13th from six-pulse drives, plus the 3rd and 9th where single-phase non-linear load dominates) is crossed. Because the resonance moves with the network and with every step the bank switches, the calculator gives every bank on a board with harmonics a reactor in series, tuned below the lowest harmonic the load makes, rather than relying on the resonance missing it.

Detuning is 7% (189 Hz, below the 5th) for six-pulse drives and rectifiers, or 14% (134 Hz, below the 3rd) with a high level of 3rd harmonic. A detuned bank is inductive above its tuning frequency, so it cannot resonate at the harmonics it is tuned below. It also tends to leave audio-frequency ripple control signals alone, where a standard bank can absorb them, so check the network's service and installation rules before choosing a standard bank.

Rating a detuned bank's capacitors

The series reactor raises the voltage across the capacitor to U ÷ (1 − p): 430 V on a 400 V board with 7% detuning, 465 V with 14%. The published practice rates a detuned bank's capacitors 20% above the network voltage: 480 V on a 400 V board, for 7% and 14% alike. Makers also pair 7% with 440 V or 525 V capacitors, so the calculator recommends 480 V and gives the nameplate at the three ratings at or above the capacitor's voltage, to match a catalogue.

A capacitor delivers its nameplate kVAR only at its rated voltage, so a detuned bank needs more nameplate than it delivers at the busbar: Qrated = Qbus × (1 − p) × (Urated ÷ U)². Delivering 75 kVAR at 400 V with 7% detuning takes 84.4 kVAR of 440 V capacitors, 100.4 kVAR of 480 V or 120.2 kVAR of 525 V. The calculator reports those with the series reactor per phase, XL = p × XC, in millihenries.

Delta capacitance, bank current and the design-current uplift

For a delta-connected bank the per-phase capacitance is C = Q / (3 × V² × ω) where V is the line voltage and ω = 2πf. The calculator works it for the installed standard bank, not the bare requirement Qc, and reports it in microfarads (µF): correcting 100 kW from 0.75 to 0.95 installs a 60 kVAR bank, about 398 µF per phase at 400 V and 50 Hz (a detuned bank's capacitance is that times 1 − p). The bank current Ic = (kVAR × 1000) / (√3 × V) is taken from the same installed bank.

A bank draws more than its nominal current in service, from capacitance tolerance, overvoltage and harmonic current: capacitors are designed to carry 1.3 times their rated current, with the parts in series oversized by 1.3 to 1.5 times. AS/NZS 3000:2018 Clause 4.15.2.3 sets the allowance for the conductors: where a capacitor is controlled by a circuit breaker, they must carry not less than 135% of the capacitor rated current, or the breaker setting, whichever is greater. The calculator applies that 135% to Ic to give the design current, and sizes the bank feeder cable and circuit breaker on it.

Automatic bank cable and breaker sizing to AS/NZS 3008.1.1

From the 135% design current the calculator auto-sizes a compliant bank feeder cable and protective device to AS/NZS 3008.1.1:2025 using a fixed reference install: copper conductor, X-90 XLPE insulation, unenclosed and spaced, a 20 m run and a 3% maximum voltage drop. The result names the cable (for example 4C 16 mm² Cu XLPE (X-90) + earth) and the matching breaker rating.

This inline estimate is intended for early sizing. For a project-specific result that accounts for the real installation method, grouping and ambient derating, the prospective fault level, the adiabatic short-circuit withstand check and the earth-fault loop impedance, hand the design off to the full ScaleSet Cable Selection calculator.

Connecting the bank: packaged PFC panel

For network-charge correction the bank is connected at the main switchboard. The single line diagram on the page shows a packaged PFC panel (a regulator, contactors and the capacitor bank, with its reactors where detuned) tapping the MSB busbar through its own circuit breaker and feeder cable, with the incomer current transformer (CT) signal feeding the regulator so the bank switches to suit the measured load. An active harmonic filter, where one is needed, is drawn on its own breaker, reading its own set of CTs, separate from the regulator's CT: on the incomer when it stands alone, and on the load side past the bank and the filter when there is a bank, so the bank is not downstream of them.

AS/NZS 3000:2018 adds two requirements for the bank itself: the circuit breakers, switches or contactors that control it must be suitable for switching capacitors (AS/NZS 3000 Clause 4.15.2.2, for example AC-6b contactors to AS/NZS 60947.4.1), and it must discharge to 50 V or less within 1 minute of disconnection (AS/NZS 3000 Clause 4.15.3.1, for capacitors rated up to 650 V). Where the load varies, an automatic stepped bank switches stages in and out to avoid a leading power factor at light load.

Power factor terms

Displacement power factor (cos φ, DPF)

The cosine of the phase angle between voltage and current at the fundamental frequency (50 Hz in Australia). IEEE 1459 calls it the fundamental power factor, P₁ ÷ S₁. It is the figure a capacitor bank changes, and the one the bank is sized on.

True power factor (λ)

Real power divided by total apparent power, harmonics included: λ = P ÷ S. IEEE 1459 and IEC call it simply the power factor; it is also called the total power factor. With a sinusoidal supply voltage it is cos φ ÷ √(1 + THDi²), the displacement power factor times the distortion factor, so it is never higher than cos φ.

Current THD (THDi)

The rms of every harmonic current as a fraction of the fundamental current (THD-F). Read it at full load from a power-quality meter; some meters also offer THD-R, a fraction of the total rms, which reads lower.

Current distortion power (D_I)

The part of the apparent power the harmonic current carries, D_I = S₁ × THDi (IEEE 1459). With a sinusoidal voltage, S² = P² + Q² + D_I². A capacitor bank removes Q and leaves D_I.

Reactive power (kVAR)

The power that oscillates between source and load in inductive equipment (motors, transformers) without doing useful work, but which still loads cables and transformers.

Capacitor bank

Capacitor stages installed at the switchboard, switched in and out by a regulator, to supply reactive power locally and raise the displacement power factor seen by the supply.

Gh/Sn

The selection ratio for a capacitor bank: Gh, the kVA of all harmonic-generating devices on the board (drives, rectifiers, UPS; a device quoted in kW taken at PF 0.7), over Sn, the kVA of the transformers feeding it. It decides standard, heavy-duty or detuned capacitors, or harmonic filtering.

Heavy-duty capacitor

A capacitor built to carry more overcurrent than the standard requires, or one rated 10% above the network voltage, for boards where the harmonic load is between 10 and 20% of the transformer rating.

Detuned capacitor bank

A bank with a series reactor on each step, sized as a fraction p of the capacitor reactance so the step is tuned below the lowest significant harmonic: 7% tunes to 189 Hz at 50 Hz, 14% to 134 Hz. Above its tuning frequency the step is inductive, so it cannot resonate with the supply there.

Parallel resonance

The harmonic order at which a capacitor bank and the supply transformer's inductance resonate, h = √(Ssc ÷ Qc). Harmonic currents near that order are amplified, overloading the capacitors and distorting the voltage.

Active harmonic filter

A power-electronic unit that measures the load's harmonic currents and injects the opposite, so the supply carries less of them. It can correct displacement power factor as well, and then carries √(Ih² + Iq²). It reads its own CTs, to the accuracy its maker sets, separate from the PF regulator's CT and with no un-isolated capacitors downstream of them, and is fed on its own breaker of at least 125% of the unit rating.

Methods reviewed by

Wisam Tozah (CPEng, NER, MIEAust) for ScaleSet Pty Ltd. The methods are tested against the standards' published tables and worked examples before every release. See how they are verified. ScaleSet is a design aid: your inputs and results are not reviewed or certified, and responsibility for the design stays with the engineer who issues it. LinkedIn. Updated .

Power factor correction questions

How do I calculate the kVAR needed for power factor correction?

Required capacitor kVAR is Qc = kW × (tanφ₁ − tanφ₂), where φ₁ is the angle of the existing displacement power factor and φ₂ the angle of the target. For a 100 kW load corrected from 0.75 (tanφ = 0.882) to 0.95 (tanφ = 0.329), Qc = 100 × (0.882 − 0.329) = 55.3 kVAR. The calculator works this out, then rounds it up to the next standard bank size on a 1 to 600 kVAR ladder: 60 kVAR here.

What target power factor should I aim for to avoid a power factor penalty in Australia?

Aim for 0.95 lagging. Many Australian network tariffs bill demand in kVA or apply a power factor penalty below 0.90, so 0.95 clears the usual threshold with margin without over-correcting. Correcting to unity, or into a leading power factor at light load, risks overvoltage and resonance. Check how your meter derives kVA: where it counts harmonic current, the true power factor is the figure billed, and a capacitor bank alone may not reach it.

Where should a power factor correction capacitor bank be installed?

To reduce network demand charges, install it at the main switchboard (global correction): a packaged PFC panel tapping the MSB busbar through its own circuit breaker, with its regulator reading a CT on the incomer, as in the calculator's single line diagram. To also cut losses in the internal cabling, correct close to large inductive loads such as motors and transformers (local correction). Where drives, rectifiers or other harmonic load is significant, the bank is detuned; the calculator chooses it from the non-linear load against the transformer rating.

Why is a capacitor bank cable sized for 135% of the bank current?

Because AS/NZS 3000:2018 Clause 4.15.2.3 requires it: where a capacitor is controlled by a circuit breaker, its conductors must carry not less than the greater of 135% of the capacitor rated current or the breaker setting. A bank draws more than its nominal current in service, from capacitance tolerance, overvoltage and harmonic current; capacitors themselves are designed to carry 1.3 times their rated current, with the parts in series oversized by 1.3 to 1.5 times. The calculator applies the Wiring Rules figure of 1.35 to the current of the installed standard bank.

How much does power factor correction reduce kVA demand and current?

At the same kW, the new kVA and line current are the old figures × (old power factor ÷ new power factor). Moving 100 kW from 0.75 to 0.95 drops apparent power from 133.3 kVA to 105.3 kVA, a 21% cut in both kVA demand and line current. On a site with harmonic current the cut in rms current is smaller, because the harmonic amps stay; the calculator reports both once the true PF or THD is entered.

What is the difference between displacement power factor and true power factor?

Displacement power factor is cos φ, the phase angle between voltage and current at the 50 Hz fundamental. True power factor is real power over total apparent power with harmonics included, λ = P ÷ S; IEEE 1459 and IEC call it simply the power factor, and it is also called the total power factor. With a sinusoidal supply voltage λ = cos φ ÷ √(1 + THDi²): at cos φ 0.75 and 40% current THD, true PF is 0.70. A capacitor bank changes cos φ only.

Does this calculator work out true power factor?

Yes, once Harmonics is switched on. The current THD is estimated from the non-linear load for a new design, or entered from a power quality meter for an existing site. The capacitor bank is always sized on displacement power factor, because that is the only part a capacitor corrects. The calculator works true power factor before and after the bank, the most a bank alone can reach, and the active harmonic filter current needed to meet a harmonic limit. It is a design estimate at one load; a site with large or varied non-linear load still warrants a harmonic study.

Why can a capacitor bank not reach 0.95 true power factor on a site with drives?

Because the bank removes reactive power and leaves the harmonic current. The best it can do, at cos φ = 1, is P ÷ √(P² + D_I²), where D_I = S₁ × THDi is the current distortion power. For 100 kW at cos φ 0.75 and 40% THDi that ceiling is 0.88. Going higher takes an active harmonic filter, which the calculator sizes to the harmonic limit the site is held to.

When does a capacitor bank need to be detuned?

By the industry's selection rule, when the harmonic-generating load (Gh: drives, rectifiers, UPS) is more than 20% of the transformer rating (Sn): standard capacitors up to 10%, heavy duty from 10 to 20%, a detuned bank from 20 to 50%, and harmonic filtering as well above 50%. Above 2 MVA the bands are set against the short-circuit power instead. A bank is also detuned whenever a standard bank would resonate with the transformer at the 5th, 7th, 11th or 13th harmonic, h = √(Ssc ÷ Qc). This calculator gives a bank reactors whenever harmonics are switched on, the safe side of both rules.

What do 7% (189 Hz) and 14% (134 Hz) detuning mean?

The percentage is the reactor's reactance as a fraction p of the capacitor's, and it sets the frequency the step is tuned to: f = 50 ÷ √p, so 7% gives 189 Hz, just below the 5th harmonic (250 Hz), and 14% gives 134 Hz, below the 3rd (150 Hz). Above that frequency the step is inductive, so it cannot resonate with the transformer there. 7% suits six-pulse drives and rectifiers; 14% is used where the 3rd harmonic is high, from single-phase electronics.

Why does a detuned capacitor bank need more kVAR than it delivers?

The series reactor raises the voltage across the capacitors to U ÷ (1 − p), so they are rated above the supply voltage (480 V on a 400 V board, 20% above the network, for 7% and 14% alike), and a capacitor gives its nameplate kVAR only at its rated voltage. The nameplate needed is Qrated = Qbus × (1 − p) × (Urated ÷ U)²: 75 kVAR at the busbar with 7% detuning and 480 V capacitors takes 100.4 kVAR of nameplate. Buying 75 kVAR of 480 V capacitors would deliver only about 56 kVAR.

Can I enter the load in amps or kVA instead of kW?

Yes. Switch the load's unit to kVA or A and type the meter reading. The calculator turns it into kW through the power factor: P = S × λ for kVA, P = √3 × V × I × λ for three-phase amps (V × I × λ single-phase). It uses the true power factor where one is entered, because a meter's kVA and amps include the harmonic current, and shows the kW it works from under the field.

How do I calculate capacitance in microfarads for a single-phase or three-phase capacitor bank?

For a three-phase delta bank the capacitance per phase is C = Qc ÷ (3 × V² × 2πf), with Qc in VAR and V the line voltage; for a single-phase capacitor it is C = Qc ÷ (V² × 2πf). The calculator reports C in microfarads for the installed standard bank, at 400 V three-phase or 230 V single-phase and 50 Hz. A 60 kVAR delta bank at 400 V is about 398 µF per phase; a detuned bank's is that times 1 − p.

How do I calculate capacitor bank current and the cable size it needs?

Bank current is Ic = kVAR × 1000 ÷ (√3 × V) for three-phase, or kVAR × 1000 ÷ V for single-phase, taken from the installed standard bank size; the cable and breaker are then sized for 1.35 × Ic, as AS/NZS 3000:2018 Clause 4.15.2.3 requires. A 60 kVAR bank at 400 V draws 86.6 A, so its design current is 116.9 A. The calculator auto-sizes the feeder cable and breaker to AS/NZS 3008.1.1:2025 on a fixed reference installation (copper, X-90 XLPE, unenclosed and spaced, 20 m, 3% maximum voltage drop). For the real installation method, fault level and earth fault loop, carry the design current into the full Cable Selection calculator.

What is an active harmonic filter sized to in Australia?

The harmonic limit the site is held to, not a true power factor. Distributors allocate a voltage distortion (THDv) limit or amps per harmonic order, under the National Electricity Rules and IEC/TR 61000.3.6; in Victoria the Electricity Distribution Code (cl 20.6.3, Table 5) holds harmonic current to a percentage of the maximum demand current and to IEEE 519-2014. The calculator sizes the filter to any of the three, order by order.

Do I need a capacitor bank as well as an active filter?

It depends on which job is bigger. An active filter can correct displacement power factor as well as remove harmonics, carrying √(Ih² + Iq²), so where the reactive current is no more than the harmonic current the filter alone does both and no bank is installed, as on a drive-heavy board already near unity cos φ. Where the reactive need is large against the harmonics, as on a mixed motor and drive site at a poor cos φ, a detuned bank is the cheaper source of kVAR beside a smaller filter, placed so it is not downstream of the filter's CTs. The page offers either choice.

Can a capacitor bank over-correct power factor?

Yes. A fixed bank sized for full load leaves the site at a leading power factor when the load drops, which raises the voltage and, on a generator supply, can push the alternator outside its excitation range and trip it. A capacitor fitted at a motor and sized above its magnetising current can also self-excite the motor after it is switched off. Size to a realistic target such as 0.95 rather than unity, and use an automatic stepped bank that switches stages in and out as the load changes.

Where the capacitor bank goes next