Detuned Reactor 5.67%, 7% or 14%: How to Choose
Choose 5.67%, 7% or 14% only after measuring the harmonic spectrum and identifying the supply frequency. A 7% reactor is commonly used to place a capacitor step below the fifth harmonic on a 60 Hz system; 14% sits below the third, while 5.67% sits nearer the fifth. The detuned reactor percentage must follow measured conditions, not a generic panel preference.
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By Eng. Ahmed Ali, Saudi Power Factor Company. Published 19 October 2026.
What does a detuned reactor percentage mean?
A detuned reactor is an inductor connected in series with the capacitor on each correction step. Its percentage describes the reactor impedance relative to the capacitor impedance at the supply frequency. That percentage sets the tuning point of the reactor-capacitor pair.
The aim is not to tune a capacitor bank into a harmonic that the plant produces. The aim is to move the combined resonance below the lowest harmonic of concern, so the step does not amplify current at that order. This is why the percentage cannot be selected from the capacitor kVAr rating alone.
On a 60 Hz system, the approximate tuning order is calculated as one divided by the square root of the percentage expressed as a decimal. The tuning frequency is that order multiplied by 60 Hz.
A detuned reactor percentage describes electrical tuning, not the percentage of plant load covered by the bank. The bank’s kVAr rating answers the correction requirement.
How do 5.67%, 7% and 14% reactors compare?
The three options place the reactor-capacitor step at different frequencies. The figures below use a 60 Hz supply and are rounded. They show why the same capacitor bank can need a different reactor depending on whether the third or fifth harmonic is the main concern.
| Reactor percentage | Approximate tuning order | Approximate tuning frequency at 60 Hz | Position relative to harmonic orders |
|---|---|---|---|
| 5.67% | 4.20 | 252 Hz | Below the fifth harmonic at 300 Hz |
| 7% | 3.78 | 227 Hz | Between the third and fifth harmonics |
| 14% | 2.67 | 160 Hz | Below the third harmonic at 180 Hz |
A 5.67% reactor is sometimes considered where the fifth harmonic is important and the third is not. A 7% reactor is often selected for typical drive and rectifier duty because its tuning point is below the fifth harmonic. A 14% reactor is considered where third-harmonic current is significant and the step needs to sit below it.
Measurement still decides the selection. The percentage also affects capacitor voltage and reactor losses, so the component ratings have to be checked together.
When is 7% usually considered for a capacitor bank?
A 7% reactor is commonly considered when six-pulse drives, rectifiers or similar loads make the fifth harmonic the main concern. On a 60 Hz system, the fifth harmonic is 300 Hz, while a 7% step tunes at about 227 Hz. The step is therefore placed between the third and fifth harmonic orders rather than at the fifth.
Not every site with a drive requires a 7% reactor. The result changes with the drive load, transformer and operating pattern.
When assessing a 7% option, record the measured harmonic components and any existing capacitor-step failures or overheating.
The detuned capacitor banks service covers automatic correction panels with reactor-protected steps. The individual detuned reactors page explains their place in a capacitor bank and the information needed for a specification.
When should 14% or 5.67% be considered?
A 14% reactor is considered where third-harmonic conditions are important. On a 60 Hz supply, its tuning point is about 160 Hz, below the third harmonic at 180 Hz. Third-harmonic current can behave differently from fifth-harmonic current because it may be associated with particular load arrangements and supply paths. The measured data must show whether this is the relevant issue.
A 5.67% reactor tunes at about 252 Hz on the same supply. It may be assessed where the fifth is the dominant concern and the third is small. Capacitor voltage, reactor current and expected step loading still need checking.
Neither option removes the need for a survey. The percentage should be part of a complete electrical design, not an isolated purchase item.
What does an assumed example show about tuning?
The following is a worked example with assumed values only. A 60 Hz plant has a 400 kVAr automatic capacitor bank planned for a busbar with drive loads. A measurement indicates that the fifth harmonic requires attention while third-harmonic current is small. This example explains the comparison; it is not a site design or a required panel rating.
| Example item | Assumed value | What it indicates |
|---|---|---|
| Supply frequency | 60 Hz | Use the 60 Hz tuning calculation |
| Required correction | 400 kVAr | Determines the capacitor-bank duty, not tuning alone |
| Dominant concern | Fifth harmonic | Compare reactor tuning below 300 Hz |
| 5.67% tuning | about 252 Hz | Below the fifth harmonic |
| 7% tuning | about 227 Hz | Further below the fifth harmonic |
| 14% tuning | about 160 Hz | Below the third harmonic as well |
For this assumed spectrum, a designer may assess a 5.67% or 7% option because both tuning points sit below the fifth harmonic. Transformer impedance, capacitor voltage rating and step arrangement still need review.
If third-harmonic current were the main concern, the 14% option would need consideration because it tunes below 180 Hz. The tuning percentage follows the measured order to be avoided.
For a related explanation of detuned and standard banks, read Detuned vs Standard Capacitor Bank: Which One You Need. It sets out why series reactors are required when capacitors share a supply with harmonic-producing loads.
What should you check before specifying a reactor?
Start with a short, documented review rather than a target percentage. The following checklist keeps the specification focused on the supply conditions that affect tuning.
- Confirm supply frequency and voltage. The tuning calculation and equipment voltage ratings depend on the actual supply.
- Obtain the harmonic spectrum. Record the relevant harmonic orders and their variation across operating periods.
- List non-linear loads. Include drives, UPS units, rectifiers, welding equipment and any planned additions.
- Check the transformer and network data. Transformer rating, impedance and the position of existing capacitors affect resonance behaviour.
- Review the correction requirement. Use the bill and load data to establish the kVAr duty and suitable step sizes.
- Inspect existing capacitor steps. Note capacitor ratings, contactors, fuses, reactors and any signs of overheating before reusing equipment.
A detuned bank provides reactive-power correction without exposing capacitor steps to an unsuitable resonant condition. If measured distortion needs reduction, a separate filtering solution may also be required.
The detuned reactor percentage should be the result of that review: 5.67%, 7% or 14% only after the harmonic data and equipment ratings point to the appropriate tuning range.
What are common detuned reactor questions?
Is a 7% detuned reactor the same as a harmonic filter?
No. A 7% reactor protects a capacitor step by moving its resonance below the fifth harmonic on a 60 Hz supply. It does not replace a filter designed to reduce a measured harmonic level at a specified point in the electrical system.
Can a 14% reactor be used for every capacitor bank?
No. A 14% reactor is chosen for a particular tuning purpose, including conditions where the third harmonic is important. It also changes the voltage and thermal duty seen by the capacitor and reactor, so its use must match the measured system conditions.
Why is 5.67% different from 7%?
The two percentages set different tuning frequencies. At 60 Hz, 5.67% tunes at about 252 Hz and 7% at about 227 Hz. Both are below the fifth harmonic, but the appropriate option depends on the measured spectrum and the network design.
Do I need measurements before replacing a failed reactor?
Yes. A failed reactor or capacitor step can be a symptom of harmonic loading, incorrect tuning, insufficient ratings or another condition on the busbar. Reviewing the measurement data before replacement helps avoid repeating the same duty problem.
What information is needed for a quotation?
Send one month of the electricity bill, the supply voltage and frequency, transformer information, a single-line diagram if available, and a list of drives or other non-linear loads. Harmonic measurements are also useful where they have already been taken.
Need a detuned reactor percentage checked for your capacitor bank? Send us one month of your electricity bill and receive a quotation within one working day. Email inquiry@saudipowerfactor.com or request a quotation.