Transformateurs à facteur K

A Rex K-Factor Rated Transformer is designed to handle harmonic currents generated by non-linear loads without exceeding its rated temperature rise.

K4 is suitable for general commercial loads with moderate harmonic distortion, while K9 and K13 are commonly used for VFDs, servers, UPS systems, and electronic equipment. K20 and K30 may be required for highly concentrated non-linear loads with significant harmonic content.

The appropriate rating depends on the load’s measured harmonic spectrum, not just total harmonic distortion (THD). For applications requiring K20 or K30, a Harmonic Mitigating Transformer may offer an alternative by reducing harmonic currents rather than simply tolerating them.

Choosing a K-factor rating below your load’s harmonic requirements can cause excessive winding temperatures, insulation deterioration, and premature transformer failure. It may also increase energy losses and reduce equipment reliability.

To avoid these issues, select a Rex K-Factor Rated Transformer based on your system’s harmonic load profile, with additional capacity for future load growth.

A 150°C-rise K13 K-Factor Rated Transformer is a complete, listed design: the K rating means the unit reaches no more than its rated rise when carrying a K13 spectrum at full nameplate kVA in a 30°C average ambient. Pairing a lower rise with the K rating is not required for the rating to be valid. The 115°C (T115) and 80°C (T80) options are chosen for other reasons. A lower rise buys thermal margin against ambient above 30°C, against short-term overload during UPS bypass or generator transfer, and against a load spectrum that drifts upward as more electronics are added over the life of the building. It also lowers total losses at rated load, which reduces electrical-room cooling in a data hall or telecom room, and extends insulation life because the hot-spot temperature is lower. What T80 does not do is convert a K4 unit into a K13 unit; the nameplate K rating is what an inspector or a harmonic study will read, so the K rating should be specified to match the load and the rise chosen for margin and efficiency. Rex’s Transformer Insulation Classes article explains the 220°C system these options are built on.

A higher K-factor rating allows a transformer to handle greater harmonic loads without overheating. While higher-rated units may have a higher initial cost, their enhanced winding design helps manage harmonic-related losses and maintain reliable operation.

Choosing the correct Rex K-Factor Rated Transformer can reduce overheating, premature insulation deterioration, and potential maintenance costs. Actual energy efficiency and operating costs depend on the load’s harmonic profile, transformer design, and operating conditions.

K-Factor transformers are designed with increased conductor cross-section, reduced core flux density, and enhanced cooling to offset additional harmonic heating. The polyester resin coating improves thermal conduction between windings and the core, stabilizing temperature rise. Core and winding losses are carefully balanced to maintain rated output even under distorted current waveforms. This design approach prevents localized hot spots, ensuring consistent temperature distribution and extending insulation life under non-linear loading conditions.

Each transformer features high-grade electrical steel cores and precision-wound copper or aluminum conductors. The core and coils are dipped in polyester resin, which provides mechanical stability, superior insulation integrity, and moisture protection. Windings are arranged to minimize stray flux and reduce eddy-current losses caused by harmonics. The design operates at a lower flux density to manage thermal stress while maintaining high efficiency. These construction details ensure dependable service life even under severe harmonic loading and continuous-duty operation.

The K-Factor rating quantifies a transformer’s capability to carry harmonic currents safely. It is determined according to IEEE C57.110, which defines how much harmonic content the transformer can handle without excessive heating. Common ratings include K-4, K-9, K-13, and K-20, with higher numbers indicating greater tolerance to harmonic loads. For example, a K-13 transformer is suitable for most office and light industrial loads, while K-20 units are used in heavy drive or data center applications. Selecting the proper K-Factor ensures safe operation and minimizes the risk of insulation degradation or premature failure.

Rex Power Magnetics’ ventilated distribution transformer terminals are rated 90°C. Conductors with at least a 90°C insulation rating at or below their 90°C ampacity rating should be utilized.

Temperature rise refers to average increase of temperature of the transformer windings at full load above the ambient temperature. when operating at full load. In addition to the average temperature rise of the windings, transformers also experience a « hot spot » temperature, which refers to the highest temperature point in the windings.

For example, a transformer with a 220°C insulation system may be designed with a 150°C average temperature rise and a 30°C hot spot allowance. This means that Above a 40C ambient, the total absolute temperature will not exceed 220°C. Transformers with lower temperature insulation systems (180°C or 200°C) will be designed with lower temperature rises (115° or 130°C) and hot spots so they can be installed in the same ambient temperature and still not exceed the temperature rating of the insulation system.

The table below shows the maximum average winding temperature rise, maximum hot spot temperature rise and maximum winding temperature for the most common insulation classes. Note that these are based on a max average ambient of 30°C during any 24-hour period and a maximum ambient of 40°C at any time.

Customers occasionally specify a transformer of a particular insulation class to be designed with an average temperature rise below the average temperature rise values shown in the table above. The benefits of doing so include:

• Longer Transformer Life: Lower temperature rise means the transformer can operate at a lower overall temperature, extending its life expectancy.
• Handling Higher Ambient Temperatures: Transformers with lower temperature rise ratings can operate safely in higher ambient temperatures without exceeding their insulation limits.
• Increased Overload Capacity: These transformers can handle continuous or short-term overloads without overheating, making them ideal for environments where transformers may be subject to occasional overloading.

Unless designed for special service conditions / environments, below are the standard service conditions for dry type distribution transformers:

• Ambient Temperature: -30°C to + 30°C (max peak +40°C)
• Relative Humidity: less than 70%
• Altitude: up to 1000m (3300 ft.) above seal level

To ensure proper operation, avoid installing transformers in environments with excessive moisture, extreme temperatures, or direct sunlight. Maintain recommended clearances and keep all ventilation panels unobstructed.

Yes, in many cases. A Rex K-Factor Rated Transformer can replace a standard transformer if the voltage, kVA, phase configuration, and electrical connections are compatible. However, installation dimensions, ventilation, neutral conductor sizing, and electrical protection should be reviewed before replacement.

As close to the non-linear loads as practical, and dedicated to them. A K-Factor Rated Transformer does its best work as the last transformation before a panel of drives, IT equipment or UPS inputs: the delta primary traps the triplen harmonics from single-phase loads, the harmonic current is confined to short secondary feeders, and the rest of the building bus sees a cleaner current. Placing it far upstream, where it feeds a mix of lighting, HVAC and receptacles along with the electronics, dilutes the harmonic content and often means a lower K rating is sufficient, but it also spreads harmonic current across more of the distribution. The usual approach is to segregate: a K-rated unit for the electronics panel, a general purpose unit for linear loads, and where drives are grouped, a Drive Isolation Transformer dedicated to them. Keep the secondary feeder short to limit harmonic voltage drop, and place the unit in a ventilated room with the 150 mm clearance to walls and adjacent equipment that Rex specifies, since its losses under harmonic load run above the efficiency test value. For a data hall the transformer is normally at the PDU; see the Data Centers page. The Knowledge Hub’s Transformer Clearance Requirements covers room layout.

On a wye secondary feeding single-phase electronic loads, the 3rd, 9th and 15th harmonic currents of the three phases are in phase with each other and add in the neutral instead of cancelling. Under a K13 spectrum the neutral can carry as much current as a phase conductor, and in an extreme single-phase rectifier panel more. The neutral conductor from a K-Factor Rated Transformer should therefore be sized as a current-carrying conductor from the measured or expected neutral current, commonly at 173 to 200 percent of phase ampacity in a full-electronics panel, rather than reduced. The neutral terminal on the transformer must accept that conductor; specify the neutral rating and, where oversize lugs are needed, the Y2 special lugs option. The secondary is a separately derived system, so it is bonded and grounded once at the transformer or first disconnect in accordance with the Canadian Electrical Code or NEC 250, with the system bonding jumper sized to the secondary conductors and the grounding electrode conductor to the code table; the neutral must not be bonded again downstream. Where the unit has electrostatic shields, their ground leads land on the same enclosure ground bus. Rex’s Transformer Grounding and Bonding Explained article and the Wye vs. Delta article cover the connection choices.

K-Factor Rated Transformers require adequate airflow to dissipate heat generated during operation. Maintain at least 150 mm (6 inches) of clearance from adjacent surfaces, except approved mounting surfaces, and keep ventilation openings unobstructed. Electrical rooms should also provide sufficient cooling to accommodate transformer losses under harmonic loads.

Yes, and the mix usually lowers the required rating, because linear load adds fundamental current without harmonics. The composite K-factor is calculated from the combined current spectrum, not by averaging the K ratings of the loads. Worked example: a 208Y/120 V panel carries 60 A of linear load (fundamental only) and 40 A of fundamental from electronic loads whose spectrum is 35 percent 5th and 20 percent 7th. Taken alone, the electronic load has K = (1 + 0.1225 × 25 + 0.04 × 49) ÷ (1 + 0.1225 + 0.04) = 6.02 ÷ 1.16 = 5.2, which would call for a K9 unit. Combined, the fundamental is 100 A, the 5th is 14 A (0.14 pu) and the 7th is 8 A (0.08 pu), giving K = (1 + 0.0196 × 25 + 0.0064 × 49) ÷ (1 + 0.0196 + 0.0064) = 1.80 ÷ 1.03 = 1.8, comfortably within the standard K4 capacity of a Rex K-Factor Rated Transformer. Two cautions apply. The mix must be stable; if the linear load is removed or the electronic load grows, the K-factor rises quickly, so specifying one K step above the calculated value is common practice. And the wye-secondary neutral still carries the triplens from the electronic share, so it is sized from that current, not the composite. The How to Calculate Transformer Load article covers the base sizing.

The standard K-Factor Rated Transformer enclosure is ventilated Type 1 or 3R for indoor use in ANSI 61 grey. The Part Number Builder offers E3R for outdoor Type 3R, E4 for non-ventilated Type 4, E4X for Type 4X stainless, E12 for Type 12, ESP for a special enclosure, and C&C for core and coil only for mounting inside a customer’s cabinet, with 304 or 316L stainless steel available. For a dusty plant or mill, Type 12 keeps airborne dust off the coils while remaining indoor-rated. Outdoors, Type 3R with anti-condensation strip heaters is the usual choice and Type 4X stainless where salt or chemical spray is present. A non-ventilated Type 4 or 4X enclosure removes the convection cooling a ventilated core and coil depends on, so the transformer inside is derated or built larger, and with a K-rated unit whose losses under harmonic load are already higher, that step-up can be significant; Rex sizes the core and coil for the enclosure at quotation. For genuine washdown or corrosive service at 150 kVA and below, an Encapsulated Transformer ordered with a K4 to K20 option may be the better product. Dimensions for each enclosure code are on the Enclosure Drawings page, and the Thermal Management in Dry-Type Transformers article explains the cooling trade-off.

The general checks in Rex’s Maintenance, Operating and Installation Manual apply: verify nameplate against the drawings, megger primary and secondary to ground and to each other, confirm the unit is dry and above 0°C using the cold start procedure if it was stored cold, torque terminals, check clearances and ventilation, and set the primary tap from a measured supply voltage. For a K-Factor Rated Transformer add the following:

• Confirm the nameplate K rating matches the design basis in the harmonic study, and that the neutral conductor and terminal size match the specified neutral rating.
• Verify the system bonding jumper is installed once, at the transformer or first disconnect, and that any electrostatic shield leads are landed on the ground bus.
• Verify thermostat or thermal sensor contacts (N.O./N.C.) are wired to the alarm or BMS point and tested.
• After loading, take a true-RMS current reading on each phase and the neutral, and if a power quality analyzer is available, record the harmonic spectrum and calculated K-factor for comparison with the nameplate.
• Record winding temperature or enclosure exhaust temperature after several hours to establish a baseline.

Keep the readings with the test report. The Transformer Commissioning article and the IOMM give the full sequence.

Dry-type transformers can generally be connected in reverse (back-fed), but there are some precautions to consider:

• Compensated Windings: Control transformers and distribution transformers below 3kVA are typically designed with an overvoltage on the secondary to compensate for voltage regulation at full load. Reverse feeding these transformers may lead to a lower than expected output voltage.
• Inrush current: The inrush current when energizing a transformer from the intended secondary terminals will be significantly higher than on the primary side as a multiple of the rated current. This high inrush current can cause nuisance tripping of the protective breaker and special considerations may need to be made.
• Voltage Taps: Given that there are typically no voltage adjustment taps on the secondary side, the transformer cannot be adjusted to account for higher or lower than nominal incoming voltages. In order to not damage the insulation or overexcite the core, the input voltage should not exceed the nominal rated secondary voltage. In the case that the incoming voltage is lower than the nominal rated secondary voltage, the taps on the primary winding can be used to boost the output voltage.
• Grounding: When the secondary (wye) of a delta-wye transformer is energized instead of the primary (delta), then the wye side of the transformer is not a separately derived service. As such, the neutral should not be connected to building ground nor should it be bonded to the transformer enclosure.

Always review applicable codes and standards and consult with the local authority having jurisdiction before reverse feeding transformers.

In dry-type transformers, the surrounding air plays a critical role in their operation. Generally, low ambient temperatures do not negatively impact an energized transformer, as the no-load losses typically generate enough heat to maintain proper conditions, even in environments as cold as -30°C. However, transformers stored at low temperatures present two primary concerns:

• Insulation Brittleness: At low temperatures, the insulation in the coils may become brittle. Expanding conductors when a cold transformer is loaded, or contracting conductors during cold storage, may cause cracks in the insulation, leading to internal faults.
• Condensation Risk: Low temperatures can cause condensation inside the transformer enclosure, as well as on the transformer coils. Energizing a transformer with condensation present on the coils can lead to internal faults and insulation damage.

Rex Power Magnetics recommends testing transformers (megger testing), warming them to above 0°C, or following a drying-out procedure if moisture is suspected. Refer to Rex Power Magnetics’ cold start procedures to ensure safe energization in cold conditions. Energizing a transformer with compromised insulation due to moisture can cause damage and potential safety hazards.

Cold Start Procedure

The minimum required clearances of a dry type transformer to walls, floors or other equipment must adhere to the local electrical code requirements.

In the absence of such requirements, Rex Power Magnetics recommends that dry type transformers be mounted so that there is an air space of no less than 150mm (6”) between the enclosures, and between the enclosure and any adjacent surface except floors. When the adjacent surface is a combustible material, the minimum permissible separation between the transformer enclosure and the adjacent surface should be 300mm (12”). Where the adjacent surface is the wall on which the transformer is mounted, the minimum permissible separation between the enclosure and the mounting wall should be 6mm (0.25”) so long as the surface is of a non-combustible material.

Any transformer which is not installed and energized immediately should be stored in a dry, clean space having a uniform temperature to prevent condensation on the windings. Dry type transformers with resin dipped or epoxy vacuum impregnated coils can be stored at ambient temperatures as low as -50C. Transformers with encapsulated or epoxy cast coils should not be stored at ambient temperatures below -20C to prevent cracking of the epoxy. Preferably, transformers should be stored in a heated building having adequate air circulation and protected from cement, plaster, paint, dirt, and water or other gases, powders, and dust. The floor on which the transformer is being stored should be resistant to the upward migration of water vapor. Precautions should be taken to prevent storage in an area that water could be present, such as roof leaks, windows, etc. Condensation or absorption of moisture can be greatly reduced by keeping the transformer enclosure 5⁰C-10⁰C above ambient temperature. This can be easily achieved by the installation and energization of space heaters (optional). If the transformer is not furnished with internal space heaters, then external, portable heaters can be used. Note: Lamps or heaters should never come in direct contact with the transformer coil insulation.

It is not advisable to store a dry type transformer outdoors, but in the case that it is unavoidable, protective measures should be taken to prevent moisture and foreign debris from entering the transformer enclosure. The plastic wrapping supplied during shipment should be left in place, and a suitable drying agent such as silica gel packs should be used. The unit should also be checked periodically for indications of condensation on the windings, coil support blocks, core, core clamping system and bus/cables.

Rex designs its dry-type transformers, including the K-Factor Rated Transformer, on UL-listed 220°C insulation systems with a hot-spot allowance that gives a design life of at least 30 years at continuous rated load in standard ambient, and over 50 years at 115°C or 80°C rise. Because the K rating is defined at rated rise under the specified harmonic spectrum, a K13 unit on a K13 load ages at the same rate as a general purpose unit on a linear load: the harmonic heating has been designed in. A standard K1 unit derated per IEEE C57.110 to carry the same load can in principle match that life, but only if the derating was calculated from the unit’s actual eddy-loss ratio and the load never grows beyond the derated value, and it leaves a fraction of the nameplate unusable. The K-rated unit also has margin in the places a derated standard unit does not: an oversized neutral connection, conductor strands that limit localized eddy heating in the outer layers, and a core with headroom against harmonic flux. Field experience with overheated standard transformers on electronic loads is what created the K-factor category. Life is shortened by the same things as any dry-type unit: high ambient, blocked ventilation, sustained overload and moisture. The Transformer Failures article catalogues the failure modes.

Selecting the right K-Factor depends on the harmonic content of the connected load. For typical office or light commercial systems, a K-4 or K-9 transformer is sufficient. For installations with a high concentration of drives, rectifiers, or electronic power supplies, K-13 or higher may be required. Rex Power Magnetics provides engineering assistance to analyze load profiles and recommend the proper rating. Choosing the correct K-Factor optimizes transformer size, efficiency, and cost while ensuring long-term reliability.

The life expectancy of a dry-type transformer is primarily determined by the insulation system and the operating temperature. According to IEEE Std. C57.96, the deterioration of insulation is directly related to the time and temperature the transformer experiences during operation. Insulation materials degrade faster at higher temperatures, so the transformer’s life expectancy is closely tied to how well it is kept within its design temperature limits.

In most transformers, the highest temperature occurs at a specific point in the windings, known as the hot spot. This area undergoes the most significant wear over time, making it the primary factor in determining the transformer’s ageing process.

All of Rex Power Magnetics’ dry-type transformers are designed using UL-listed insulation systems with a maximum hot spot temperature that ensures a design life of at least 30 years under standard operating conditions (continuous rated load, typical ambient temperatures, and no sustained overloads). Transformers designed with reduced temperature rise can extend this design life expectancy to over 50 years, as operating at lower temperatures slows the insulation’s ageing process.

Factors That Affect Life Expectancy:

• Temperature: As discussed, the most significant factor is the operating temperature of the transformer. Operating continuously at higher temperatures reduces the expected lifespan.
• Humidity and Condensation: Humid environments or condensation can affect the insulation material and lead to premature failure. Proper storage and maintenance help mitigate this risk.
• Short Circuit Events: Sudden surges or short circuits can damage internal components and shorten the transformer’s life.
• Overloading: Continuous overloading beyond the transformer’s rated capacity generates excess heat, which accelerates insulation degradation.
• Environmental Conditions: Extreme environments, such as exposure to dust, moisture, or chemicals, can also lead to earlier-than-expected failure.

By following proper installation and maintenance practices, such as avoiding overloading and ensuring the transformer operates within its designed ambient temperature, you can significantly extend its lifespan. Rex Power Magnetics’ high-quality transformers are built for durability, ensuring reliable performance for decades under standard conditions.

The K-Factor Rated Transformer spec table lists CSA C22.2 No. 47, CSA C9, IEEE C57.12.01, IEEE C57.12.91, IEEE C57.110, IEEE Std 519 and NEMA ST-20, and each covers a different thing:

• CSA C22.2 No. 47 is the Canadian safety standard for air-cooled transformers, the basis of CSA certification and the reference an inspector uses under the Canadian Electrical Code.
• CSA C9 covers dry-type transformer performance and test requirements in Canada.
• IEEE C57.12.01 sets general requirements for dry-type distribution and power transformers, including insulation classes, temperature limits, BIL and service conditions.
• IEEE C57.12.91 is the test code that defines how ratio, losses, impedance, dielectric and temperature-rise tests are performed.
• IEEE C57.110 defines the capability of a transformer to carry non-sinusoidal current, which is the basis of the K rating.
• IEEE Std 519 sets the system distortion limits the harmonic study works to.
• NEMA ST-20 defines the sound-level ratings and general dry-type transformer conventions.

The unit is also UL listed for the U.S. market, and efficiency is regulated separately by DOE 10 CFR Part 431 and NRCan SOR/2018-201 with Ontario Regulation 404/12. Certification marks and scopes are on the Certifications page; the Knowledge Hub’s Understanding Transformer Nameplates shows how they appear on the unit.

A standard quotation for a K-Factor Rated Transformer needs kVA, phase, primary and secondary voltages and connections, frequency, required K rating, temperature rise, shield option, enclosure type and any accessories such as thermal sensing, strip heaters or surge protection. To confirm the K rating rather than take it on faith, Rex asks for the harmonic study or a metered current spectrum at the panel, the load list showing linear and non-linear shares, the expected neutral current, the ambient and altitude, and whether the unit will see generator or UPS transfer overloads. The design becomes custom, and lead time and review time extend, when the request includes a K30 rating, a specified impedance, a non-catalogue kVA or voltage, multiple secondary windings, a primary above 600 V class, non-ventilated Type 4 or 4X at larger ratings, seismic certification, a witnessed temperature-rise test, or a guaranteed sound level below the NEMA ST-20 rating. Custom sizes for specific kVA ratings and open core and coil construction are listed options on the product page and are handled routinely. Submit the package through Get a Quote or the local representative found with the Agent Locator; the Custom vs. Standard Transformers article explains what triggers engineering review.

Routine maintenance on a K-Factor Rated Transformer is the same as any ventilated dry-type unit: de-energize and clean dust from coils and vent screens, retorque terminals, inspect for discoloured insulation and check strip heaters and thermostats. The harmonic-specific investigation begins when the unit is warmer or noisier than at commissioning. First re-measure the load spectrum with a power quality analyzer and compute the K-factor; the most common cause is that the electronic load has grown or the linear load that diluted it has been removed, so the unit is now beyond its nameplate K. Second, measure neutral current with a true-RMS clamp; a neutral running near phase current indicates a triplen load the unit may not have been sized for. Third, check phase balance, since single-phase electronic loads unevenly distributed overload one winding. Fourth, confirm the tap matches the present supply voltage; a supply that has drifted high overexcites the core, which raises both temperature and hum. Fifth, look for blocked ventilation, added heat sources in the room, or an enclosure change. A thermal camera scan of the coils and connections under load locates hot spots. If the spectrum exceeds the nameplate, the options are to reduce load, redistribute it, or replace with a higher K rating. Rex’s Transformer Maintenance checklist and the IOMM give intervals, and Contact reaches Rex’s technical support.

Yes. Rex Power Magnetics designs custom K-Factor transformers for specific harmonic profiles and installation requirements. Options include special voltage ratios, impedance levels, temperature-rise ratings, electrostatic shields, and enclosure types. Engineering teams can also incorporate multiple secondary windings or harmonic-filtering features to meet unique system conditions. Every custom transformer is built and tested to the same standards as catalog models, ensuring compliance, reliability, and efficient operation under complex non-linear loads.