Increasing transformer efficiency

Transformers are fundamental components in electrical distribution systems for commercial buildings.

By Troy Thompson, PE, RCDD, LEED AP, DLR Group, Overland Park, Kan. March 5, 2015

This article has been peer-reviewed.Learning objectives

  • Learn the history of transformer efficiency regulation.
  • Understand how transformer efficiency is optimized.
  • Know how to select a transformer type for specific applications.

The most efficient way to transport power from point A to point B is to use the highest voltage available.Higher voltage equates to lower currents for a constant amount of power. Losses are a result of current and wire resistance (P = I2 * R). Therefore, decreasing current has a much greater return in reducing power losses than increasing wire size to decrease resistance.

A transformer is required at each point along the distribution path where a change in voltage is necessary. Power produced at a utility company power plant will change voltage numerous times via transformers before reaching the end user.

Figure 1: Pad-mounted transformers such as those shown in the photo are used with underground electrical power distribution to convert higher voltages to lower voltages. Courtesy: CFE MediaUtility companies typically use medium voltages (5 kV or 15 kV) to distribute power to customers.Commonly used local distribution voltages are 4,160 V, 12.47 kV, and 13.8 kV, while transmission lines that traverse greater distances use higher voltages such as 69 kV and higher. The voltage may be boosted at the production source to a higher voltage for transmission and then reduced to a medium voltage at a neighborhood utility substation. Voltage is reduced again (typically 480 V) at a utility pad-mounted transformer to provide service to an individual building, then reduced to 208 Y, 120 V via a dry-type transformer in a building electrical room (see Figure 1).

The concept of using the highest voltage available also applies to power distribution within buildings. In large commercial buildings, 480-V, 3-phase, 4-wire power is commonly used to serve large mechanical equipment. Lighting is typically served at 277 V while 120 V power is needed for receptacle loads.

Today, as design teams and building owners strive for high-performance, net zero buildings, maximizing transformer efficiencies is essential. After a transformer is placed in a building and energized, it begins consuming energy and is never turned off. Even when a building is unoccupied and all loads on the transformer are turned off, the transformer itself continues to consume energy 24 hr/day. The losses of a transformer contribute to heat in the building. More efficient transformers require less cooling to maintain a desired room temperature, which in turn saves more energy.

Standards and regulations history

Over time, both industry and government regulatory agencies-National Electrical Manufacturers Association (NEMA) and U.S. Dept. of Energy (DOE), respectively-have worked in tandem to advance the efficiency of transformers. In 1992, the DOE commissioned a study of transformer efficiency. The result of the study revealed that on average, low-voltage dry-type transformers are loaded to only 35% of the nameplate rating. Because equipment is sized for maximum peak demand using the conservative diversification factors allowed by the National Electrical Code (NEC) and loads can fluctuate based on time of day or time of year, the results of the DOE study were not surprising to design engineers. In the 1990s, many transformers were designed for maximum efficiency at much higher loading, when in reality most transformers will never experience 100% load. The 35% load identified by the DOE study provides a clear baseline for improving performance.

The next advance occurred in 2002. NEMA published its TP-1: Guide for Determining Energy Efficiency for Distribution Transformers. This standard defined minimum efficiency levels required for NEMA Class 1 efficiency for low-voltage dry-type transformers, medium-voltage dry-type transformers, and liquid-filled distribution transformers.

For common 3-phase dry-type distribution transformers, the minimum required efficiencies ranged from 97.0% for a 15 kVA transformer to 98.9% for a 1,000 kVA transformer. NEMA also published its TP 2: Standard Test Method for Measuring the Energy Consumption of Distribution Transformers, and the TP 3:Standard for the Labeling of Distribution Transformer Efficiency. These standards specified testing and labeling requirements related to NEMA TP-1 transformers.

Following the publication of the NEMA TP 1 standards, a variety of compliant products became available on the market. Design professionals embraced these standards and included the requirement for NEMA TP 1 compliant transformers, tested to the NEMA TP 2 standard in specifications, shortly after the standards were published.

Then in 2005, the DOE passed the Energy Policy Act of 2005. This legislation required all dry-type transformers rated 600 V or less and manufactured and installed in the U.S. to meet the NEMA TP 1 requirements as of Jan. 1, 2007. Prior to this regulation taking effect, the DOE did not regulate the energy efficiency levels of transformers. The DOE soon followed with regulations for liquid-immersed and medium-voltage dry-type distribution transformers. In 2010, the DOE published a formal Code of Federal Regulation for energy efficiency levels for liquid-immersed and medium-voltage dry-type distribution transformers. The regulation was coded as 10 CFR 431. The next advance occurred in 2009 when NEMA launched its NEMA Premium Efficiency Transformer Program. This required transformers dubbed NEMA Premium to have 30% fewer losses than the DOE 10 CFR 431 regulation, which was still being formalized at that time. Most major transformer manufacturers bought into the program and agreed to make transformers meeting this new NEMA Premium standard available to the marketplace.

At this time, the efficiency levels in the current regulations were based on the 2002 TP-1 standard created by NEMA. With an industry agreed-upon standard in place and manufacturers committed to creating product, it seemed inevitable that the DOE would follow the NEMA Premium standard as it had done with NEMA TP-1.

Table produced by DLR Group Data was obtained from US Government Electronic Code of Federal Regulations and NEMA.However, this was not to be. On April 18, 2013, the DOE published a final ruling (78 FR 23335) amending its 10 CFR 431 energy efficiency standards for low-voltage dry-type, liquid-immersed, and medium-voltage dry-type distribution transformers. The new standards will be placed into effect Jan. 1, 2016. The new efficiency levels are similar to the NEMA Premium levels, but are not a consistent 30% decrease in losses across the board (see "Candidate and trail standard levels" on page xx). For low-voltage dry-type transformers, the percentage decrease in losses varies from 29% to 36% depending on size (see Table1).

It is anticipated that manufacturers will evolve their current NEMA Premium lines to meet the DOE’s new 10 CFR 431 requirements while striving to maintain compliance with the NEMA Premium standard. The DOE predicts that the amended energy efficiency standards will save consumers up to $12.9 billion for equipment sold from 2016 to 2045.

The current state

Currently, the efficiency of low-voltage dry type transformers must meet the current 10 CFR 431, which also is the NEMA TP-1 standard. NEMA Premium products that are available on the market use copper windings and have temperature rise ratings of 176 F. The cost of a NEMA Premium transformer could be as much double of the cost of a standard NEMA TP 1 transformer with aluminum windings and a 302 Frise rating. NEMA Premium products will save energy, with the amount of savings and the duration of payback depending directly on the utility rate. In just over one year’s time, all distribution transformers will be required to meet the amended 10 CFR 431 requirements and NEMA TP-1 will be a legacy.

Loads and specification

Standards and regulations for transformers define minimum efficiency levels under certain test conditions,including percentage of nameplate loading and temperature. During operation in the real world, these parameters are not fixed because the load will change as devices cycle on and off. As the load varies, the efficiency will decrease proportional to the percentage of nameplate rating for which it was optimally designed.

The types of transformer losses are core (no-load) losses and coil losses, also referred to as load losses.The no-load losses are a function of core construction and are independent of the load. Load losses vary proportionally to the square of the load. NEMA standards define minimum efficiency levels, but do not define a maximum no-load loss component. Equipment manufacturers must determine the best method to achieve desired efficiency levels. Therefore, two transformers, both with the same efficiency under the defined test conditions, could have different efficiencies under other conditions.

There are a number of items to consider when specifying a transformer. Options for a low-voltage dry-type distribution transformer include:

  • Voltage, primary, and secondary
  • Single or 3-phase
  • Configuration for 3-phase transformers: delta or wye
  • Grounded or ungrounded
  • Winding material: copper or aluminum
  • Insulation temperature
  • Rise temperature
  • Harmonic mitigating or K rated
  • Efficiency: NEMA TP 1, NEMA Premium, 2016 DOE.

Although aluminum is less expensive than copper, NEMA TP 1 level performance can be achieved with either copper or aluminum windings.

Insulation temperature for dry-type distribution transformers is typically 428 F. The insulation temperature must be above the temperature of windings. If the temperature of the windings exceeds the rated temperature of the insulation, the insulation will begin to break down, reducing the life of the transformer.

Options for rise temperature include 176 F, 239 F, and 302 F. The cost of transformers increases as the temperature rise decreases. Transformer temperature rise is defined as the temperature rise of the windings above the ambient temperature when loaded to 100% of the nameplate rating. For example, the windings in a 302 F transformer in a room with an ambient temperature of 104 F could rise to 374 F. This is acceptable because the temperature is below the accepted 428 F insulation temperature. If the transformer is placed in an unconditioned space, such as outside where the ambient temperature could become extreme, one should consider a lower temperature rise rating to ensure longevity of the transformer.

There are a variety of transformers with various temperature rise ratings that meet the current 10 CFR431 (NEMA TP-1) standard. No-load losses will vary for the different temperature rise ratings. The 176 Frise, 75 kVA transformer will have 1,356 kW in load losses, while the 302 F rise transformer would have 2,518 kW in load losses. The 302 F transformer is more efficient at 25% and less efficient at 50% load and above when compared to a similar 176 F transformer. If the average loading on a transformer is expected to greatly exceed the typical 35% loading, then a 239 F or 176 F transformer may be the best choice.

If the load is known to produce harmonics, K-rated or harmonic mitigating transformers should be considered. K-rated transformers were introduced in the 1990s as the adverse effects of nonlinear loads were recognized. As offices became populated with computers and electronic devices, some electrical systems became overtaxed, and harmonics made these situations worse. The K-rated transformer was designed to tolerate the presence of harmonics and the additional heat generated by these loads. At onetime, transformers with K factors of 4, 9, 13, 20, 30, 40, and 50 were available. Today, K factors of 4 and 13 have emerged as standard solutions as widespread use of K-rated transformers has fallen out of favor with specifiers. The challenge of harmonics appears to have subsided with the understanding that transformers only carry an average 35% load following the 1992 DOE study.

Harmonic mitigating transformers are typically very efficient, exceeding the efficiency levels set in NEMA TP 1, although they are exempt from meeting this requirement. The application of harmonic mitigating transformers remains controversial, with some manufacturers only recommending such transformers to correct a known existing problem, while others recommend including harmonic mitigating transformers for new construction projects.

Other considerations in transformer selection

A low-voltage dry-type transformer designed for optimum efficiency at 35% of nameplate load has greater impedance than a transformer manufactured for optimal efficiency at 100% loaded conditions. A beneficial result of greater impedance in a transformer is reduced available fault current on the secondary side of the transformer. Reduced fault current equates to a safer working condition and potentially lower cost of over current protection devices on the load or secondary side of the transformer.

As NEMA Premium transformers are implemented, care should be taken when sizing the primary over current protection device. These transformers have a greater inrush current than the previous generation of transformers, which should be taken into account. Sizing the over current protection device at 125% of the primary current may result in nuisance tripping of the transformer’s line side breaker. A better solution is available in accordance with NEC, Article 450, in which one can size the line side breaker at up to 250% of the transformer’s primary rating. The actual inrush of the specific transformer should be checked during the coordination study to ensure that the breaker or fuse operating curve falls between the transformer inrush point and the transformer damage curve.

Candidate and trail standard levels

When the DOE began evaluating efficiency levels for distribution levels prior to the Energy Policy Act of 2005, it developed a series of efficiency levels for consideration. These levels were called Candidate Standard Levels (CSL). CSLs evolved to Trail Standard Levels (TSLs) as part of the evaluation process. CSL 1, which matches NEMA TP-1, was ultimately chosen as the required efficiency levels for the regulation put into effect in January 2007. CSL 3 seems to have found its way into specifications as ameans to require transformers that exceed the current mandated minimum standard. CSL 3 is not astandard intended to be used as a product specification. The CSL is an option that was considered andnot selected to be the standard. CSL 3 is similar but not equal to the efficiency levels proposed in theNEMA Premium standard or the amended 10 CFR 431 efficiency levels to be required in 2016.


Troy Thompson is a principal with DLR Group‘s Overland Park, Kan., location where he leads the electrical design practice. He joined DLR Group in 1992 and has designed complex electrical systems fora variety of commercial and private clients. He is an active member of Building Industry Consulting Services International and is a registered communications distribution designer.