Transformer efficiency: Minimizing transformer losses


Figure 2: This graph compares core and load losses of 80-C rise and 150-C rise transformers. Courtesy: Lovorn Engineering Assocs.Distribution transformers and TP-1

Transformers that have primary voltages of 34.5 kV or less and secondary voltages of less than 600 V also must meet the efficiency ratings of TP-1 at a linear loading of 35%. However, TP-1 covers only 3-phase distribution transformers between 15 kVA and 1,000 kVA, so the larger transformers are not addressed by this standard. In addition, the distribution transformers are traditionally designed to be loaded to between 50% and 75%. As noted previously on the smaller, dry-type transformers, loadings that exceed the 35% TP-1 point will have significantly greater losses than the tabularized values. So while the goal of TP-1 was very lofty, it does not apply as well to actual installations. 

Historically, it was common to see distribution transformers that had impedance ratings of 5.75%. As the electrical utilities have worked to reduce their operating costs, the impedance for distribution transformers has dropped to values as low as 1.5% impedance. Since the utilities typically absorb the transformer losses as a part of their operating costs, reducing the impedance percentage from 5.75% to 1.5% has saved more than 70% of their losses at the transformer. This turned out to be very convenient because TP-1 was requiring that these transformers have higher efficiencies at the same time that the utilities were attempting to reduce their operating costs. 

This process had a negative side effect that was not immediately evident but had a major impact on the electrical engineer’s design: the available fault duty on the secondary of the transformer. A 1,000 kVA transformer with 5.75% impedance will have an available fault duty of 21,000 A at 480 V, assuming an infinite bus on the primary side. Given the same criteria for a 1.5% impedance transformer would result in an available fault duty of 80,000 A. The same transformer operating with a 120/208 V secondary will have available fault duties of 48,000 A and 185,000 A, respectively. This operating efficiency improvement has a major impact on the electrical system design, particularly at the lower secondary voltage of 120/208 V (see “Sizing stepdown transformers”). 

While TP-1 did not address transformers that were rated larger than 1,000 kVA, there have been similar reductions in their impedances to affect matching savings for these larger transformers. As one would anticipate, available fault currents for a 2,500 kVA transformer are dramatically, though proportionally, larger. At 480 V, the fault duties would increase from 52,000 A to more than 200,000 A for a 1.5% impedance transformer. Thank goodness transformers of this size do not commonly come with a 208-V secondary, because the fault current would approach 500,000 A.


In the engineer’s quest to reduce energy consumption, matching the transformer to its anticipated load is critical in achieving that goal. By applying a 150-C rise transformer to a lightly loaded linear circuit, the losses noted in TP-1 will be very close to the actual losses. However, heavier transformer loadings will suggest the engineer design around one of the lower temperature-rise transformers, such as the 115 C or 80 C transformers. When there are significant harmonically rich loads that are to be fed by a dry transformer, the lowest losses are likely to be achieved by using K-rated transformers that are sized for the anticipated harmonic currents.

Injudicious transformer selection can exceed the losses shown in TP-1 by 300% to 400%, resulting in a negative return on investment for the increased cost of the higher efficiency transformers.

Know the loss data, loading when specifying transformers

In researching this article, the author found it quite interesting that published loss data for all of the major manufacturers queried was virtually nonexistent. When asking about losses for operating points other than the 35% loading for TP-1, there appeared to be nothing available. Also, loss data for transformers operating at 80-C rise, 115-C rise, and K-rated transformers were also unavailable. Asking your local transformer sales representative for loss data at the operating point to which you are designing and for the type of transformer that you are designing around may save your client many dollars in energy savings. However, inserting the standard 150-C rise transformer into your design, while planning on operating the transformer at a point other than 35% loading and with a significant percentage of nonlinear loads, could cost your client significantly over the life of a transformer.

Sizing stepdown transformers

One cautionary note on stepdown transformers: When transforming from 480 V to 120/208 V, these low-loss, dry-type transformers can sneak up on your design. With the higher impedances of yore, an engineer usually did not have to worry about having higher interrupting duty branch circuit breakers when they were connected downstream from a dry transformer and the rest of the distribution system was a fully rated system. With the lower impedances, transformers as small as 112.5 kVA can have available fault duties that would require the use of breakers with an interrupting duty of more than 10,000 A. When using dry transformers such as a 300 kVA, 480/120/208 V, available fault duties can exceed 40,000 A, requiring your electrical design to use 65,000 AIC breakers. It is better to break the 120/208 V load into small divisions so that the maximum transformer size does not exceed 75 kVA with an impedance of at least 2% and the lower interrupting breakers (read: less expensive) may be used. 


Lovorn is president of Lovorn Engineering Assocs. He is a member of the Consulting-Specifying Engineer editorial advisory board.

<< First < Previous Page 1 Page 2 Next > Last >>

The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
A new approach to the Skills Gap; Community colleges may hold the key for manufacturing; 2017 Engineering Leaders Under 40
Doubling down on digital manufacturing; Data driving predictive maintenance; Electric motors and generators; Rewarding operational improvement
2017 Lubrication Guide; Software tools; Microgrids and energy strategies; Use robots effectively
The cloud, mobility, and remote operations; SCADA and contextual mobility; Custom UPS empowering a secure pipeline
Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Mobility as the means to offshore innovation; Preventing another Deepwater Horizon; ROVs as subsea robots; SCADA and the radio spectrum
Power system design for high-performance buildings; mitigating arc flash hazards
Research team developing Tesla coil designs; Implementing wireless process sensing
Commissioning electrical systems; Designing emergency and standby generator systems; Paralleling switchgear generator systems

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
The maintenance journey has been a long, slow trek for most manufacturers and has gone from preventive maintenance to predictive maintenance.
Featured articles highlight technologies that enable the Industrial Internet of Things, IIoT-related products and strategies to get data more easily to the user.
This digital report will explore several aspects of how IIoT will transform manufacturing in the coming years.
Maintenance Manager; California Oils Corp.
Associate, Electrical Engineering; Wood Harbinger
Control Systems Engineer; Robert Bosch Corp.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
click me