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 1 2 Next > Last >>

Anonymous , 06/27/13 11:00 AM:

My opinion on footnote at the bottom the June 2013, Article "Minimizing Transformer losses" gives me the opinion that the author did not try very hard to get transformer loss data. I was able to find data from several vendors on the internet without to much effort. For example,

One item that should have mentioned was the definition of efficiency.
Paraphrasing the efficiency definition in NEMA TP-1,

Efficiency = Power OUT/Power IN

%EFF = 100 x kVA x Output/(kVA x Output + kW losses at Output loading)

A reader could have been mislead in believing percent base is the transformer full load rating. In the article 1000W at 35% loading on a 75kVA transformer would be 1000*100/(0.35*75000)=3.8% losses and not 1.3% as stated.

As mentioned the transformer loss are the sum of the no-load losses and load losses. The load losses are proportional to the square of the load on the transformer and not protortional to the load. Since most of the load losses are I^2*R, transformer with low impedance and resistance tend to have lower load losses. Since the curve 'total losses(kW)' verses 'Transformer loading*%)' did not state the basis for the curve it could be missleading. For the curve to be useful the percent coil resistance would have to be the same. Generally the lower the %impedance to lower the full load losses. On small transformers the user does not have much control over the impedance since the small transformers tend to be off the shelf types. Sometimes the same size transformer from different vendors can significantly different impedance. Lower impedance also means higher fault currents which is another item that has to be considered.
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2013 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
A cool solution: Collaboration, chemistry leads to foundry coat product development; See the 2015 Product of the Year Finalists
Raising the standard: What's new with NFPA 70E; A global view of manufacturing; Maintenance data; Fit bearings properly
Sister act: Building on their father's legacy, a new generation moves Bales Metal Surface Solutions forward; Meet the 2015 Engineering Leaders Under 40
Cyber security cost-efficient for industrial control systems; Extracting full value from operational data; Managing cyber security risks
Drilling for Big Data: Managing the flow of information; Big data drilldown series: Challenge and opportunity; OT to IT: Creating a circle of improvement; Industry loses best workers, again
Pipeline vulnerabilities? Securing hydrocarbon transit; Predictive analytics hit the mainstream; Dirty pipelines decrease flow, production—pig your line; Ensuring pipeline physical and cyber security
Upgrading secondary control systems; Keeping enclosures conditioned; Diagnostics increase equipment uptime; Mechatronics simplifies machine design
Designing positive-energy buildings; Ensuring power quality; Complying with NFPA 110; Minimizing arc flash hazards
Building high availability into industrial computers; Of key metrics and myth busting; The truth about five common VFD myths

Annual Salary Survey

After almost a decade of uncertainty, the confidence of plant floor managers is soaring. Even with a number of challenges and while implementing new technologies, there is a renewed sense of optimism among plant managers about their business and their future.

The respondents to the 2014 Plant Engineering Salary Survey come from throughout the U.S. and serve a variety of industries, but they are uniform in their optimism about manufacturing. This year’s survey found 79% consider manufacturing a secure career. That’s up from 75% in 2013 and significantly higher than the 63% figure when Plant Engineering first started asking that question a decade ago.

Read more: 2014 Salary Survey: Confidence rises amid the challenges

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.