Selecting energy-efficient transformers

Engineers should know the design concepts for selecting and sizing transformers to help achieve energy efficiency.


Learning objectives

  • Understand the different types and uses of transformers.
  • Know how to select and size transformers.
  • Understand the concepts of transformer protection.

This article has been peer-reviewed.Transformers are perhaps among the most overlooked components within an electrical distribution system. However, they play a key role in our everyday lives. Every house is fed by a single transformer. In our offices, a single transformer may serve the computers for the entire floor. In hospitals, the operating rooms and intensive care units are typically fed from two separate transformers.

If a transformer fails or is improperly sized, catastrophic outages, which are not quick and easy to fix, could occur. Also, transformers don’t shut off; they continue to use current and generate heat 24 hr a day, seven days a week.

Transformer selection

Transformers are available in many different flavors. High- and medium-voltage transformers (primary voltage greater than 600 V) are available as dry type units, but are more commonly liquid filled or liquid immersed. That liquid is commonly petroleum-based oil, but many companies are starting to offer similar products based on biodegradable seed oil. Commercial buildings operate on low-voltage (primary voltage less than 600 V) predominately, and typically use dry type transformers to step down the voltage from 480 V to 208 Y/120 V or 240/120 V. This article focuses primarily on dry type transformers. However, many of the concepts presented apply to higher voltage transformers as well.

Figure 1: Transformers are available in a variety of sizes and distribution voltages, and can be installed indoors or outdoors. All images courtesy: TLC Engineering for ArchitectureTo select a dry type transformer, you need to answer three simple questions:

  1. What is the purpose of the transformer?
  2. How do I want the transformer to perform?
  3. What options should I select?

Generally, there are three purposes of a transformer: Change the voltage, isolate power systems, and harmonic accommodation. Voltage can be decreased or increased. These transformers are available in either delta- or wye-connected primary or secondary, depending on the distribution voltage and system requirements. They may be single- or 3-phase, and are available in a variety of sizes (see Figure 1). Isolation transformers can be used in health care facilities to minimize the risk of stray currents in the electrical system, or even on a single load that has very sensitive electrical requirements.

How the transformer performs boils down to temperature and efficiency. Transformers are listed with a rate of temperature rise, typically 80 C, 115 C, or
Figure 2: This graph shows transformer rate of temperature rise above ambient, typically listed with 80 C, 115 C, or 150 C temperature rise ratings.150 C. This temperature rating is the rise above ambient (see Figure 2). The surface temperature of a transformer with an 80 C rise is significantly less than
one at a 150 C rise.

How engineers approach energy efficiency is evolving. Every other year, the U.S. Dept. of Energy (DOE) revises many of the energy standards that regulate our industry. Transformers manufactured today, and since 2007, are required to meet the criteria defined under NEMA TP-1 2002. In April 2013, a new rule was adopted that implements new transformer standards, effective Jan. 2016. NEMA TP-1 2002 introduced two significant changes to energy efficiency considerations: Minimum efficiency was defined for each transformer size, and the point where that efficiency was measured changed from full load to 35% of transformer capacity. As a part of the origin of NEMA TP-1, research was performed to determine that most low-voltage distribution transformers are, on average, only 35% loaded. Many manufacturers currently offer a NEMA premium efficiency transformer, which was created prior to the implementation of the April 2013 final rule (see Table 1). Please refer to the NEMA class I efficiency chart for further information on the past, present, and future ratings of transformers.

Source: TLC Engineering for Architecture

The decision of which transformer to provide becomes a decision of which options to specify. Questions that engineers should ask include:

  • Are the transformer windings aluminum or copper?
  • What kind of enclosure do you need for your application?
  • Is it outdoors, or is a NEMA 1 enclosure acceptable?
  • What are the specific requirements of the manufacturer in terms of required space around the transformer?

Some manufacturers require a 3-in. clearance around transformers, while some require a 6-in. clearance. Unfortunately, sometimes 3 in. can make a big difference in designing electrical rooms.

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Fabo , Non-US/Not Applicable, Nigeria, 03/21/14 04:14 AM:

Why are we, from Nigeria not applicable
BRANDON , VA, United States, 04/21/14 12:32 PM:

The article mentions that when replacing a 150 kVA transformer that the room be 1 hour fire rated. However, most commercially available standard transformers come with an insulation class of 220 degrees. Therefore they meet the second exception of 450.21(B) and do not need to be in a fire rated room. Something to consider when preparing the transformer specification. It is likely easier to get 220 class insulation (if not standard already) than fire rating an existing room.
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