U.S. Department of Energy - Energy Efficiency and Renewable Energy

Federal Energy Management Program – Energy-Efficient Products

How to Buy an Energy-Efficient Distribution Transformer

Information about energy-efficient distribution transformers in this section includes the following:

Efficiency Recommendation Cost-Effectiveness Example Where to Find

Buyer Tips Safety For More Information

Also provided is a portable document format version of How to Buy an Energy-Efficient Distribution Transformer (PDF 127 KB, 2 pp). Download Adobe Reader.

^a Energy efficiency of distribution transformers is defined by NEMA's Standard Publication TP-1 (see "For More Information") as output kVA divided by the sum of output kVA plus losses, at a specified percent load and reference temperature.
^b Low voltage transformers have a primary voltage of 1200 volts or less; efficiency is measured at 35% of nameplate load, at 75°C.
^c Medium voltage transformers have a primary voltage greater than 1200 volts; efficiency is measured at 50% of nameplate load, at 75°C for dry-type transformers and 85°C for liquid-filled.
^d Liquid-filled transformers typically use oil as a combination coolant and insulating medium. They are generally higher in efficiency but are most frequently installed outside.

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^a Annual energy loss is based on 50% of nameplate load in the medium voltage example (top), and 35% of nameplate load in the low voltage example (bottom).
^b Lifetime energy cost savings is the sum of the discounted value of annual energy cost savings based on average usage and an assumed transformer life of 25 years. Future electricity price trends and a discount rate of 3.4% are based on Federal guidelines (effective from April 2000 to March 2001). The assumed electricity price is $0.06/kWh, the Federal average electricity price in the U.S.

Using the Cost-Effectiveness Table: In the example shown above, a 1500 kVA, three phase, medium voltage transformer at the recommended efficiency level of 99.0% is cost-effective if its purchase price is no more than $22,300 above the price of the base model. Similarly in the 25 kVA, single phase, low voltage example, the Recommended model, with an efficiency of 98.0%, is cost-effective if its purchase price is no more than $900 above the price of the base model.

EPA provides software to assist in evaluating the cost-effectiveness of transformers for both new and replacement applications (see For More Information).

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Where to Find Energy-Efficient Distribution Transformers

The Federal supply source for transformers is the General Services Administration (GSA). GSA offers transformers through Schedule 61-V(B), Power Distribution Equipment. Whether you buy from GSA, or use a commercial supplier or contractor (e.g., an A&E firm), be sure to specify a transformer that meets the efficiency recommendation for that capacity and type.

The efficiency levels in this recommendation are the same as those in NEMA's TP-1 standard. The Consortium for Energy Efficiency (CEE) (see For More Information) and the EPA/DOE program identify low-voltage distribution transformers that meet the efficiency criteria specified in this Recommendation and TP-1.

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Buyer Tips

Many types of modern office equipment and devices (personal computers, printers, copiers, fax machines, electronic lighting ballasts) and some industrial equipment (adjustable speed motor drives) impose non-linear loads, which can introduce power wave form distortion. If your transformer serves equipment such as these, consider using a "K-rated" or "drive isolation" transformer designed for specific types of non-linear loads. Simply replacing an existing transformer with a standard unit rated at a higher kVA level is generally not a good approach for these applications. Both first cost and operating cost are frequently higher with larger capacity standard units, which are not designed to accommodate harmonics and other problems imposed by non-linear loads.

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Safety

Safety codes require that indoor transformer installations meet stringent fire-protection and spill containment conditions. Some "less-flammable liquid-filled" transformers can meet code requirements for indoor uses when applicable fire protection and spill containment conditions are met.

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For More Information

• National Electrical Manufacturers Association (NEMA) publishes Standards Publication TP-1-1996, Guide for Determining Energy Efficiency for Distribution Transformers.
  Phone: (800) 854-7179
• Consortium for Energy Efficiency (CEE) provides information on transformers that meet this recommendation and utility programs promoting low-voltage transformers.
  Phone: (617) 589-3949
• Environmental Protection Agency (EPA) has transformer model listings. EPA also provides computer software that performs cost-effectiveness analyses for transformers.
  Phone: (888) 782-7937
• Lawrence Berkeley National Laboratory provided supporting analysis for this recommendation.
  Phone: (202) 646-7950

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