DOE tightens efficiency standards for distribution transformers

New standards mean that distribution transformers are probably getting pricier, but are they getting more efficient?


On Oct. 12, 2007, the U.S. Dept. of Energy (DOE) issued in the Federal Register , 10 CFR Part 431, Energy Conservation Program for Commercial Equipment, Distribution Transformers Energy Conservation Standards; Final Rule . This regulation covers all distribution transformers manufactured after Jan. 1, 2010, and defines the minimum efficiency standards in the design of distribution transformers under federal law. However, not everyone agrees on whether these new standards will produce the intended effect; meanwhile, the window to order less-expensive transformers is closing.

To meet DOE transformer efficiency requirements, manufacturers will likely need to increase the amount of materials in affected transformers. Increased materials inevitably lead to increased prices. Prices for DOE-compliant transformers are likely to increase by 25% to 30%.

However, affected transformers delivered before the deadline are not subject to the efficiency requirement, and time is of the essence for ordering lower-priced units. When specifying distribution transformers, take quoted delivery times into consideration. Upcoming quotes will be for the DOE-compliant transformers.

According to the definition in Federal Register 71:81 (April 27, 2006), p. 24995, a distribution transformer has an input voltage of 34.5 kV or less; has an output voltage of 600 V or less; is rated for 60 Hz operation; and has a capacity either of 10 kVA to 2,500 kVA for liquid-immersed transformers, or 15 kVA to 2,500 kVA for dry-type transformers.

The distribution transformer definition does not include:

  • Autotransformers

  • Drive (isolation) transformers

  • Grounding transformers

  • Machine-tool (control) transformers

  • Nonventilated (dry-type) transformers outside the specified voltage range

  • Rectifier transformers

  • Regulating transformers

  • Hermetically sealed transformers

  • Special-impedance transformers

  • Testing transformers

  • Transformers with a tap range of 20% or more

  • Uninterruptible power supply transformers

  • Welding transformers.

According to Copper Development Assn. Inc . (CDA), about 1 million distribution transformers are produced and sold annually in the United States, and virtually all electric power in the country passes through at least one of them before it's consumed. Mandated minimum efficiencies are listed in the National Electrical Manufacturers Assn . (NEMA) TP-1-2002: Guide for Determining Energy Efficiency for Distribution Transformers, also known as the TP-1 standard.

“TP-1 transformers are several percentage points more efficient than lower-cost, standard-grade (302 F-rise) transformers,” according to a report posted on the CDA Web site ( “By contrast, the final rule for medium-voltage distribution transformers calls for, on average, efficiencies that are between about 0.2 and 0.3 percentage-points higher than TP-1 levels, measured at 50% loading. The final rule raises the efficiency bar very little above TP-1, but the rule's real value to national energy conservation is that it forbids the installation of standard-grade transformers, which are several percentage points less efficient.”

CDA further explains that “efficiency standards for low-voltage TP-1 transformers are set at 35% of full load. Efficiency peaks at and slightly above this loading level. It is important to understand that TP-1-rated transformers gain their high efficiency because they contain better-quality core steels. The steels reduce core losses, which are present whenever a transformer is energized%%MDASSML%%-loaded or not. Core losses remain relatively constant with the degree of loading.

“Winding losses, on the other hand, vary roughly with the square of the winding current (I²R) and therefore increase with the degree of loading. Winding losses are reduced by using lower-resistance copper winding conductors, preferably in the entire transformer but occasionally (to reduce cost) only on the secondary side,” the CDA report said.

According to independent consulting engineer Larry Spielvogel, the concepts driving transformer-loading and efficiency legislation are backward. Spielvogel said that many engineers tend to specify oversized transformers—sometimes to as much as twice the required load. The DOE mandate increases the amount of copper used in transformers. While this action increases transformer efficiency at light loads, it reduces efficiency at higher loads. If transformers were actually loaded properly, this mandated increase in materials will actually decrease transformer efficiency%%MDASSML%%-the opposite of what the legislation is intended to accomplish.

Also, some facilities are designed with multiple transformation stages, which are usually unnecessary, according to Spielvogel. For example, a community college accommodated anticipated expansion in its design. Consequently, multiple stages of step-down transformation were specified. Spielvogel said that the facility experienced more than 14% loss due to multi-stage transformer losses alone.

Spielvogel recommends that engineers size transformers realistically for the anticipated load while making provisions for space and connections in case building load changes due to growth or expansion. He also recommends that engineers minimize the steps of transformation when specifying distribution transformers.

It also a good idea to re-examine previous projects. “No one goes back to examine loading on previous jobs,” Spielvogel said. “Users should report on the difference between the load on a transformer and the size of the transformer.”

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