DOE rules impact transformer efficiencies

Building designers must keep the Dept. of Energy’s new standards when selecting and evaluating distribution transformers.


Jan. 1, 2010, marked the beginning of a new era for transformers in the United States. New rules were put into place that changed the efficiency, size, and price of distribution transformers commonly found in today’s nonresidential buildings and industrial centers.

The decision makers that design those buildings will feel the impact. Consulting engineers should be aware of the changing landscape of distribution transformers and how these standards affect their proposal.

Improved efficiency standards

Experts forecast that energy consumption will increase 40% by the year 2030, as shown in Figure 1. Building infrastructure to expand power capacity is expensive and takes years to implement. At the same time, for the past several years the U.S. Dept. of Energy (DOE) has been seeking ways to reduce CO2 emissions by eliminating unnecessary power plant construction. One approach is to reduce wasted energy by improving the transmission efficiency from the source to the demand.

Distribution transformers are a key part of this electric pipeline. Previously, transformers were subject to an open market, in which lowest initial price usually wins, or they had to meet loss targets contained within customer’s specifications and then compare total ownership costs. These targets varied by location,% loading, and their given electric rates. DOE’s action essentially leveled the playing field for transformers in setting new limits for both liquid-filled and dry-type, medium voltage transformers.

In 2006, the DOE studied raising the minimum acceptable efficiency by consulting with transformer manufacturers to determine what options were available. Initially six trial standard levels were investigated. After much debate, two standards emerged as the standards of choice. They agreed upon certain levels that were technologically feasible, economically justified, and would result in significant energy savings. The final rule came out in October 2007 to give manufacturers time to make the transition.

On Jan. 1, 2010, new efficiency standards were implemented, as shown in Tables 1 and 2. The standards affect both liquid-filled transformers and dry-type transformers base rated at 2500 kVA and below (except for some unique, specific applications). All new transformers produced and imported to the U.S. must be in compliance.

DOE estimates the standard will save approximately 2.74 quadrillion British Thermal units (Btu) of energy over the next 29 years, equivalent to all the energy consumed by 27 million American households in a single year. From a green perspective, the cumulative greenhouse gas emission reductions would be approximately 238 million tons of carbon dioxide, the equivalent of removing 80% of all light vehicles from U.S. roads for one year.

These figures are impressive and the ecological impact is significant, but how will the standards change each building project moving forward?

Changed transformer design formulas

The new standards mean new design evaluation formulas will be considered. For example, for a liquid-filled single-phase unit, the minimum efficiency is approximately equal to an evaluation formula of $4.50 for every Watt loss with no load on the transformer and $1.00 for every Watt loss of transformer load loss. For a three-phase unit the approximate evaluation becomes $3/W lost no-load and $1/W lost loaded. Again, these are only approximations as there is no direct translation between the evaluation formula and efficiency. Compliance with the minimum efficiency requirement is design specific.

For those unfamiliar with how transformer designers achieve the higher efficiencies, manufacturers have to either select better grades of grain-oriented silicone electrical core steel or use more pounds of their current steel choice. The manufacturer decides how to optimize the design by weighing in all the factors and components that make up a transformer.

If the analysis suggests that it is best to reduce losses by using better steel, the better grades cost more per pound on face value alone. Steel prices can increase by 15%, so if these transformers use upwards of 3,000 lbs. of steel, this can add up in a hurry. As a rule of thumb the core alone is one-third of the weight of a DOE-compliant, liquid-filled transformer.

If the latter design approach is chosen, the cores get bigger. As they grow so do the coils, which means more copper or aluminum and insulation are needed. If the core and coil get bigger, then the surrounding structure is larger. For dry-type transformers, it’s the enclosures. For liquid-filled transformers, the tank and fluid volume grows. Figure 2 is a 2,500 kVA design comparison before and after the DOE standards are applied.

In another dry-type design--cast coil--dies are used to mold the epoxy resin encapsulating the core and coil. New dies must be made to incorporate the bigger designs. This adds significant tooling cost for the manufacturer.

Impacted transformer selection

What this means to the consulting engineer is higher first cost of the transformer—averaging 15% more—and different geometries of the units for a given kVA rating. Numerous combinations of materials can be selected when building a transformer. Core steel, windings, and installation come in a variety of thicknesses, grades, and electrical qualities. As discussed earlier, this in turn affects the size of the core and coil assembly, overall size/weight, and cooling requirements. For dry-types the relative size and weight of the unit will increase roughly 15%, whereas a liquid-filled unit’s weight will increase roughly 5%. Building engineers will have to consider new envelope space and supporting structure to accommodate these changes.

Realized long-term benefits

This is not all bad news for the end user. Having more efficient transformers equates to less heat generated by losses. Less heat means a lighter load on the heating, ventilating, and air conditioning system, as much as 40% to 50%. This has a dramatic effect on the monthly electric bill for the life of the equipment. For example, a 43% increase in efficiency could reflect an annual savings of $6500/kVA.

Moreover, in some cases designers can eliminate several-ton air conditioners, which are expensive pieces of capital equipment. This results in a huge savings on both upfront cost and space within the building. Adding more rentable space for tenants increases building owners’ revenues significantly.


The DOE decision focused on new purchased items, those built after Jan. 1, 2010, but one segment overlooked is the replacement of those “inefficient” transformers currently in service. In most case units that are 20 to 25 years old might have a few years of service left in them, but the customer will need to evaluate how much money it could save by planning ahead and replace them sooner.

The savings from replacing an older liquid-filled transformer with a new, DOE efficient design calculates to a 5-year payback period when you also factor in the HVAC electric cost. Furthermore, replacement of an aging dry-type transformer with a contemporary high fire point liquid-filled design could reduce operating costs by 50%. Liquid-filled offerings are inherently more efficient than similar air-cooled designs, as shown in Tables 1 and 2.

Evaluate carefully

As we move forward with alternative, renewable energy sources, the cost per watt to provide this power will vary greatly. Some areas will increase their rates while others will remain the same. The DOE will also push for higher standards in the years to come. Every watt saved with improved efficiency will help the environment. Both designers and owners should invest the time to properly evaluate their options as the changes described today have long-term financial impacts for tomorrow.

- Kampa is a senior product specialist for distribution class, substation-style transformers at Cooper Power Systems. Focusing on product marketing functions, he supports commercial, pricing, and strategic initiatives and manages key OEM accounts and select regions of the U.S. and Canada. He holds a bachelor’s degree in mechanical engineering from the University of Wisconsin-Madison.

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