Proper Transformer Sizing and Copper Windings Mean Lowest Total Owning Cost, Fastest Payback

Transformers are used in virtually every commercial and industrial building, from the service transformer reducing the distribution voltage to a more usable voltage for the building, to step-down transformers serving individual floors, to small transformers for individual apparatus or functions. Typically a transformer is a long-lived device that can be in service for decades.

Over such a long life span, the operating cost of a transformer can greatly exceed the initial price, so selection of the right transformer for economic performance involves looking at proper size (capacity) and efficiency.

And efficiency means looking at both the core steel and the winding material.

Transformer Losses

In the simplest terms, there are two components to transformer losses: core losses (also called no-load losses); and coil losses (called load losses).

The core losses originate in the steel core of the transformer, caused by the magnetizing current needed to energize the core. They are constant, irrespective of the load on the transformer (thus the name no-load). They continue to waste energy as long as the transformer is energized. No-load losses do, however, vary with the size (kVA) of the transformer, and the core steel selected; hence the emphasis on proper sizing.

The coil losses (load losses) originate in the primary and secondary coils of the transformer, and are a result of the resistance of the winding material. That's where selection of copper windings can make a difference in losses.

Proper Sizing

Transformers are sometimes placed into a speculative setting in advance of occupancy, so the engineer does not necessarily know the load that will be placed on the unit. As the installer is often not the party paying the electric bill, there can be a tendency to oversize the transformer capacity relative to the load it will actually see. Since the no-load loss is a function of the kVA capacity of the transformer, careful selection of the transformer capacity closer to the intended task will ensure lowest core loss.

Energy Star (TP-1) Transformers May Not be Efficient Enough

The Energy Star label is applied to transformers that meet a certain minimal standard for efficiency known as NEMA TP-1 (NEMA stands for the National Electrical Manufacturers Association.) The full name of the standard is "Guide for Determining Energy Efficiency for Distribution Transformers"1 or "NEMA Standards Publication TP-1-1996." It is intended to promote the manufacture and use of energy efficient transformers by establishing minimum efficiency standards, albeit with certain assumptions built-in. It contains a simplified method for evaluating the first cost of transformers along with the costs of core and load losses. It also presents tables of minimum transformer efficiencies based on kVA size, voltages, and liquid or dry-type.

Unfortunately, there is nothing especially efficient nor cutting-edge about transformers that meet TP-1. Yes, they are an improvement over so-called "standard" transformers, still made and sold widely, and they are conditionally required in certain states for new construction. However, many transformers are available from various manufacturers that exceed the efficiency levels of TP-1, and may provide a fast payback of their purchase price.

The efficiency standards in NEMA TP-1 are based on certain assumptions that may result in the selection of less-than-optimally efficient transformers. One key assumption is that low voltage (600-volt class) dry-type (typical commercial or industrial) transformers are loaded at 35 per cent of their nameplate rating. For medium voltage and liquid-filled transformers the assumed loading is 50% of nameplate rating. Another underlying part of the economic rational for the standard is an assumed electricity cost of 6 cents per kWh.

Both these assumptions may be too low for industrial and commercial users, who often can more accurately predict their load requirements, or who may be paying more than 6 cents per kWh, particularly at peak times. In fact, recommended loading for economic sizing of a transformer is typically around 75% of nameplate (35% load, if constant, means the transformer is oversized and wasting core loss as well as well as higher purchase price.)

The table below, provided by Olsun Electrics, compares a "standard efficiency" 75 kVA transformer to an aluminum-wound TP-1 model, a copper-wound TP-1 model, and a "premium efficiency" copper-wound unit, at various loading levels. As the table shows, choosing a more efficient, copper-wound transformer that exceeds the minimal efficiencies of TP-1 (and Energy Star) can pay back its price premium in as little as one year.

Noteworthy is the fact that the TP-1 (Energy Star) efficiency, copper-wound unit, loaded at 75% of its nameplate capacity (column 7), saves over $88 per year compared to an aluminum-wound TP-1 model (column 6), but costs only $85 more initially. At only 50% loading, the copper TP-1 unit (column 11) saves about $50 per year compared to the same aluminum unit (column 10). No-load loss (core) is reduced from 350 to 320 watts because the greater conductivity of copper windings allows a smaller core to be used, so energy continues to be saved even at light loading levels.

For even greater savings, the premium efficiency, copper-wound unit saves over $401 per year at 75% loading (column 8), compared to the aluminum TP-1 model (column 6), and costs only $1235 additional. In fact, over a 20-year life (neglecting the time value of money), the total owning cost of the premium efficiency, copper-wound model is $12,399.60 compared to $25,447.00 for the standard efficiency model. The 20-year total ownership cost to buy and operate the premium efficiency transformer is less than one-half the cost of the standard model.

Payback time comparison for 75kVA Dry-Type transformers
Standard (Aluminum)TP-1 (Aluminum)TP-1 (Copper)Premium (Copper)Standard (Aluminum)TP-1 (Aluminum)TP-1 (Copper)Premium (Copper)
% of name plate load
Core loss (w)
Conductor loss (w)
Total loss (w)
Efficiency (%)
Transformer cost ($) 1336 1979 2064 3214 1336 1979 2064 3214
Additional cost compared with standard unit ($) 643 728 1878 643 728 1878
Energy cost/year ($) 1964.69 1363.76 1220.27 725.42 1205.55 860.93 772.63 459.29
Annual energy cost saving compared with
standard unit ($)
600.94 744.42 1239.28 344.62 432.92 746.26
Payback period (yrs) 1.07 0.98 1.52 1.87 1.68 2.52
Standard (Aluminum)TP-1 (Aluminum)TP-1 (Copper)Premium (Copper)Standard (Aluminum)TP-1 (Aluminum)TP-1 (Copper)Premium (Copper)
% of name plate load Core loss (w)
Conductor loss (w)
Total loss (w)
Efficiency (%)
Transformer cost ($) 1336 1979 2064 3214 1336 1979 2064 3214
Additional cost
compared with
standard unit ($)
643 728 1878 643 728 1878
Energy cost/year ($) 663.48 502.21 452.54 268.58 1205.55 322.54 292.50 185.80
Annual energy cost saving compared
with standard unit ($)
161.27 210.94 394.90 69.90 99.95 206.65
Payback period (yrs) 3.99 3.45 4.76 9.20 7.28 9.09
Courtesy: Olsun Electrics, Richmond, IL.

  1. Standard and Aluminum TP-1 units are 150C rise, copper TP-1 unit is 115C rise, Premium unit is 80° C rise.
  2. Loss values at 100%, 75% and 50% nameplate load are at reference temperature.
  3. Loss values at 35% nameplate load are at 75°C in accordance with TP-1.
  4. Energy cost assumed to be $0.07/kWh.

Specifying to Minimize Owning Cost

Whenever possible, always compare competing transformer models by asking for the load and no-load losses, in watts, and look at the total cost of ownership. If possible, perform life cycle cost analysis (discussed elsewhere on this Web site). Remember that no-load losses are constant whenever the transformer is energized. Specifying copper windings can minimize both the load loss and the no-load loss, by allowing for a smaller core. If the load is known or can be predicted, choose a transformer that will be loaded to about 75% of its nameplate rating. Oversizing the unit increases the no-load losses, as well as the purchase price, unnecessarily.

If the actual losses in watts are not available, and you are seeking the transformer with the lowest losses, choose a transformer with 80° C rise, core of grade M 6 steel or better, and copper windings. Specifying a lower temperature rise transformer results in a unit with higher overload capability. For example, an 80°C rise dry-type unit using 220ºC insulation, has 70°C reserve capacity. This allows the 80°C unit to operate with an overload capability of 15-30% without affecting the transformer life expectancy. Also, a cooler running transformer means a more reliable unit, and more up-time. Back to Top


1. National Electrical Manufacturers Association, Rosslyn, VA, 1996, Guide for Determining Energy Efficiency for Distribution Transformers. No. TP-1.