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Electrical, Power

Transformer selection and sizing

Selecting and sizing electrical distribution transformers is fundamental for electrical designers and engineers

By Robert Sander and Mario Caraveo, NV5, Las Vegas December 18, 2019
Figure 4: This is a sample K-factor transformer nameplate. Note that the K-factor is clearly identified. Courtesy: NV5

Learning objectives

  • Learn design concepts for selecting and sizing electrical transformers.
  • Review the codes and considerations associated with transformer design.
  • Evaluate a case study profile to highlight best practices.

In the United States, transformers are governed by the NFPA 70: National Electrical Code; in this case, the latest version of the code published in 2017 will be used. Transformers are fundamental components in many different commercial, industrial and residential electrical systems. They allow for the working voltage to either be “stepped” up or down. Transformers are able “step” voltage up or down by using the magnetic field produced passively by the current carrying windings.

The most basic version this concept can be illustrated by two copper loops of different size, one inside of the other with no contact. If one of these loops has current passing through, it then an induced voltage is seen at the terminals of the other loop. The voltage and current in the second loop is proportional to the voltage and current associated with the first loop. The amount of loops or windings can be changed to create a variety of voltages to work with.

Transformer windings are made of aluminum or copper. Aluminum is the common selection because it is less expensive while offering similar electrical characteristics to copper. Aluminum is lighter in weight than copper, but is typically larger in physical size.

Power distribution transformer power rating sizes are standardized throughout the industry. The most common type of application within a commercial facility are three-phase delta primary to wye secondary step-down type transformers. Industry standard sizes for 480- to 120/208-volt wye transformers are commonly 15, 30, 45, 75, 112.5, 225, 300 and 500 kilovolt-amperes.

There are also single-phase 277-or 480-volt transformer sizes available at 5, 7.5, 10, 15, 25, 37.5, 50, 75 and 100 kilovolt-amperes. This is not a comprehensive list, but illustrates the variety and range that is commercially available.

Figure 1: This harmonic mitigating transformer does not have vibration-isolation pads. Typically, these are noted during inspection by authorities having jurisdiction or field observations by engineer. Courtesy: NV5

In general, three-phase transformers are the most commonly used for electrical designer application and selection. Single-phase transformers tend to be used for special applications or voltages. An example may be unit equipment that specifically requires 240 volts single-phase where the service voltage is 120/208 volts wye three-phase. For a special case like this, it is common to only provide a single-phase transformer for the equipment because there won’t be a multitude of loads served by it. When a single-phase transformer is used for general distribution, it can cause phase imbalances when using a three-phase utility. Otherwise, if a property is served by single-phase and a transformer is used (e.g., for isolation) then a single-phase transformer would be appropriate.

All transformers are required to have a nameplate with the information described in NEC 450.11(A)(1-8). This information includes the name of manufacturer, rated kilovolt-amperes, frequency, primary and secondary voltages, impedance of transformers 25 kilovolt-amperes or larger, required clearances for transformers with ventilating openings, amount and kind of insulating liquid where used. For dry-type transformers, temperature class for the insulation system.

Transformer sizing

The first step to sizing a transformer is to determine the load that will be served, either at the branch circuit, feeder or service level. This starts with estimating or calculating the demand load using NEC Article 220 and then applying and applicable demand factors. Based on the types of loads served, demand factors will reduce the calculated load to determine appropriate sizing of the transformer. ‘

This calculated design load represents the base load or starting point for transformer sizing. Once you have determined the base load, depending on the type of project, a few considerations will need to be made when determining the final size of the transformer. These considerations include the future flexibility, available physical space, cost and project type.

Future capacity or expansion for a property is one of the most crucial considerations for sizing. This is important because both an undersized and an oversized transformer operate at lower efficiencies and could cause degrading damage to equipment over time. It is crucial to understand the owner’s intended use for the facility. There are instances where the property is not likely to expand and as such, owners may not require capacity for future loads or equipment.

Figure 2: A sample nameplate from a 1,500 kilovolt-amperes dry-type transformer lists all parameters related to the transformer construction and capabilities. Courtesy: NV5

However, some owners may not use their space to full capacity at project completion (e.g., a pharmaceutical lab filled to half occupancy) and it would be prudent to allow capacity for future expansion at the transformer. Such considerations for expandability should be discussed and coordinated by the design consultant with ownership to suit their needs.

Additionally, depending on the project type (e.g., new construction, tenant improvement, remodel) there may not be physical space for expansion. The addition of a transformer to an existing property can be costly dependent on location and size. The location of an added transformer requires coordination for ventilation, spacing for code-required clearances and may require structural bracing. Furthermore, transformers produce excess heat that the mechanical engineer must evaluate existing systems to determine if they will support sufficient cooling.

Another consideration is the weight; some smaller transformers weigh less than 1,000 pounds and can be incorporated with minimal structural coordination. These considerations should be evaluated before the addition of a transformer to an existing electrical system. Typically, it is easier, in terms of cost and coordination, to accommodate a larger-sized transformer in a new construction structure, but a remodeling project may prove to be more costly and require more coordination.

Lastly and important for an owner to consider, is the transformer’s cost. Usually, the larger the size of the transformer, the higher the equipment and installation cost. Often, for larger transformer sizes, they can also incur additional design and structural costs. For example, a 225 kilovolt-amperes dry-type transformer placed at an upper floor and typically weighs 2,000 to 4,000 pounds and would require structural engineers and architects to consider the weight and additional bracing needed to support the equipment load.

In general, as with most other aspects of electrical engineering, it is best to be conservative and oversize at the early stages of a project until further design development and final determination is made considering all the preceding items. It is worth noting that it is easier to downsize a transformer later in design, for coordination purposes, rather than upsize the transformer after the preliminary stages of design.

Information regarding transformer installation is found in the NEC, Article 450. Article 450.3(A) and (B) provide tables for maximum rating or setting of overcurrent protection for transformers with voltages for both, equal to/less than and larger than 1,000 volts. The numbers given in the tables are percentages of the transformer-rated current which is derived by taking the transformer’s kilovolt-ampere rating and dividing it by the voltage of the feeder.

Needless to say, the primary and secondary feeders of a transformer will have different current requirements corresponding to their voltage with one exception — transformers used for power isolation. Primary protection allows an engineer to make a simpler design, but using a combination of single and secondary protection allows for greater flexibility in the use of a transformer’s current rating. One could use the full rating of a transformer as long as the feeders are still adequately protected according to these tables.

Figure 4: This is a sample K-factor transformer nameplate. Note that the K-factor is clearly identified. Courtesy: NV5

Types of transformers

Once a transformer size is determined, consider the application of and types of loads that will be served by the transformer. In commercial design, there are a few commonly used types of transformers with characteristics as described below:

Dry-type transformers use ambient air to cool the core and windings. These transformers tend to be larger than liquid-filled transformers, but are generally less expensive in materials and installation costs.

The two commonly used dry-type transformers are encapsulated and ventilated. Nonventilated or encapsulated are sealed completely with surface area cooling, suited for wash-down areas and corrosive, combustible or other harmful conditions. Ventilated dry-type transformers are made with openings that allow air to move through the inside, are larger in dimension, use different insulation materials and contain an enclosure for the windings providing physical protection for the equipment and for personnel.

Liquid-insulated transformers use liquid for cooling and to act as an insulator for the cores. Mineral oil and bio-based oils are the most commonly used liquids. Liquid-insulated transformers allow better cooling that translates to a more compact transformer than a dry type.

However, these transformers require periodic oil analysis, but are considered less costly for repairs. Bio-based oils are less flammable and are environmentally friendly in the case of a leak. Less flammable is considered for liquids with a fire point of not less than 300°C. Exterior pad-mounted utility transformers are typically used with mineral oil and are considered combustible. For transformers less than 35 kilovolts, indoor installations may require minimal requirements such as an automatic sprinkler system or liquid containment area with no combustibles stored inside the room.

NEC 450.23 covers the requirements for indoor and outdoor installations for these liquid-insulated types. Additionally, nonflammable fluid-insulated transformers that use a dielectric fluid that is nonflammable require a transformer vault to be installed indoors per NEC 450.24. Oil-insulated transformers must be installed in a transformer vault per NEC 450.26 when indoors.

Figure 5: An example of a pad-mounted oil-insulated 300 kilovolt-amperes transformer is installed outdoors on a dedicated equipment pad. Courtesy: NV5

Special applications

K-rated and harmonic mitigating transformers typically are used for harmonic, nonlinear loads such as computer/servers with switch-mode power supplies, gaming slot machines, LED lighting, motors or variable frequency drives. HMTs can be used to correct the harmonic issues generated by the nonlinear loads.

K-rated transformers, on the other hand, do not mitigate harmonics, but rather allow for a more robust system to tolerate the harmonics. Transformer failure from harmonics are caused by excessive and/or constant overheating of the coils leading to a faster degradation of the coils’ insulation. Electrical systems with excessive harmonics can cause electronic components to fail due to a distorted sinusoidal wave.

The major difference between K-rated transformers and HMTs is that K-rated transformers are built to handle the stresses and strain of nonlinear loads depending on the level. Meanwhile, HMTs are physically constructed in such a way to reduce or mitigate harmonic currents from downstream devices to keep disruptive currents from traveling electrically upstream of the transformer.

Most electronic equipment nowadays is powered by switch mode power supplies. SMSPs convert sinusoidal alternating current to constant direct current using rectifiers and capacitors that draw short and sharp bursts of current, which alter the original AC sinusoidal wave. This altered wave is now a nonlinear load and has odd harmonics that can become harmful to the transformer by increasing the current in the windings resulting in excess heat in the transformer coils. HMTs suppress or reduce the effects of these odd harmonics, in particular the third harmonic that is additive on the neutral conductor.

Table 1: The efficiency of a low-voltage dry-type distribution transformers is governed by the Department of Energy. Courtesy: NV5

Transformer design considerations

Location: An important factor to consider is physical location of the transformer. The type of environment/building material where the transformer is located and the surrounding occupancies or rooms adjacent to the transformer should be considered.

For example, an oil-insulated transformer installed indoors requires spill containment areas that are typically more costly. Specifically, for oil-insulated transformers, a vault room would be required by NEC Article 450.26, unless at least one of six exceptions are met. There are advantages and disadvantages for using a transformer vault depending on any number of variables, however they require special attention and tend to add significant cost, which should be taken into consideration. Although they are not governed by the same building construction regulations mandated by the NEC, utility companies commonly use oil-insulated transformers.

Additionally, when locating a transformer, consider its physical location in the building and the area it is intended to serve and distribute power to. A 277/480 volt-delta transformer is better suited for longer runs on medium-sized buildings due to voltage drop. To avoid sizing larger feeders for longer runs, it’s better to use a higher voltage to distribute power as needed.

A 120/208 volt-wye is common for nonindustrial applications at the branch circuit level, but the lower voltage makes it subprime for long-distance distribution. Medium-voltage properties, where the voltage-to-ground is 1,000 volts or more, carry power from clusters of buildings throughout the site.

Noise: Noise also should be considered, depending on the type of building occupancy. The constant vibrations from the transformer may cause an undesirable audible hum for the client or occupants. In a hotel tower occupancy, for example, transformer rooms in the upper floors where guestrooms are located may need sound-proofing or acoustical treatment to mitigate noise from the electrical space.

This room treatment may be avoidable if the transformers are placed at grade level or on the roof in a location that gives adequate separation from the transformers and guests. Another solution could be to provide vibration-isolation pads that reduce the noise to a level acceptable to the client. An acoustical engineer or consultant may be involved to assist with this noise mitigation.

There room construction is required to meet requirements as outlined in NEC Article 450 Part II. Specifically, dry-type transformers installed indoors require at least 12 inches of separation from combustible material for transformers rated less than 112.5 kilovolt-amperes, per NEC 450.21(A). For dry-type transformers larger than 112.5 kilovolt-amperes, the room requires a fire-resistant construction of at least one hour per NEC 250.21(B).

However, there is an exception that commonly applies: those with Class 155 or higher and completely encapsulated except ventilation openings do not need to be located in one-hour rated rooms. Figure 1 represents one of these transformers; as such, the room it resides in does not require a one-hour fire-resistant rating.

Energy-efficiency requirements

Energy efficiency for dry-type distribution transformers is governed by the U.S. Department of Energy. As such, compliant transformers are labeled with DOE-2016 to mark their compliance since Jan. 1, 2017. Dependent on the capacity of the transformer and its quantity of phases, the efficiencies range from 97.0% to 98.9% using 35% of the nameplate-rated load. In addition to the DOE requiring their label for commercially available transformers, many authorities having jurisdiction require transformers specified to meet these requirements.

Not all projects will follow the exact methodology as described here, but may expand to make further considerations. No two properties are the same and as such, no two projects will be the same. It is the responsibility of the design engineer to make the appropriate decisions and consult with their client to suit their needs.


Robert Sander and Mario Caraveo, NV5, Las Vegas
Author Bio: Robert Sander is an electrical engineer in training at NV5. He has experience in electrical design of high-rise hospitality and PV. Mario Caraveo is an electrical designer at NV5. Experience includes electrical design for commercial offices, retail, hospitality, industrial, mixed-use, residential and health care.