Selecting, sizing transformers in nonresidential buildings

Transformers are a critical piece of any electrical infrastructure. Knowing how to properly size and select these transformers will allow the building to function with minimal interruptions.

By Stephen Berta, EI, and Robert R. Jones Jr., PE, LEED AP, NV5, Las Vegas December 12, 2018

Learning Objectives

  • Learn about the different types and sizes of transformers in use in nonresidential buildings.
  • Understand the different ratings and performance characteristics associated with transformer selection.
  • Efficiently select and size a transformer for a specific set of design conditions.

Transformers are a ubiquitous component of any nonresidential distribution system. With many different sizes, types, and listed uses, it can be quite complicated to pick the correct transformer for a specific application.

Transformers are a passive component of electrical distribution systems and are used throughout electrical systems to change the voltage, they will either step-down a voltage or step-up a voltage with almost zero power loss; the average transformer is roughly 98% to 99.5% efficient. Typically, step-down transformers are seen in commercial buildings to bring the voltage from a higher utility-distribution voltage (7 to 25 kV) to a lower utilization voltage (120/208 V, 277/480 V, etc.). This is done for safety and efficiency; it is much more efficient for transmission to take place at high voltages with smaller conductor sizes and it is much safer for a lower utilization voltage at the end-user equipment.

There are myriad transformer types; however, there is only a handful used commonly. NFPA 70: National Electrical Code (NEC) 2017 outlines the installation requirements for these transformers and provides minimal guidance on sizing the transformers.

Transformer ratings

Transformers are rated using industry-accepted terms, such as capacity (kilovolt-amperes rating or kVA rating), voltage, temperature rise, and insulation class.


Transformers are rated to carry a load, which is indicated by their kilovolt-amperes nameplate ratings at the rated output voltage and frequency. These are standard within the industry and include: 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, and 1,000 kVA. There are several transformers larger than this on the utility level; however, they are not common, with the exception of large projects with campus-style distribution schemes.


A transformer will have a primary and secondary voltage rating along with the wiring configuration. In U.S. facilities, the overwhelming majority of transformer installations are a step-down 480-V delta primary to 120/208-V wye secondary. Other voltage ratings and configuration types exist for special applications.

Unless there are strong technical reasons otherwise, most transformer installations should have a wye output. Whether the neutral is required for the downstream distribution system, this configuration provides a point to intentionally create the reference to ground; this is important as transformers are separately derived systems in which the ground reference is lost on the primary side and must be re-established on the secondary. A corner-grounded delta is possible, and often encountered in older installations, but has several disadvantages. The grounded phase must be labeled throughout the system, a higher line-to-ground voltage exists on two phases resulting in a reduced fault-current rating on circuit breakers, and circuit breakers must be marked 1- to 3-phase.

Energy efficiency

The U.S. Department of Energy (DOE) mandates transformer efficiencies. The most recent regulation, commonly referred to as DOE-2016, was adopted starting Jan. 1, 2016, and is a requirement for all transformers produced in or imported into the U.S. Manufacturers are permitted to sell all remaining inventories; most transformers at this time are compliant with DOE-2016.

Sound level

All transformers will give off both vibrations and audible noise caused by magnetic expansion and contraction of the cores. These vibrations cannot be entirely eliminated but may be mitigated with measures such as vibration-isolation pads and acoustically dense room construction.

Insulation class (also known as temperature class)

This rating describes the maximum temperature, in Celsius, at which the windings may operate without damage to the insulation. Several common insulation classes are 105, 150, 180, and 220.

It should be noted that NEC 450.21(B) requires a fire-resistant room (1-hour fire-rated construction) for transformers that are larger than 112.5 kVA with an insulation class below 155°C. The typical transformer specified today has an insulation class of 220°C and falls within exception No. 2.

Temperature rise

This is the average change in temperature at the windings from a no-load to full-load situation. Typically, this is expressed in degrees Celsius. This rating is usually standard based on the insulation class.

Standard dry-type temperature rise is 80°C, 115°C, or 150°C.

Standard oil-filled temperature rise is 60°C.

Hot spot allowance

The windings of a transformer core are not evenly heated during operation, instead the interior of the windings is hotter than the surrounding areas because they are farther from any ventilated openings. This is a set number as defined by industry standards and is associated with the insulation class. For example, a Class 105°C transformer will be allotted a 10°C hot spot allowance.

Ambient temperature

This rating specifies the average temperature of the space that the transformer will occupy over a 24-hour period. Typically, this rating is 40°C.

It should be noted that typically the maximum operating temperature for a transformer is:

Maximum temperature (°C) = ambient temperature + temperature rise + hot spot allowance


Each transformer is required to have a permanently affixed label indicating the requirements outlined in NEC 450.11(A). These are: manufacturer, rated kilovolt-amperes, frequency, primary and secondary voltages, impedance (if greater than 25 kVA), required clearances, quantity and type of insulating liquid (if any), and temperature class (see Figure 1).

Common transformer types

Liquid-filled transformer

These units are filled with a liquid that acts as the cooling fluid and dielectric medium between the transformer cores. The most common types of liquids in use are mineral oil and less-flammable bio-based oils. Mineral oil is commonly used for exterior pad-mounted utility transformers and is considered combustible, with a flashpoint of less than 300°C. Typically the flashpoint of mineral oil is around 155°C. Bio-based oils do not contain petroleum and are made from vegetable oils. These bio-based oils have a much lower flashpoint—around 330°C—and are much more environmentally friendly; in the event of a leak, they will biodegrade within a month under normal conditions.

Indoor use is restricted to vault rooms and the installation must comply with NEC 450.26. These vaults are characterized by having exterior access, 3-hour fire-rated construction, liquid containment, and exterior ventilation per NEC 450.42. FR3 is a less flammable liquid with a flashpoint of around 316°C and is preferred for liquid-filled transformers located within the building. Installation requirements for transformers insulated with less flammable liquids are addressed under NEC 450.23. Indoor installations that comply with NEC 450.23(A)(1) do not require a vault or sprinkler protection.

Dry-type transformer

These units are air-cooled and are not liquid-filled. As these units rely on air to cool the core and windings through convection, they are typically larger than their liquid-filled counterparts. Within the dry-type family there are two specific subtypes: ventilated and nonventilated. Ventilated dry-type transformers have openings within their enclosures and allow air movement from the exterior of the enclosure to the coils within the enclosure. Nonventilated (or sealed) dry-type transformers are completely sealed and allow cooling through the surface area of the enclosure. These units are well-suited for wash-down areas as well as corrosive, combustible, or otherwise harmful conditions.

It should be noted that NEC-2017 currently has a typographical error in section 450.23(A) where it appears to limit indoor installations to vault rooms. However, this has been corrected in errata number 70-17-6, issued July 5, 2018. This corrects NEC 450.23(A)(1)(e), which requires less flammable liquid-insulated transformers installed indoors to be supplied with automatic fire suppression and liquid containment. NEC 450.23(A)(1)(f) requires that the less flammable oil-insulated transformer is installed in a vault to be subsets of NEC 450.23(A) instead of being supplemental requirements.

This correction now allows three options for indoor installation in lieu of the single way that the code appears to outline. For transformers less than 35,000 V, all three installations are acceptable: installation with liquid confinement and no combustibles stored in the room, installation with automatic fire suppression, or installation in a vault room. For transformers larger than 35,000 V, there is only one code-compliant installation, which is constructing a vault room.

Liquid-filled transformers operate with a standard temperature rise of 60°C above ambient. These transformers can sustain bursts of overload up 50% above a nameplate rating for short periods of time. The amount of time and overload capacity is directly related to the previous continuous loading and the winding temperature.

Resin cast coil

A cast coil or epoxy cast coil transformer is a dry-type transformer constructed with the primary and secondary windings fully encapsulated in a resin, which protects the transformer from moisture, corrosion, or other aggressive contaminants. These transformers have a standard ventilated dry-type enclosure.

The properties of this insulation provide a higher short-circuit strength and can sustain repeated short-duration overloads.

Harmonic mitigating transformer (HMT)

These transformers are specially designed to cancel out problematic harmonic currents (often these are the 3rd, 9th, 15th, or triplen harmonics) resulting from nonlinear loads. These transformers accomplish this through phase shifting and zero-sequence flux cancellation within the cores, preventing the harmonics from being reflected back to the primary winding of the transformer

K-rated transformer

These transformers are commonly used in applications involving harmonic currents, but they will not cancel out the harmonics in the same process as an HMT. Instead, this transformer has a derated core and will withstand the heating effects of harmonic currents. Additionally, this type of transformer will not prevent the harmonics from propagating up through the distribution system through the transformer primary.

The industry commonly refers to K-rated and HMT transformers as alternatives to each other because they are commonly used to address the same issue. However, the HMT will remedy the issue while the K-rated transformer is merely a solution to the transformer damage and does not address the harmonic currents for the entire distribution system.

Load types and sizing

When selecting a transformer, load calculations are completed as outlined within the NEC. Sizing of panelboards and branch circuits is typically done through calculations outlined in NEC Article 210, Branch Circuits, and NEC Article 230, Services. This calculation or summation of calculations shall be used in sizing the transformer. For example, a building that has a total NEC demand load of 60 kVA on the 120/208-V, 3 phase, 4-wire system.

To size the transformer, it is recommended to allow 25% future growth to the circuit and then adopt the next standard transformer size up. In this example, 60 kVA of demand load shall be multiplied by a factor of 1.25, resulting in a load of 89 kVA. Obviously, 89 kVA is not a standard transformer size and it is recommended to use the next standard size, which is 112.5 kVA.

Once the size of the transformer has been decided, the type of transformer shall be selected. The next logical step is to determine the load types that are in use on the downstream system. If the system has a high number of resistive or linear loads, the selection is quite easy and sends us to a standard dry-type or oil-filled transformer (depending on location, size, etc.). However, if there are several nonlinear loads, such as computer/servers with switch-mode power supplies, gaming slot machines, LED lighting, motors, or variable frequency drives (VFDs), an HMT should be considered.

Systems with harmonic loads commonly are designed with K-rated transformers in lieu of an HMT. It is important to note that an HMT will correct the issue while a K-rated transformer will only withstand the heating of the transformer core. The installation of a K-rated transformer is a solution if the system as a whole can withstand the harmonic currents without failures and the only point of concern is the premature failure of the transformer due to overheating the windings.


A specific location can make or break transformer selection. Depending on the space in which the transformer is intended to be installed, certain options may be completely off the table. For example, if you are in an indoor corrosive environment, the most effective options are either a nonventilated or cast coil transformer. Conversely, it may be too costly to use a liquid-filled transformer in a standard indoor application due to potential vault-construction requirements and oil containment.

Consideration also should be given to the location of transformers in relation to surrounding occupancies and replacement. Transformers produce an audible low humming and vibration that can propagate through the structure and surrounding spaces. It is common to furnish a transformer with vibration-isolation pads and flex-metal conduit for the final connection; these precautions prevent the vibrations from entering the building structure. However, the hum of the transformer will still be audible in surrounding spaces.

Vault room

When designing with a liquid-insulated transformer for installation within the building footprint, a vault room is often required by NEC 450.26. Less flammable liquid-insulated transformers also can fall under the constraints of a vault room, but this is slightly less common—and when selecting a less flammable liquid-insulated transformer, the intent is often to avoid construction of a vault room.

The room requirements are outlined in NEC 450, Part III, beginning with 450.41. There are several requirements outlined in Part III, such as ventilating the vault with outside air via ductwork or a flue, 3-hour fire-resistant construction, a 4-in. concrete floor, and oil containment for the largest transformer in the room. A typical 3-hour fire-resistant room will be constructed with 6-in.-thick reinforced concrete, which can add considerable cost and complexity to a project.

There is an exception to allow for 1-hour fire-resistant construction, with the use of sprinklers, carbon dioxide, or halon systems. These rooms typically are designed for utility transformers and services; in these cases, both the NEC and the utility’s requirements shall be followed.


Dry-type transformers are the most commonly used transformers for indoor installation, as there are no restrictions on their location. As previously mentioned, if the transformer is larger than 112.5 kVA with an insulation class below 155°C, a 1-hour fire-resistant room shall be required.

However, this is atypical of most transformers specified today. Additionally, less flammable oil-filled transformers may be installed indoors with minimal requirements, such as either an automatic sprinkler system or liquid containment with no combustibles stored inside the room for transformers less than 35 kV. It is not uncommon to see smaller dry-type transformers installed on a wall-mount assembly, hung from the slab in an open-ceiling room, even hung from the slab with an open ceiling or above a drop ceiling. Per NEC 450.13(B), a transformer installed in a hollow building space shall not exceed 50 kVA.

Overcurrent protection

Transformers must be protected like any other component of a building’s electrical infrastructure. An entire article could be written discussing protection strategies, sizing, and requirements. These requirements are outlined in NEC 240.21 and 450.3. It is common practice to use NEC Table 450.3(B) and provide both the secondary and primary protection based on this table.

In conclusion, there are many details that come into play when selecting and sizing a transformer. Once a specific end use and location have been determined, the selection process can begin while considering and aggregating the topics outlined in this article. It is important to note that some end users may have their own requirements in addition to those listed here.

Author Bio: Stephen Berta is a project consultant at NV5. He has experience in electrical design of hospitality, gaming, K-5 education, and data centers. Robert R. Jones Jr. is the associate director of electrical engineering for NV5’s Las Vegas office. He has experience in multiple market sectors including hospitality, commercial, medical, and government projects.