Selecting, sizing transformers for commercial buildings

While commercial building designs change, their electrical loads remain fundamentally unchanged. Properly sizing and selecting transformers ensures that these loads are accommodated.

By Ralph Baeza, PE, LEED AP, TLC Engineering for Architecture, Miami June 9, 2011

Transformers, along with other power distribution apparatus, remain a fundamental component in electrical systems distribution for commercial buildings. This article presents several useful design concepts for selecting and sizing transformers in the design of electrical systems for commercial buildings.

Transformers change voltage levels to supply electrical loads with the voltages they require. They supply the required incoming electrical service to the buildings. Transformer primary and secondary voltages can be 2,400; 4,160; 7,200; 12,470; and 13,200 for 15-kV Class, and 120, 208, 240, 277, and 480 for 600-V Class.

Transformers are located either outdoors or inside buildings in an electrical room or other areas as permitted by code. The electrical phase characteristics associated with the transformer’s primary side is 3-phase, 3-wire or Delta connected. The secondary is 3-phase, 4-wire or Wye connected.

Construction types

There are different construction types for transformers used in commercial buildings. Our understanding of their general characteristics will allow the designer and end user to make the proper selection for the electrical system application. Following are some of the transformer types available in the industry along with a few of their characteristics:

Ventilated dry-type transformers are ventilated by air, use larger space for clearance, and use different insulating materials to augment the dielectric strength of the air. They contain an enclosure surrounding the windings for their mechanical protection and the safety of personnel. This type is the most common to be used in the building indoor electrical system distribution. See Table 1 for typical dry-type transformer ratings, dimensions, and weights.

Sealed dry-type transformers are similar to dry type in most of their characteristics. The difference is they contain an enclosed tank with nitrogen or other dielectric gas to protect the windings. They can be installed outdoors or indoors. They are useful in areas with a corrosive or dirty atmosphere.

Cast-coil transformers are constructed with the primary and secondary windings encapsulated in reinforced resin. They can be installed where moisture or airborne contaminants exist.

Nonventilated dry-type transformers are similar to the ventilated type but are totally enclosed. This type can be installed in areas with corrosive or dirty atmospheric conditions where it would be impossible to use a ventilated-type transformer.

Oil-filled transformers are constructed with the windings encased in an oil-tight tank filled with insulating mineral oil. It is good practice to regularly test this type of transformer in order to determine dielectric breakdown, which affects its useful life.

Application types

There are different ways in which transformers are installed and used as part of a commercial building electrical system. These application types include:

Indoor distribution transformers are used with panelboards and are separately mounted to supply the specific electrical load requirements in a system-specific application within the system distribution. Several transformer types rated higher than 600 V for oil insulated type, higher than 35,000 V for dry type, and other transformers rated higher than 600 V are required to be located in vault rooms, which must be built with fire-rated enclosures depending on the transformer type and applicable local authority requirements, when indoors. Transformers that are not over 600 V and are part of the indoor building electrical system distribution have both primary and secondary voltages below 600 V with the most common voltage level change from 480 V to 208 Y/120 V.

Pad-mounted transformers are installed outside and are considered the first option for supplying service entrance voltage to the building electrical system based on the project size and requirements. They typically have primary voltages higher than 600 V and secondary voltages lower than 600 V with compartments for the associated protective devices assembled in an integral tamper-resistant and weatherproof unit.

In addition, the size of the commercial facility will determine the appropriate approach for designing the electrical distribution system for the specific application. In this electrical system design, the transformer can be used as part of a substation, primary unit substation, secondary unit substation, or network configuration.


The electrical size of the transformer load is rated in kVA. This rating provides the associated power output delivered for a specific period by the loads connected to the transformer on the secondary side of the equipment. The loads, which are calculated as part of the building electrical system design phase, are shown in the construction documents’ respective equipment schedules in VA or kVA.

A general approach to determining transformer capacity and selecting the proper rating for the design application is to obtain the calculated design load from the respective electrical schedule and add 20% spare capacity for future load growth to be shown in the equipment schedule, unless otherwise directed by the facility based on design parameters. For example, the code-based demand load of a 208 Y/120 V, 3-phase, 4-wire panelboard is 42 kVA, which does do not include spare capacity for future growth. Therefore, the transformer size required for converting the system voltage from 480 V, 3-phase, 3-wire to 208 Y/120 V, 3-phase, 4-wire is:

Transformer size in kVA = 42 kVA x 1.25 = 52.5 kVA

Therefore, a 75 kVA transformer would be selected for this application out of the available standard ratings for a 480 V primary to 208 Y/120 V secondary. The most common building industry standard ratings are 3, 6, 9, 15, 30, 37.5, 45, 75, 112.5, 150, 225, 300, 500, 750, and 1,000 kVA.

The above simple calculation meets the intent to achieve the normal life expectancy of a transformer, which is based on the following basic conditions:

  • The transformer is equal to or less than its rated kVA and rated voltage.
  • The average temperature of the cooling air during a 24-hour period is 86 F.
  • The temperature of the cooling air at no time exceeds 104 F.


Transformer selection starts with the kVA rating required to supply the loads connected in the electrical system. Another consideration for indoor distribution transformers is the type of load: linear or nonlinear. Linear loads include resistive heating and induction motors; nonlinear loads are produced by electronic equipment that contributes to the distortion of the electrical power signals by generating harmonics. The harmonics resulting from nonsinusoidal currents generate additional losses and heating of the transformer coils, which reduce the transformer life expectancy.

Indoor transformers for nonlinear loads can be selected with a K rating, which allows the transformer to withstand nonlinear conditions in the electrical system. K-rated transformers do not mitigate or eliminate harmonics. However, they do protect the transformer itself from damage caused by harmonics. For harmonic mitigation, K-rated transformers can be combined with harmonic filters or chokes. For linear load applications, transformers should be selected with lower core losses. Other factors that should be considered in selecting transformers are voltage ratings for both primary and secondary, voltage taps, efficiency, impedance value, type of cooling and temperature rise, voltage insulation class, basic impulse level, and sound level.

Practical applications

In the past two years, two large projects in Miami Dade County have been built: the Florida International University football stadium and Miami International Airport South Terminal. Both projects included dry-type 480 V, 3-phase to 208 Y/120V V step-down transformers (in NEMA 2 enclosures), ranging from 15 kVA to 112.5 kVA in the electrical system distribution design.

The 18,688-seat FIU football stadium was designed with about 12 transformers as part of the electrical system distribution in order to supply general-use receptacles, small motors, and other loads in the stadium building structure and the attached field house building. The MIA South Terminal expansion was designed with about 50 transformers with similar intent as the stadium’s but a more diverse group of loads for the 208 Y/120 V 3-phase, 4-wire system, which also included lighting loads, signage, telecommunication, security systems, and other loads part of this building project (Figure 1).


The installation of power transformers and transformer vaults must comply with the requirements of National Electrical Code (NFPA 70) article 450 and specific local authority having jurisdiction requirements. Some principles to consider for transformer installation include locating them in isolated rooms with proper ventilation, clearances, and accessibility. Otherwise, they can be installed on open walls or steel columns or above suspended ceilings.

In addition, there are other specific requirements based on the transformer type, such as weatherproof enclosures for dry-type transformers installed outdoors or a transformer vault room for oil-insulated transformers installed indoors. In addition, a good design and installation require the proper transformer feeder and overcurrent protection device size based on NEC articles 240, 250, 450, and applicable sections of Article 310 (Figure 2).

Looking ahead

Transformers remain a fundamental component of electrical distribution systems. Equipment operation characteristics will continue to change. However, their operating principles will remain with the same. The industry trend is to continue building transformers with less core losses, and that comply with Energy Star efficiency requirements. 

Baeza is a principal and senior electrical engineer at TLC Engineering for Architecture in Miami. He is a registered professional engineer with more than 29 years of experience in electrical engineering, project management, building design, and construction.