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Specifying a transformer requires significant understanding of the environment in which it will reside, the input and output needs and power stability, in order to provide trouble-free performance that meets your application’s exact requirements.

There are a few specifications you may currently know your transformer requires, such as the input and output voltage and if it is single-phase or three-phase, as well as whether it is 50 or 60hz and what the necessary kVA size will be.

Beyond the fundamental requirements, you may be unsure of what specifications your transformer demands for reliable operation. Here are several key components to review when specifying a transformer.

Industrial control transformers

If you have an electromagnetic device that requires an above average voltage regulation when actuated, an industrial control transformer can accommodate the momentary current inrush. They provide the high degree of secondary voltage stability required for the 30-50 milliseconds upon start-up that demands 3-10 times the normal operating current. Industrial control transformers are frequently used for motor starters, contactors, solenoids and timers.

Contrastingly, a general purpose transformer is primarily utilized for low voltage applications, such as lighting. General purpose transformers are to be utilized in accordance with the National Electrical Code, NFPA 70.

Insulation class or system

Ranging from 105 degrees Celsius to 220 degrees Celsius, five classes of insulation systems exist: A (for 135 degrees Celsius), B (for 130 degrees Celsius), F (for 155 degrees Celsius), H (for 180 degrees Celsius) and C (for 220 degrees Celsius).

However, one class or system is not necessarily superior or more recommended. Each one maintains a life expectancy of approximately 20 years, as well as an efficiency level that’s nearly identical.

Class F and H insulation systems are in much higher demand than Class A and B, due to the fact that transformer size and weight has been vastly reduced. A “Class A” transformer that would weigh approximately 1,000 pounds can be reduced to approximately 450 pounds by using Class H insulation with 150 degrees Celsius rise.

Cost benefits can be a substantial driver when determining which class or system to select. Class insulation systems for higher temperatures are more expensive, such as for a larger transformer that necessitates a higher temperature system.

Isolating vs. insulating transformers

An isolation transformer is the separation/barrier between the primary and secondary windings. The terms isolating and insulating are interchangeable, as they do not indicate two separate transformer types.

Isolation/insulating transformers are a general purpose or industrial control transformer with two windings, transforming all the incoming voltage rather than a portion of it.

A non-isolation transformer is an autotransformer constructed with only one winding and capable of only transforming a portion of incoming voltage. An autotransformer is smaller and built with less material than an insulating transformer with the same kVA and rating.


A shielded transformer is designed with a metallic shield between the primary and secondary windings to attenuate transient noise as well as provide a safety precaution. If the insulation fails between the primary and secondary windings, the fault will travel to the shield and then the ground. The primary application for shielded transformers is for hospital utilization.

Ambient temperature

Ambient is the air temperature surrounding the transformer. If a transformer is designed for 40 degrees Celsius ambient, that will need to be considered or the engineer will need to derate the transformer.

Transformer enclosure type

Enclosure type is dependent upon the transformer’s application and location. If the transformer is housed in the equipment enclosure along with another component, no transformer enclosure is required. However, if the transformer is a standalone unit and/or exposed to outside weather, a NEMA Type II or NEMA 3R enclosure is required, respectively.

Other applications may require a different type of enclosure than NEMA Type II or NEMA 3R, as it entirely depends on the transformer’s application and location. Enclosure locations are defined in NEMA Standard 250, UL 50 or UL 508 and CSA C22.2 No. 94.

Working with SNC

SNC can help direct you in determining what transformer specifications are necessitated from your specific application’s requirements. Throughout the entire process, from requesting a quote to designing and building, we will explain any specifications and regulations not fully understood, as well as answer any questions.

When you request a quote from us, we ask that you provide a list of any electrical and construction specifications you currently know you need. If you decide to work with us, you will then fill out a more detailed form describing the design you currently know will be required.

Contact us today for more information on how we can help you determine your transformer specifications to help ensure smooth, durable operation.