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What Size Transformer Do I Need?

Nov. 16, 2021


What is the purpose of step-up and step-down transformers?

What is a step-up transformer?

A step-up transformer is a transformer in which the secondary voltage is greater than the primary voltage. This transformer "raises" the voltage applied to it. For example, a step-up transformer is required for the use of 220v products in countries where 110v power is available.


What is a step-down transformer?

It is the opposite of the above and would be used to run, for example, a 110v product in a country with a 220v supply.

Dry-type Transformer

Dry-type Transformer     

How does a step-up transformer or step-down transformer work?

A transformer consists of two or more insulated wire coils wound around an iron core. When voltage is applied to one coil (usually called the primary or input), it magnetizes the core, which induces a voltage in the other coil (usually called the secondary or output). The ratio of the number of turns in the two sets of windings determines the amount of variable voltage.

An example is 100 turns for the primary and 50 turns for the secondary, a ratio of 2 to 1.


A transformer is nothing more than a voltage ratio device.

For step-up or step-down transformers, the voltage ratio between the primary and secondary will reflect the "turns ratio" (except for single-phase less than 1 kva with a compensated secondary). A practical application of this 2 to 1 turns ratio is a step-down of 480 to 240. Please note that if the input is 440 volts, the output will be 220 volts. The ratio between input and output voltage will remain the same. Transformers should not be operated at voltages higher than the nameplate rated voltage but can be operated at voltages lower than the rated voltage. It is therefore possible to use standard transformers for some non-standard applications.

Oil-immersed Transformer

 Oil-immersed Transformer     

Single-phase transformers of 1 kva and larger can also be connected in reverse to step-down or step-up voltages. (Note: Single-phase step-up or step-down transformers of sizes smaller than 1 KVA should not be connected in reverse because the secondary winding has extra turned to overcome the voltage drop when a load is applied. If connected in reverse, the output voltage will be less than desired.)

Step-up and step-down transformers have a long life.

The main components of a voltage transformer are the core and the coil of the transformer. An insulation layer is placed between the turns of the conductors to prevent them from short-circuiting each other or grounding. This is usually made up of polyester film, Nomex, kraft paper, varnish, or other materials.


As the transformer has no moving parts, it usually has a life expectancy of between 20 and 25 years.


What are the rules for step-up transformers?

Step-up transformers are sold according to their kilowatt-volt-ampere (KVA) rating, often referred to as the "size" of the transformer. There are two rules that apply when determining the appropriate size of step-up transformer required: the Electrical Code and Watt's Law.

Electrical specifications

According to the National Electrical Code, step-up transformers must have a certain level of current protection. The load centre served by the transformer must be fitted with a main circuit breaker, which must not be greater than 70% of the maximum current capacity of the transformer.

Dry Type Transformer

Dry Type Transformer     

Kilovolt-ampere ratings

All transformers have a KVA rating. This rating indicates the maximum capacity of the transformer in kilowatts (kW). In calculating the practical meaning of KVA, Watt's law is applied. In Watt's law, the power (P) is equal to the output voltage (E) multiplied by the amperage capacity (I). Using this formula P = E x I and its direct derivatives I = P / E and E = P / I, all transformer properties can be calculated. For example, if a transformer is rated at 10 KVA and has an output of 240 volts, its current capacity is 41.67 amperes (10,000 watts / 240 volts = 41.67 amperes).



Watt's Law also applies when selecting a step-up transformer for an application. If the transformer needs to step up from 240 volts to 480 volts and you require a maximum current capacity of 40 amps, you must first calculate the number of amps required to comply with the electrical code. If the code requires a maximum of 70% of the transformer capacity, multiply 40 by 1.43. The product will be 57.2 amps, of which 70% is 40.0 amps. Knowing that you need 57.2 amps (I) and 480 volts (E) of output power, you can apply P = I x E. Therefore, 57.2 x 480 = 27,456 watts or 27.456 KVA. This would be the minimum size transformer required for this application.

Let us help you find the best transformer for your needs. Welcome to contact us today or visit our website to learn more about transformers.

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