Transformer Sizing Calculator

Created by Luciano Miño
Reviewed by Hanna Pamuła, PhD and Steven Wooding
Last updated: Jun 05, 2023

Our transformer sizing calculator allows you to obtain the minimum single or three-phase transformer size required based on the load you need to power. If you need a device that converts from different voltages so it can power another device, then a transformer will fulfill that role perfectly.

In this short text, you will learn what a transformer is, what kVA stands for on a transformer, and how to calculate transformer load capacity from a given kVA using our transformer kVA calculator.

What is a transformer?

From its definition, a transformer is a device that transforms something. What exactly? Voltage. It takes an input voltage and produces a different output voltage. We can use it to create a step-up or step-down in voltage. This ability can be useful in many ways, such as lowering the voltage from the power lines so it can be safely used inside a house. And how does it do that? Before answering that, let's introduce the basic concepts required to understand what's really happening inside a transformer.

If you're already familiar with the concepts, skip ahead and feel free to use our transformer sizing calculator!

Magnetic Induction

Magnetic induction is the process in which a changing magnetic flux induces a current on a conductor. Faraday's law of induction, which you can read more about in our faraday's law calculator, describes it:

emf=dΦdt\quad \text{emf} = {-\frac{d\Phi}{dt}}

Where emf\text{emf} is the electromotive force that drives the current through a conductor. In other words, a current will be produced in the conductor if the magnetic field that goes through it changes in time. This effect can occur if the magnetic field changes intensity or if the conductor changes position, for example.

Now, what will be the direction of the current? As we know from our magnetic field of straight current-carrying wire calculator, a current produces a magnetic field, and Lenz's law tells us that, 'the emf induced by the changing magnetic flux, will have a direction such that the magnetic field it creates, opposes the change in flux'.

Transformers utilize this concept to convert voltages. Let's see how in the following section.

How does a transformer change voltages?

Let's analyze the following picture:

Illustration of a single phase transformer.
Illustration of a single-phase transformer.

A single phase transformer consists of two windings: the primary (left) and secondary winding (right). When an alternating current passes through the primary winding, a changing magnetic flux occurs in its interior. If a magnetic core is added, it will direct the flux through the secondary winding, which will induce a current on it (remember, a changing magnetic flux induces an emf). The resulting relation between the voltage on each winding is described below:

VsVp=NsNp\quad \frac{V_{s}}{V_{p}}=\frac{N_{s}}{N_{p}}

So, by changing the number of turns in the winding on each side, we can control the change in voltage between them. How simple is that!

While a transformer is a really efficient device, and we use an ideal transformer in our transformer sizing calculator, there exist multiple sources of power losses, for example:

  • Eddy currents. When the changing magnetic flux goes through the magnetic core, it induces a current on it. These currents will then produce heat. That's why the core consists of multiple plates coated with an insulating material to keep the field lines from penetrating through them.
  • Iron losses or hysteresis loss. When the flux changes direction, power is required to magnetize and demagnetize the core.
  • Heat loss through the windings. When electricity circulates through the windings, the material is heated increasingly with the current.

A 3-phase transformer uses the same principles as the 1-phase transformer. However, 3-phase transformers are primarily for industrial use. Our calculator works as a 3-phase transformer calculator as well!

🙋 While our transformer sizing calculator is awesome, you won't learn how to build a transformer with it. So, unless you're really sure about what you're doing, please don't go and try to build it on your own. Although fun, it can be dangerous.

What does kVA stand for on a transformer?

The rating unit for a transformer is the kVA, kilovolt-ampere. It displays how much load current and voltage the transformer can withstand.

If the load connected is resistive (such as heaters or incandescent lights), it will consume active power, or power used for generating real work, which is measured in kW. When an inductive load is connected instead (motors or speakers), reactive power is consumed. This power doesn't produce actual work and is measured in kVAR, kilovolt-ampere reactive.

The apparent power combines both, its unit is the kVA (read more about this in our kva calculator), and we use that as the transformer rating's unit because it doesn't distinguish between loads so that you can use either. Its formula is:

App. Power=Pact2+Preact2\text{App. Power} = \sqrt{P_{act}^{2}+P_{react}^{2}}

How do you calculate kVA for a transformer?

The formula is straightforward. We just need the load's current and voltage requirements (it's indicated in the device's label). Then we multiply them and later divide the result by 1000 to express it in kVA.

The formula for a single-phase transformer is:

kVA=IV1000\quad \text{kVA}= \frac{I \cdot V}{1000}

And for a three-phase transformer:

kVA=IV31000\quad \text{kVA}= \frac{I \cdot V \cdot \sqrt{3}}{1000}

Where II and VV are the connected load's current and voltage, respectively.

Now you know how to calculate kVA for a transformer and are ready to use our transformer kVA calculator!

How to calculate transformer load capacity

We already know how to obtain a transformer's kVA rating based on load, but how do we go the other way around? Do we need some other transformer amp calculator? Let's see how to calculate transformer load capacity:

  1. Obtain the transformer kVA rating.
  2. Get your load's required voltage.
  3. Rearrange the kVA equation to leave the current on one side and our parameters on the other. Resulting in: I=1000  kVAVI= \frac{1000 \ \cdot \ \text{kVA}}{V}
  4. Plug in the data and solve!

Or easier: use our calculator! It will automatically solve for the missing parameters, that way, it can work as a:

  • Transformer sizing calculator
  • Transformer amp calculator
  • 3-Phase transformer calculator

If you want to obtain the requirements for a specific kVA transformer, use our ideal transformer calculator instead!

Luciano Miño
Transformer type
Load current (I)
Load voltage (V)
Minimum kVA required
You can include spare capacity with the advanced mode!
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