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A Complete Guide to Properly Size a Transformer

Selecting the wrong transformer size for the required application could adversely affect the entire power distribution system. Here’s a closer look at how to size a transformer and consider other characteristics that may impact performance.

Understand the Units of Measurement

A transformer’s full load ampacity helps you determine the size required to handle a particular load. Transformer sizes are usually expressed in kilovolt-amperes or kVA. Additionally, some models, particularly smaller ones, are sized as volt-amperes or VA. One kVA equals 1,000 VA. A transformer with a 1.0 KVA or 1,000 VA rating can handle 100 volts at 10 volts of current.

Look at the Transformer Schematic

Transformer schematics are visual representations of the components used in circuit designs. The electrical load connected to the transformer’s secondary winding requires a certain load voltage contained in the schematic.

The schematic will also detail the current flow, or load phase current, required for the electrical load. Alternatively, you can calculate the current flow by dividing the input voltage by the input resistance.

Calculate the Kilowatts for the Load’s Power Requirements

Now that you know the load voltage and load phase current, it’s time to convert them into kilowatts. Do that by multiplying the load voltage by the load phase current and dividing the result by 1,000.

Convert the Kilowatts to Kilovolt-Amperes

Next, divide your kilowatts figure by 0.8. That’s the typical power factor of a load. The final number you get from that calculation represents the kilovolt-amperes.

Remember that transformer loads will vary depending on what you run at a given time. The goal is to calculate the maximum load your transformer might handle at once. Some people also recommend adding 20% to the figure to accommodate unplanned growth.

Know the Considerations Associated With Starting the Transformer

One of the things to keep in mind when sizing a transformer is that starting it will usually require more current than running it. That’s why it’s a good idea to include a start factor in your calculations.

First, multiply the voltage by the amperage. Then, accommodate for an additional start factor of 125% by multiplying the figure you get by 1.25. Moreover, if you plan to start the transformer more than once per hour, it’s best to figure an even larger start factor percentage to accommodate that usage.

Think About Future Expansion Needs

When it’s highly likely that people will need to expand their transformer setups, they often choose transformers with open-delta connections rather than standard closed-delta connections. This approach allows using two transformers to accommodate the current load.

However, people can get a 58% capacity increase over a closed-delta connection by adding a third transformer later.

Learn the Specifics of kVA Ratings

A transformer’s kVA rating is usually shown as a whole number from 3-1,000 if you’re shopping for the most transformer ratings. However, some transformers have ratings of more than 2,000 kVA. Moreover, you’ll also see some differences in the kVA ratings depending on if you’re getting a single-phase or three-phase transformer.

Single-Phase Transformers and Their Ratings

A single-phase transformer has a single line of alternating current (AC) power. You can usually find them as encapsulated, ventilated and enclosed nonventilated types. The encapsulated versions work well for general indoor and outdoor loads. Ratings generally span from about 50 VA to 25 kVA.

Ventilated single-phase transformers are also suitable for indoor and outdoor use. Some companies use them to handle their lighting needs. The transformers in this group usually have slightly higher ratings, such as 25-100 kVA.

Three-Phase Transformers and Their Ratings

As you might guess from their name, three-phase transformers usually have three lines of power. Each one has a 120-degree difference in phase compared to the other two.

Encapsulated three-phase transformers usually have ratings from 3-75 kVA. When you size a transformer and want a ventilated three-phase type, expect the rating to reach significantly higher than what’s been described and sometimes surpass 1,000 kVA.

Finally, there are enclosed nonventilated units, which may be either the one or three-phase types. They’re ideal for dusty environments. The associated ratings are typically from 25-500 kVA.

Important Considerations After You Size a Transformer

Choosing the correct transformer size for your needs is undoubtedly an important step, but your planning should not end there.

Transformer Oil

Selecting the right transformer oil will ensure your equipment keeps running smoothly and has a long life. Naphthenic fluid for transformers has a moderately soluble sludge that does not leave deposits on windings. Paraffinic oil offers high antioxidation stability, which helps extend the overall service life. Choosing the right kind of transformer oil is important for general upkeep concerns.

Transformer Placements on Concrete Pads

The kVA rating helps determine the correct size of a concrete base if you plan to install your transformer on one. When the rating is 75-500 kVA, ensure that the pad size measures 5.5 by 6.5 feet and is 10 inches thick.

When you have a transformer rated at 500-2,500 kVA, the pad thickness can stay at 10 inches. However, increase the other measurements to 8 feet by 9 feet.

Another popular option is to attach the transformer to a pole. Regardless of these specifics, the transformer placement should not pose hazards to people working in the vicinity.

Real-Time Monitoring Options

After you size a transformer and start using it, you may wish to install accessories that help ensure the equipment operates safely. Some companies sell products that connect to the transformer and give real-time metrics during use.

One popular possibility concerns temperature management. Some products also have built-in cooling mechanisms that prevent thermal overloads.

Manufacturer Guidance Can Provide Additional Tips to Size a Transformer

This guide helps you go through the steps to size a transformer without help. However, your chosen manufacturer could be an excellent resource if you’d like additional assistance. Such companies often have charts and calculators to help customers make well-informed decisions before purchasing.

Remember that you will need to make new calculations to size a transformer whenever something about the application changes. For example, using it to run different equipment requires another calculation. Also, if you plan to run a transformer outdoors, consider what steps you might need to take to protect it from corrosion.

Understanding what you require from a transformer will make it easier to get the right size and take other precautions to keep it running well. It’s an especially good idea to get professional guidance before making your purchase if you’re buying a transformer for the first time.

Emily Newton is the Editor-in-Chief of Revolutionized Magazine. She has over three years experience writing articles for the tech and industrial sectors. Subscribe to the Revolutionized newsletter for more content from Emily at
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