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How to store transformers

Planning on keeping a transformer in storage before energizing it? Learn how to properly store your transformer, so that it is ready for use when you need it.

April 4, 2024

Maddox's transformer storage yard in Greer, South Carolina

Planning on storing a transformer long-term before energizing it? 

This article is a guide on the when, where, what, and how of transformer storage. In it, we cover distribution transformers below 69kV.

When should I consider long-term storage?

If you plan to keep a transformer unenergized for 6 months or more, you need to create a storage plan.

Where should I store my transformer?

A reliable and efficient storage plan starts with a good location. When possible, backup units should be...

  1. Near the installation site for rapid deployment
  2. Under cover when possible (especially for dry-types)
  3. Not subjected to wide temperature swings

What type of transformers do I store?

Ventilated dry-type and sealed liquid-filled transformers need long-term storage plans. You can store them both for extended periods, but the maintenance for each varies. Let’s break down in greater detail the how for both types of transformers.

How to store transformers

Liquid-filled transformers

Padmount and substation transformers are designed for outdoor installation. Their sealed tank makes for simpler storage and maintenance.

There are four keys to storing liquid-filled transformers:

  1. Keep the nitrogen blanket at 2-3 psi
  2. Store transformers with their factory-filled amount of oil
  3. Avoid drastic temperature changes 
  4. Perform acceptance and maintenance testing at the end of the storage period

Let’s open up these four points a bit more…

Padmount transformers being stored outside
Padmount transformers stored outside in Maddox's Battle Ground, WA yard.

Monitor Nitrogen Levels

All new and reconditioned transformers from Maddox ship with a pressurized nitrogen blanket. Typical values range between -2 psi and +5 psi. The nitrogen blanket should be positively charged for long-term storage situations. If the pressure gauge reads negative, consider filling the nitrogen port until it reaches 2-3 psi. Ensure the pressure holds for at least eight hours before storing the unit. 

Conduct routine checks to ensure the pressure gauge reads 2-3 psi. Our recommendation is to check transformers quarterly, which allows you to monitor nitrogen levels at varying ambient temperatures.

Ideally, positive pressure will be maintained on the unit. However, negative pressure may exist if the transformer is stored in an area where the temperature varies greatly. An example would be a high desert environment where the daytime and nighttime temperatures vary widely.

In any case, it is technically less important that the unit always has positive pressure, but rather that it is able to hold both positive and negative pressure.

The worst pressure reading is 0. If it stays at 0, it’s free breathing, and you might have an air leak somewhere in the transformer. Check out this video on leak troubleshooting.

A padmount transformer pressure vacuum gauge

Oil Levels

Maddox units ship filled with the proper amount of transformer oil. This oil should remain in the unit during storage. Check the oil level prior to installation.


At the end of the storage period, perform standard maintenance and acceptance tests. Any unit that has been in long-term storage should be tested before going into service.

At a minimum, testing protocol would include TTR, insulation resistance (aka Megger), and winding resistance.

A low megger reading might indicate the presence of moisture in the transformer fluid.

We recommend testing the oil for moisture levels before putting a spare unit back into service.

Storing Liquid-filled Units Indoors

Liquid-filled transformers can be (and often are) stored indoors, but this is not a requirement at all. For instance, Maddox keeps hundreds of liquid-filled transformers stored outdoors all year long. That said, we don’t necessarily plan on keeping units on hand for more than a year before deploying them, while end users seeking to keep a mission-critical spare on hand may end up storing a transformer for decades.

In those cases, some end users consider the ideal place to store their liquid-filled transformers indoors. 

Having the transformer completely protected from the elements would keep the protective coating (paint) from breaking down and would also slow the degradation of plastic and rubber components like gaskets, gauges, and some types of bushings.

But here are some things to keep in mind if you are considering storing a liquid-filled transformer indoors…

  1. Floor capacity: Most standard warehouse floors have a 3-4” concrete slab, which will easily crack under the weight of larger liquid-filled units. You should first understand the floor's weight capacity and make sure it can handle the weight of the transformer - plus the weight of the forklift used to move it.
  2. Environmental implications: Liquid-filled transformers are big tanks filled with oil. You need to understand the environmental and insurance implications of having that much oil stored indoors in the event of a leak or a fire.

UV Rated Substation Transformer Components

One last thing to consider when storing transformers outdoors has to do with unit substation style transformers. 

If you plan to store a unit substation-type transformer outdoors, check with the manufacturer to ensure that the gauges and bushings are UV-rated.

If the substation transformer was designed for indoor use, the bushings and gauge face plates might not be rated for long-term storage in direct sunlight. Over time, the gauge plates may fog up like old car headlights, and the enamel on the porcelain bushings might degrade to the point where they are not effective insulators.

Generally speaking, if a unit substation transformer has bushings on the top, the intention is for them to be exposed to the elements, and you can assume they are UV-rated. However, if a transformer has the bushings on the sides which are made to be in sealed air terminal chambers, they probably aren’t made to be stored in direct sunlight, and they will lose their insulating properties over time.

Shrink-wrapping or finding some other way to seal these types of bushings up may be a solution.

Reconditioning Spare Transformers Prior To Deploying Them

If your liquid-filled transformer has been stored for a decade or more, you want to consider having a transformer repair company recondition the transformer. This does a few things…

  1. Ensures that any degraded gaskets are refreshed. Gasket failures are common in older transformers.
  2. Gives it a fresh protective coating to stave off rust.
  3. Provides an up-to-date report on all standard electrical and oil diagnostic tests.
  4. Refreshes the warranty so that the transformer is under warranty during its most critical phase of life—at the very beginning.

Contact us if you need a quote to recondition an old transformer.

Dry-type transformers

Since dry-type transformers are not sealed, they require more storage considerations.

Follow the checklist below to properly store your dry-type transformers:

  1. Keep internal components and windings dry and free of dust
    • Store inside a clean climate controlled building when possible
    • Cover with a tarp when storing outside
  2. Keep the enclosure free of critters, dust, and debris
  3. Perform acceptance and maintenance testing at the end of the storage period (Learn about transformer testing)
Medium voltage dry type transformers under tarps

Keeping your transformer dry

Dry-type windings are more susceptible to moisture (due to the vented enclosure). For long-term storage, keep the transformer dry and properly ventilated. This is best done in a temperature controlled building. Space heaters and/or desiccant packets help remove moisture inside of an enclosure. Make sure you check the packets during regular maintenance intervals. Covering the enclosure with a tarp adds protection for units stored outdoors.

Cleaning the enclosure

Vents in dry transformers also allow for the ingress of dirt, debris, and rodents. Long-term storage plans should include regular cleaning of the enclosure’s inside area. If the unit is stored where critters may be an issue, rodent screens should be used to block vent openings.


Perform standard maintenance and acceptance tests before putting stored dry transformers into service. Perform one or more tests to check the moisture content in and around the windings.

How do I know if my transformer is dry enough?

This is a frequently asked question when it comes to storing dry-types. One way to check is through an insulation resistance (Megger) test. Well insulated transformers should measure at least 1MΩ per volt. For example, a reading of 500MΩ on a 480 volt unit is acceptable.

Maddox technicians taking a megger test on a dry type transformer.
Maddox technicians taking a megger test on a dry type transformer.

If your transformer reads less than that, it is not dry enough. Larger dry-types with high levels of moisture may need to be cured. At Maddox, we have bake-out ovens that can remove this moisture. Fill out this form if you need this done.

A transformer core in Maddox's bake out oven.
A transformer core in Maddox's bake out oven.

Final Thoughts 

Storing spare transformers is an excellent way to prepare for the unexpected. Follow the above steps in this article to ensure your transformer is ready for service when you need it.

At Maddox, our customers are our number one priority. We have thousands of transformers in stock and the fastest lead times in the industry. We make sure you get what you need, when you need it the most. Fill out the form below if you need a transformer and one of our reps will reach out within the hour.

Maddox padmount transformer loaded on truck

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