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The Benefits of FR3® Fluid in Transformers

FR3® fluid is the go-to cooling fluid in transformers. Learn all about FR3® fluid, and why you should choose it over mineral oil for your transformer.

Written by:
Nathan Stenzel

January 17, 2024

A padmount transformer filled with FR3® Fluid being loaded on a truck

Liquid-filled transformers depend on oil for their smooth operation. This transformer oil keeps the transformer cooled, insulated, and lubricated. Not all transformer oil is created equal, though. While mineral oil has powered the world since the 1900s, it’s not without its issues. Natural esters like FR3® fluid have quickly become the fluid of choice in oil-filled transformers.

What is FR3® fluid?

FR3® fluid is a natural ester made from seed oils—just like the vegetable oil in your kitchen. It is a safer, more sustainable alternative to the standard mineral oil found in many older transformers. FR3® fluid has been around since the 1990s and is the primary alternative insulating fluid to mineral oil in the United States.

A Maddox transformer filled with FR3 fluid powering a datacenter in South Carolina
A Maddox transformer filled with FR3® fluid powering a datacenter in South Carolina

FR3® fluid vs. Mineral Oil

For decades, the go-to fluid in oil-filled transformers was mineral oil. However, FR3® fluid offers many significant advantages over mineral oil. Here’s a rundown of why you should consider FR3® fluid for your liquid-filled transformer.

Flash point Fire point Biodegradability Toxicity Cost Chemical Base
Mineral Oil ~155°C ~165°C Poor Low to none Low Petroleum
Natural Esters (FR3® Fluid) ~330°C ~360°C 100% None Higher Plant
A tote of FR3® fluid filling a padmount transformer tank

Benefits of FR3® fluid

Reduced Fire Risk

The flash point of mineral oil is 155°C. FR3® fluid, however, has a much higher flash point of 330°C. FR3® fluid is also classified as a “non-propagating” fluid, meaning it is self-extinguishing and won’t burn continually if it does catch fire. The upshot is that there is a far lower fire risk with FR3® fluid than with mineral oil. In fact, FR3® fluid is used in millions of transformers, and, to date, there have been zero reported pool fires in transformers filled with FR3® fluid. Even if a catastrophic failure occurred in an FR3-filled transformer, it is highly unlikely that a fire would start.

Besides reducing the risk of fire, FR3® fluid eliminates the significant costs associated with fire safety precautions. NEC standards permit FR3-filled transformers to be installed indoors without costly fire-suppression sprinkler systems. Additionally, they can be installed with a minimum wall clearance of only 3 feet, allowing for greater flexibility in installation locations.

Life Expectancy

FR3® fluid has a higher saturation point than mineral oil. Why does this matter? Transformer fluid can build up moisture during normal operation. FR3® fluid draws this moisture out of the paper insulation and absorbs it, allowing the insulation to operate at a higher temperature and extending the transformer’s life expectancy. This makes FR3-filled transformers much more immune to dielectric failure and decreases transformer capacity due to moisture. Mineral oil-filled transformers cannot retain this moisture: as the temperature of the transformer increases, the moisture travels from the paper insulation to the fluid. Once the temperature decreases, all that moisture will return to the paper. 

Because of its higher saturation point, FR3® fluid can also slow the aging process of insulation paper, increasing the lifespan of the insulation system by as much as 5–8 times.

Transformer Performance

In addition to extending the lifespan of the transformer’s insulation, transformers with FR3® fluid are also able to operate up to 20°C warmer than mineral oil-filled units. This significantly increases the transformer’s loading capability, up to a 20% increase in rated kVA.

Maddox reconditioned padmount transformers filled with FR3® fluid headed to a customer’s project in Minnesota

Environmental Impact

Since it’s derived from seed oils, FR3® fluid is entirely biodegradable. It has over 99% biodegradation in just 28 days. FR3® fluid contains no petroleum, halogens, silicones, or sulfurs. Additionally, it is entirely non-toxic when it does biodegrade—it won’t pollute the soil or groundwater. This means that if your transformer springs a leak, you won’t incur high remediation costs for the spill.

Mineral oil, however, is hydrocarbon-based and non-biodegradable. Because it does not quickly bio-degrade in soil and water, mineral oil-filled transformers could require costly containment measures, not to mention the expensive cleanup work in the case of a spill.


Speaking of cost, FR3® fluid costs roughly twice as much as mineral oil. This may seem like a significant drawback, but the additional costs of using mineral oil can be even more significant. From additional safety measures necessary to prevent environmentally destructive oil spills to the higher fire risk and the attendant costs of fire suppression systems, mineral oil is not always worth the cheaper price tag.

Padmounts filled with FR3® fluid waiting in Maddox's Battle Ground, WA yard to be shipped to customers


Even though mineral oil was the go-to for over a century, transformers and the electrical grid are far better served by FR3® fluid. From safety to performance to life expectancy, FR3® fluid offers nothing but benefits. If you’re looking for an oil-filled transformer and want to know more about FR3® fluid and how to buy the right transformer, fill out the form below.

Maddox padmount transformer loaded on truck

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