Transformer fluid choice: mineral oil, ester or silicone?

Choose mineral oil for outdoor units, natural ester for sealed units with fire or spill risk, synthetic ester for free-breathing units that need a high fire point, silicone for high-temperature indoor designs. This holds for new liquid-immersed transformers filled to IEC 60296:2020, IEC 62770:2024, IEC 61099:2010 or IEC 60836:2015, subject to fire code and maker.

• A less flammable insulating liquid has a fire point of at least 300 °C, the property ASTM D5222-23 links to certain requirements of US National Electrical Code section 450.23, which it cites as Article 450-23 (ASTM D5222-23, 2023).

• Mineral transformer oil has a fire point around 165 °C, which does not qualify it where a less flammable fluid is required (Maddox, 2023).

• The viscosity of one natural ester fluid is 4 times that of mineral oil at an operating temperature of 80 °C (Virginia Transformer, 2022).

• Water solubility in three natural esters was around 15 times that in mineral oil, and the synthetic ester dissolved around 40 times more water (Tenbohlen et al., CIGRE, 2008).

This guide covers the fluid choice for new liquid-immersed power and distribution transformers. It does not cover in-service limits for used fluid, low-viscosity ester-derived liquids (IEC 63012), cables and capacitors, or units whose maker fixes the fluid in the design and warranty.

Which conditions decide the transformer fluid choice?

Five site conditions decide the transformer fluid choice: fire exposure, spill exposure, how the tank breathes, the coldest energizing temperature, and the cooling design. Fire exposure comes first. In the US, oil-insulated transformers installed indoors shall be installed in a vault (US OSHA 29 CFR 1910.305(j)(5)(iv), as of 2026-10-14), and a less flammable liquid needs a fire point of at least 300 °C (ASTM D5222-23, 2023). Spill exposure matters near water, drains, farmland or buildings, where biodegradability and containment come into play. Breathing matters because IEC 62770:2024 restricts natural esters to sealed units or to conservators protected from the atmosphere by a membrane (IEC 62770:2024, clause 1). A free-breathing unit with an open conservator therefore narrows the choice to mineral oil, synthetic ester or silicone. The coldest energizing temperature matters because IEC 62770:2024 added the lowest cold start energizing temperature (LCSET), with advice on the cold behavior of natural esters (IEC 62770:2024, foreword item d). The cooling design matters because ester fluids are more viscous than mineral oil, so one maker selects radiators and fans from a thermal calculation that considers ester viscosity and selects the winding oil ducts with care (Virginia Transformer, 2022). Write these five conditions down before you compare fluids.

Which transformer fluid types fit where?

The four transformer fluid types are mineral oil, natural ester, synthetic ester and silicone, and each has its own IEC product specification for unused fluid. Each specification sets the properties of the fluid as delivered, not its limits in service. The table below shows each fluid type, its specification, where it fits and its main limit.

The key row is natural ester: IEC 62770:2024 specifies unused natural esters and restricts them to sealed units or membrane-protected conservators. Mineral oil remains the reference fluid. IEC 60296:2020 defines two groups of transformer oil, Type A and Type B, based on their performance (IEC 60296:2020, foreword). Synthetic organic esters "have high flash and fire points and are therefore relatively difficult to ignite" (IEC 61099:2010, clause 4). Silicone transformer liquid also has high flash and fire points, and if combustion occurs its heat release rate is much lower than that of hydrocarbon oils (IEC 60836:2015, clause 4.1).

Which transformer fluid fits your conditions?

The transformer fluid that fits your conditions follows from the fire, spill, breathing and temperature conditions you wrote down. Start with the row that matches the strictest condition at the site, because a fire code or a breathing limit removes options before cost or habit enters the choice. The decision table below gives the fluid type to check first for each set of conditions; confirm it with the transformer maker before ordering.

A common case is the third row: a sealed distribution transformer near a building or a watercourse points to natural ester to IEC 62770:2024. A free-breathing unit with the same fire risk points to synthetic ester instead. The CIGRE ageing tests found that natural ester viscosity rises sharply in open systems (Tenbohlen et al., CIGRE, 2008),.

Which transformer fluids count as less-flammable or K-class?

Transformer fluids with a fire point of at least 300 °C count as less flammable, and this group includes natural esters, synthetic esters, silicone and high fire point hydrocarbon oils. ASTM D5222-23 specifies the US threshold: a less flammable insulating liquid differs from conventional mineral oil by "possessing a fire-point of at least 300 °C", which is needed for certain requirements of National Electrical Code section 450.23 (ASTM D5222-23, 2023). Under the IEC classification of insulating liquids (IEC 61039), the fluid code carries a letter for fire point. IEC 60836:2015 shows the effect: silicone transformer liquid is classified L-NTUK-8360300, while a capacitor liquid with a fire point of less than 300 °C is classified L-NCUO-8360300 (IEC 60836:2015, clause 3). Natural ester transformers carry a KNAN cooling class marking on the nameplate (Maddox, 2023). Reported fire points are about 165 °C for mineral oil, 360 °C for a natural ester and about 370 °C for silicone (Maddox, 2023; Virginia Transformer, 2022). Mixing matters: mixing a less flammable hydrocarbon liquid with a lower fire point mineral oil may result in fire points of less than 300 °C (ASTM D5222-23, 2023). Fire point is the deciding number, so check the fire point the supplier reports, not the flash point.

How does fluid viscosity affect transformer cooling and cold start?

Fluid viscosity sets how fast the fluid flows through windings and radiators, so a more viscous fluid such as an ester cools a naturally cooled transformer less readily and starts harder in the cold. One transformer maker reports that natural ester viscosity is almost 4 times that of mineral oil at 80 °C (Virginia Transformer, 2022). A CIGRE study of natural ester transformers judged that the main drawback seems to be a more than five times higher viscosity than mineral oil, because winding hydraulic resistance under natural oil (ON) cooling is roughly proportional to viscosity (Tenbohlen et al., CIGRE, 2008). The same study found the heat conductivity of the natural ester about 40 % higher than that of mineral oil, so no general statement on hot-spot temperature could be made; each design has to be calculated (Tenbohlen et al., CIGRE, 2008). The two ratios come from different fluids and temperatures, so use the maker's data for the chosen fluid. For cold start, a natural ester cannot move freely through the transformer until the unit warms up under load, and both mineral oil and natural ester units need special startup procedures in cold climates (Maddox, 2023). IEC 62770:2024 added the lowest cold start energizing temperature (LCSET) to its natural ester specification for this reason (IEC 62770:2024, foreword item d). Ask the transformer maker to confirm the cooling rating and the cold start procedure for the chosen fluid.

Which transformer fluid suits indoor or environmentally sensitive sites?

For indoor sites, a listed less flammable transformer fluid with a fire point of at least 300 °C suits best, and for environmentally sensitive sites a biodegradable ester suits best, provided containment is still planned. Indoors in the US, an oil-insulated transformer shall be installed in a vault, and a high fire point liquid-insulated transformer rated over 35 kV installed indoors shall also be in a vault (US OSHA 29 CFR 1910.305(j)(5)(iii) and (iv), as of 2026-10-14). NEC section 450.23 sets the conditions for installing a less-flammable liquid-insulated transformer indoors; for example, a unit rated under 35,000 V may go indoors where both an automatic fire extinguishing system and a liquid confinement area are provided (EC&M, 2023). Check these conditions with the authority having jurisdiction. Near water or farmland, biodegradability separates the fluids. IEC 62770:2024 describes natural esters as liquids "with suitable biodegradability and lower environmental impact" (IEC 62770:2024, clause 1). IEC 61099:2010 lists biodegradation among synthetic ester properties and references the OECD 301 ready biodegradability guideline (IEC 61099:2010, clause 5.2). Silicone is not readily biodegradable (Maddox, 2023). Biodegradable does not mean exempt from spill rules. In the US, the oil pollution prevention rule defines oil to include vegetable oils, synthetic oils and mineral oils, and lists transformers as oil-filled operational equipment (US EPA 40 CFR 112.2, as of 2026-10-14). Plan spill containment for an ester-filled unit as you would for a mineral oil unit, and check the local rule in your jurisdiction.

What should you ask a transformer fluid supplier?

Ask a transformer fluid supplier six things: which specification and edition the fluid meets, which additives it contains, the measured fire point and its method, the less flammable listing, the miscibility and retrofill limits, and the maintenance guide that applies. First, ask for a certificate to the active edition: IEC 60296:2020 for mineral oil, IEC 62770:2024 for natural ester, IEC 61099:2010 for synthetic ester or IEC 60836:2015 for silicone. Second, ask about additives: IEC 61099:2010 notes that additives, if used, should be mentioned by the supplier to the user if requested (IEC 61099:2010, clause 3.2). Third, ask for the fire point by the Cleveland open cup method, which IEC 62770:2024 now uses alone for natural esters (IEC 62770:2024, foreword item f). Fourth, for a US indoor installation, ask whether the fluid is listed for less flammable use under National Electrical Code section 450.23 (ASTM D5222-23, 2023). Fifth, ask what fire point remains after a retrofill, because ASTM D5222-23 warns that blending a less flammable liquid with lower fire point mineral oil may give fire points below 300 °C (ASTM D5222-23, 2023), and residual oil in a retrofill raises the same question, and IEC 62770:2024 excludes natural esters blended with other insulating liquids (IEC 62770:2024, clause 1). Sixth, ask which maintenance guide applies in service; IEC 61099:2010 points to IEC 61203 for synthetic esters (IEC 61099:2010, clause 1).

When should you call a specialist about transformer fluid choice?

Call a transformer fluid specialist when the choice involves a retrofill, a free-breathing unit, a code listing or a cold site, because each of these changes the answer in the decision table. A retrofill from mineral oil to ester leaves residual mineral oil in the paper and tank, so check the fire point of the blend against the 300 °C threshold. ASTM D5222-23 states this for less flammable hydrocarbon liquids: mixing them with lower fire point mineral oil may result in fire points of less than 300 °C (ASTM D5222-23, 2023). A free-breathing unit that the owner wants to fill with natural ester needs a breathing change first, since IEC 62770:2024 restricts natural esters to sealed or membrane-protected units (IEC 62770:2024, clause 1). An indoor or rooftop unit needs the fire code and the fluid listing checked together with the authority having jurisdiction. A cold site needs the transformer maker to confirm the cold start procedure and the LCSET of the fluid. A cooling upgrade needs the maker's thermal calculation, since ester viscosity changes how windings and radiators cool (Tenbohlen et al., CIGRE, 2008). Lubinpla, an applied AI company that structures specialty chemicals technical reasoning, works such a case from your own TDS, SDS, CoA and site history and returns one cited document for review, with your approval at each step. Bring the transformer nameplate, the conservator type, the site temperature range and the fire code requirement to the first conversation.

FAQ

Can you mix natural ester with mineral oil in a transformer?

Mixing happens in every retrofill, but the blend is not covered by the natural ester specification: IEC 62770:2024 places natural esters blended with other insulating liquids outside its scope (IEC 62770:2024, clause 1). ASTM D5222-23 states this for less flammable hydrocarbon liquids: mixing them with lower fire point mineral oil may result in fire points of less than 300 °C (ASTM D5222-23, 2023), so ask the supplier for the fire point of the blend.

What is the difference between a natural ester and a synthetic ester?

A natural ester is a vegetable or other biological oil made of triglycerides (IEC 62770:2024, clause 1). A synthetic organic ester is made by chemical processing of fatty acids and polyols (IEC 61099:2010, clause 1).

How much more water do esters hold than mineral oil?

Esters dissolve far more water than mineral oil. In one comparison, water solubility in natural esters was around 15 times that in mineral oil, and a synthetic ester dissolved around 40 times more (Tenbohlen et al., CIGRE, 2008). Judge ester water content against the ester's own maintenance guide, not against mineral oil limits.

Should older transformers be tested for PCBs before a fluid change?

One US transformer supplier advises that units with a manufacture date prior to 1980 should be tested for polychlorinated biphenyls (PCBs) (Maddox, 2023). Test before draining, and follow the SDS and, in the US, 40 CFR Part 761 (PCBs), or the PCB rule in your jurisdiction, for handling and disposal of the old fluid (US EPA 40 CFR Part 761, as of 2026-10-14).

Working a case like this? Lubinpla works it from your own documents and site history, step by step with your approval. Free to start, no card.

Lubinpla does not sell chemical products and names no brands in recommendations. Our reviewer also leads a specialty chemicals distribution company.

Sources checked 2026-10-14.

Sources

1. IEC. IEC 60296:2020 Fluids for electrotechnical applications, Mineral insulating oils for electrical equipment. Edition 5.0, 2020. https://webstore.iec.ch/en/publication/61025

2. IEC. IEC 62770:2024 Fluids for electrotechnical applications, Unused natural esters for transformers and similar electrical equipment (preview). Edition 2.0, 2024. https://vde-verlag.de/iec-normen/preview-pdf/info_iec62770{ed2.0}b.pdf

3. IEC. IEC 61099:2010 Insulating liquids, Specifications for unused synthetic organic esters for electrical purposes (preview). Edition 2.0, 2010. https://www.vde-verlag.de/iec-normen/preview-pdf/info_iec61099{ed2.0}b.pdf

4. IEC. IEC 60836:2015 Specifications for unused silicone insulating liquids for electrotechnical purposes (preview). Edition 3.0, 2015. https://www.vde-verlag.de/iec-normen/preview-pdf/info_iec60836{ed3.0}b.pdf

5. ASTM International. ASTM D5222-23 Standard Specification for Less Flammable High Molecular Weight Hydrocarbon Mineral Electrical Insulating Liquids. 2023. https://store.astm.org/d5222-23.html

6. US OSHA. 29 CFR 1910.305 Wiring methods, components, and equipment for general use. Accessed 2026-10-14. https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.305

7. US EPA. 40 CFR Part 112 Oil Pollution Prevention (eCFR). Accessed 2026-10-14. https://www.ecfr.gov/current/title-40/chapter-I/subchapter-D/part-112

8. Tenbohlen S., Koch M., Vukovic D., Weinläder A. et al. Application of vegetable oil-based insulating fluids to hermetically sealed power transformers. CIGRE A2-102, 2008. https://www.ieh.uni-stuttgart.de/dokumente/publikationen/2008_Cigre_Vukovic_Application_of_vegetable_oil-based.pdf

9. Mani B., John J. K. Transformer Design and Manufacturing for Natural Ester Fluids. Virginia Transformer, 2022. https://www.vatransformer.com/wp-content/uploads/2022/11/white-paper-on-transformer-design-and-manufacturing-for-natural-ester-fluid.pdf

10. Gulick B., Spiller M., Stenzel N. Transformer Oil. Maddox, 2023. https://maddox.com/resources/articles/transformer-oil

11. Lamendola M. NEC: Transformers, Part 2 (overview of NFPA 70 National Electrical Code, Article 450, Part II). EC&M, 2023. https://ecmweb.com/national-electrical-code/article/21270660/nec-transformers-part-2

12. US EPA. 40 CFR Part 761 Polychlorinated Biphenyls (PCBs) Manufacturing, Processing, Distribution in Commerce, and Use Prohibitions (eCFR). Accessed 2026-10-14. https://www.ecfr.gov/current/title-40/chapter-I/subchapter-R/part-761

Written by the Lubinpla team from the sources listed and checked against them before publication.