Transformer types: dry-type vs liquid-filled field guide

The transformer, and the type decision

A transformer changes one AC voltage to another by magnetic coupling. Current in the primary winding makes a changing magnetic field in the core, that field induces a voltage in the secondary winding, and the ratio of turns sets the ratio of voltages. No current flows directly between the two windings. They share a magnetic field, not a wire, which is also why a transformer isolates the secondary from the primary.

That much is physics and it does not change. What changes, and what this guide is about, is the type. Dry-type or liquid-filled. Ventilated, cast resin, or sealed. Mineral oil or a less-flammable fluid. Self-cooled or fan-boosted. The type is a packaging and cooling decision, and you make it from where the unit lives, the voltage and kVA it handles, the efficiency the owner pays for over twenty years, and the fire and environmental rules of the space.

Get the type wrong and the transformer still works on day one. It fails you later. A liquid-filled unit put indoors without a vault or a listed less-flammable fluid is a code violation and a fire problem. A dry-type pushed onto a hot outdoor pad it was never built for cooks. The sizing and the installation details for the common indoor dry-type live in the companion dry-type guide, and where this unit sits in the building distribution is the distribution-equipment guide. This guide is the type decision that comes before both.

The basics: primary, secondary, turns ratio, kVA

The two windings are the primary, which takes power in, and the secondary, which delivers it at the new voltage. The turns ratio is the whole trick. Wind twice as many turns on the primary as the secondary and you get roughly half the voltage out and, ignoring losses, twice the current. A step-down transformer trades volts for amps. A step-up does the reverse.

kVA, not kW, is the rating, because the transformer does not care about the load's power factor. It carries volt-amps. The core is laminated silicon steel to hold the magnetic field while limiting eddy losses, and the coils are copper or aluminum wound around it. Those two losses, the core loss that runs whenever the unit is energized and the winding loss that rises with load, are what the efficiency standards target later in this guide.

The one thing to hold onto: the windings are not electrically connected. The energy crosses as a magnetic field, so the secondary is a separately derived system that you ground on its own. Sizing the kVA from the load and grounding that secondary is the companion dry-type guide's job. This guide assumes you have a kVA number and are choosing what kind of transformer delivers it.

Dry-type transformers

A dry-type transformer cools itself with air and a solid insulation system, with no liquid anywhere in it. Air moves through or around the coils, the solid insulation handles the dielectric job that oil does in a liquid unit, and there is nothing in the case to leak, catch fire, or test. That is the reason it is the standard transformer inside an occupied building.

Dry-type is the indoor distribution unit you see most: the 480 V to 208Y/120 V step-down in an electrical room feeding panels, sized through the low hundreds of kVA. It can sit close to the load it serves, which shortens the secondary run and helps voltage drop, and it needs no containment basin or vault because there is no liquid to contain. Lower fire risk is the headline, and it is real.

What it gives up is cooling. Air carries heat away worse than oil, so for the same kVA a dry-type runs hotter, loses a little more, and tolerates overload for shorter before the insulation pays for it. It also dumps that heat into the room it sits in, which becomes the room's problem. The kVA sizing, the Article 450 protection, the separately derived grounding, and the clearance and ventilation for one of these is the whole subject of the companion dry-type guide. Here it is one option in the comparison.

Dry-type subtypes: ventilated, cast resin, sealed

Dry-type splits into three builds by how the windings are insulated and protected, and the split matters most in dirty or humid air.

Ventilated is the general indoor unit. The windings are open to air for cooling, usually vacuum-pressure impregnated (VPI) with a varnish, or VPE with an epoxy, to seal the surface against moisture and dust. It is the cheapest dry-type and the right call for a clean, dry, conditioned electrical room.

Cast resin, also called cast coil, seals the entire winding inside a void-free epoxy casting poured under vacuum. Nothing of the conductor is exposed. That makes it stand up to humidity, condensation, salt air, corrosive atmospheres, and frequent short-circuit stress far better than a varnished coil, and it carries a higher fire rating. It also costs more, commonly on the order of 30 to 50 percent over a comparable VPI unit, and it is heavier. You buy cast resin for the environment, not for the conditioned electrical room down the hall.

Sealed gas-filled units enclose the core and coils in a hermetic tank under an inert gas such as nitrogen, keeping the windings away from the outside air entirely. They suit harsh, wet, or contaminated locations where you want the protection of a sealed case without liquid.

Liquid-filled transformers

A liquid-filled transformer immerses the core and coils in a dielectric fluid that does two jobs at once: it insulates the windings and it carries their heat out to the tank walls and radiators where it sheds to the air. Liquid moves heat far better than air, and that one fact drives everything good and everything cautious about this type.

Because the fluid cools so well, a liquid-filled unit of a given kVA runs cooler, loses less, and holds up under overload longer than the dry-type equivalent. It ages slower and tends to last longer in service. It is also how you build the large ones. Above the few-hundred-kVA range, and at medium voltage, liquid-filled is the practical and usually the only choice. The utility transformer on the pad outside, the substation unit, the pad-mount feeding a campus, almost all of it is liquid.

The cost is the liquid itself. It can leak, it can burn, and it has to be contained and maintained. That is why you find liquid-filled units outdoors, on pads, in vaults, or in fenced yards by default, and why putting one inside a building triggers a specific set of code requirements covered below. The fluid that buys you the cooling is the same fluid that buys you the containment basin and the oil-testing program.

The liquid: mineral oil and less-flammable fluids

The fluid is a real choice, and it comes down to fire and environment. Three families cover almost everything.

Mineral oil is the traditional fill: cheap, well understood, and a good coolant and insulator. It is also flammable, with a fire point around 165 degrees C, so a mineral-oil unit is the one the fire and containment rules are written around.

Less-flammable fluids are the high-fire-point alternatives, classed by a fire point at or above 300 degrees C. Two matter on most jobs. Natural and synthetic esters, such as the natural ester FR3 and the synthetic ester MIDEL 7131, are vegetable-derived or synthetic, biodegradable, non-toxic, and carry the highest fire points of the common fluids, well above 300 degrees C. Silicone is the older less-flammable fluid, also high fire point and chemically stable, but not biodegradable. Both let you do things mineral oil cannot, including some indoor and rooftop installs that would otherwise demand a vault.

The ester fluids have pushed hard into the market because they answer fire and spill at once: high fire point for the code, biodegradable for the environment, and they tolerate moisture and heat well enough to extend insulation life. If a liquid-filled unit has to go somewhere a mineral-oil unit cannot, the fluid is usually the reason it can.

What is the difference between a dry-type and liquid-filled transformer?

A dry-type transformer cools with air and solid insulation and contains no liquid; a liquid-filled transformer immerses its windings in oil or a dielectric fluid that cools and insulates. That single difference sets the rest. Dry-type wins on fire safety and indoor placement with no containment. Liquid-filled wins on efficiency, overload, and life, and owns the larger and outdoor work.

Lean dry-type when the unit lives inside an occupied building, the kVA is moderate, and avoiding fluid, vaults, and oil maintenance is worth the slightly higher losses and shorter overload. Lean liquid-filled when the unit is outdoors or large or medium-voltage, when efficiency over a long service life pays back, and when there is room for containment. The table lays the trade-offs side by side; the sections after it work each one in detail.

Fire, code, and where each type is allowed

The fire rules are why the type decision is also a code decision, and they live in NEC Article 450, the transformer installation article. The framework, simplified and with the section numbers to confirm against your adopted edition: a dry-type indoor installation is the default-permitted case, and the rules scale with kVA and voltage for clearances and fire-resistant separation. An ordinary mineral-oil liquid-filled transformer indoors is the restricted case. It generally requires a transformer vault, a fire-rated room built to the Article 450 vault provisions, unless the installation qualifies for an exception.

The big exception is the less-flammable, high-fire-point liquid. Article 450 has provisions for listed less-flammable liquid-filled transformers that allow indoor and rooftop installation without a full vault under stated conditions, often with a liquid-confinement area and limits on size and location. This is the practical reason ester and silicone fluids exist on the spec: they move a liquid-filled unit into spaces a mineral-oil unit cannot legally go.

Do not run any of this from memory on a submittal. The article numbers, the kVA thresholds, the vault construction, and the less-flammable allowances shift between code cycles, and the jurisdiction adopts a specific edition with its own amendments. Confirm the requirement against the adopted NEC edition and the AHJ, and confirm the fluid's listing. The blunt version: a mineral-oil transformer set inside a building without a vault is a violation and a fire waiting for an ignition source, and the inspector will catch it.

Containment travels with the liquid. An outdoor liquid-filled unit sits over secondary containment, a basin or a curbed pad sized to hold the fluid volume plus rainfall, so a leak or a rupture does not run into soil or a storm drain. This is an environmental requirement as much as a fire one, and a spill of mineral oil is a reportable event with a cleanup bill attached. The ester fluids soften this side too: biodegradable and non-toxic, they lower the environmental stakes of a leak, though the containment still gets designed and built.

Cooling classes: ONAN, ONAF, AA, and FA

The nameplate carries a cooling class code that tells you how the unit moves heat and whether fans add capacity. Liquid-filled units use the IEEE four-letter codes. ONAN means oil natural, air natural: the oil circulates by convection and the air over the radiators is still. ONAF means oil natural, air forced: fans blow across the radiators, and the forced air lets the same transformer carry more load. A unit rated ONAN/ONAF carries two kVA numbers, the self-cooled base and the higher fan-on rating.

Adding fans buys real capacity. Depending on the size and the standard, the forced-air stage commonly adds on the order of 15 to 33 percent over the self-cooled rating, which is why a 1000 kVA self-cooled unit is nameplated higher with fans running. Confirm the actual numbers on the nameplate against IEEE C57; do not assume a fixed percentage.

Dry-type uses the older letter codes. AA is air-cooled, self-cooled by natural convection. FA, written AA/FA, adds fans for a forced-air stage and a higher rating, the same idea as ONAN/ONAF on the dry side. The point for selection: a transformer with a forced-cooled stage gives you headroom you can switch on, but only when the fans, the controls, and the temperature sensors are installed and working. A fan stage nobody wired is a rating you do not actually have.

Pad-mount, distribution, and power transformers

Pad-mount describes where a transformer sits, not what it is. A pad-mount transformer is a liquid-filled unit in a locked, tamper-resistant steel enclosure set on a concrete pad at grade, built for placement in a public or accessible spot with no fence. It is the green box on the lawn of a commercial site or a campus, and inside it is almost always liquid-filled because it lives outdoors and often at medium voltage. Pad-mount is a configuration; the unit in it can be a distribution or a power transformer.

Distribution versus power is a size and voltage split. Distribution transformers are the smaller, lower-voltage units that step down to the utilization voltage a building actually uses, commonly cited up to around 500 kVA and below roughly 33 kV, and they are built for good efficiency at part load because they run lightly loaded most of the day. Power transformers are the large units at substation and transmission voltages, rated well above that range and optimized for efficiency near full load. The line between the two is not a hard wire and the references vary, so treat the numbers as the common framing, not a definition to cite.

For the building electrician, almost all the work is distribution-class: the dry-type inside, the pad-mount or unit substation feeding the service. Power transformers are the utility's and the substation designer's world. Where the service enters and how it lands on the building distribution is the distribution-equipment guide's subject.

What is a K-rated transformer?

A K-rated transformer is built to carry harmonic-rich current from non-linear loads without overheating. Non-linear loads, the switching power supplies in computers, the rectifier front ends of VFDs and UPS systems, and LED drivers, draw current in pulses, not a clean sine wave. Those harmonics add heating in the windings and core, especially eddy-current heating that rises fast with frequency, far beyond what the plain kVA suggests.

The K-factor is a number that rates how much harmonic heating the transformer is designed to handle. UL recognizes standard ratings of K-1, K-4, K-9, K-13, K-20, K-30, K-40, and K-50, tied to IEEE C57.110. A K-rated unit gets there with heavier and often parallel conductors, a doubled or upsized neutral for the triplen harmonics that add up in the neutral, and reduced core flux density, so it runs cool on a load that would cook a standard transformer. K-13 is the common pick for general office and mixed non-linear load; K-20 for heavily non-linear environments like data centers and large UPS-backed loads.

The alternative to a K-rated unit is to oversize a standard transformer and derate it, running it well below nameplate so the harmonic heating fits inside the margin. That works, but it is usually larger, less efficient, and not clearly cheaper than buying the K-rating. Put a standard transformer on a harmonic load at full nameplate and it overheats and ages early, and the failure gets blamed on everything but the missing K-rating. The harmonic side of load sizing is its own subject; here the point is that the load's harmonic content is part of choosing the transformer.

Isolation, shielding, and voltage taps

Two more options change what the transformer does for the load, not how it is cooled.

An isolation transformer is one used specifically to break the direct electrical connection between source and load, which every two-winding transformer does inherently, but the term gets used when that isolation is the point: breaking ground loops, separating a sensitive system from upstream noise, and giving the secondary its own derived neutral and ground. Add an electrostatic shield, a grounded metallic layer between the primary and secondary windings, and you also block the high-frequency noise and transients that would otherwise couple across the inter-winding capacitance. A shielded isolation transformer is the common feed for sensitive electronics, medical, lab, and IT loads where clean power and a clean ground matter.

Taps are the voltage-adjustment fix. Most distribution transformers carry no-load taps on the primary, typically two steps above and two below nominal at 2.5 percent each, that you set with the transformer de-energized to compensate for a supply that runs consistently high or low. They are not load tap changers; you pick the tap at commissioning, measure the secondary under load, and move it once if the voltage is off, not on the fly. The rookie move is to leave every unit on the nominal tap and then chase a low secondary that a one-tap change would have fixed. Set the tap to the supply you actually have, then confirm with a meter under load.

Efficiency and the DOE standard

Efficiency is not a marketing number on a distribution transformer; it is a federal minimum. The DOE 2016 standard, in 10 CFR Part 431, sets minimum efficiencies for low-voltage dry-type distribution transformers built on or after January 1, 2016, with the efficiency measured at 35 percent of rated load and a reference winding temperature, which reflects how lightly these units actually run. A transformer sold into that scope has to meet the level for its kVA, so the floor is the law, not a target. DOE finalized a rule in April 2024 that raises those minimums, with a compliance date of April 2029, pushing part of the market toward amorphous-core designs, so confirm the level in force for the manufacture date of the unit you are specifying.

Efficiency comes from lower losses, and there are two. No-load, or core, loss runs every hour the unit is energized, load or no load, and it is set by the core steel and the design. Load, or copper, loss rises with the square of the current. Because a distribution transformer is energized continuously but loaded only part of the time, the no-load loss often dominates the lifetime energy bill, which is why the standard tests at part load and why low-loss core materials, like better grain-oriented or amorphous steel, are where the efficiency gains live.

Liquid-filled units are generally more efficient than dry-type at the same rating, because the fluid cools the windings better and lets the designer run a lower current density. Over a twenty-year life the loss difference is real money, and on a large or continuously loaded unit it can outweigh a higher purchase price. Confirm the efficiency basis and the applicable standard for the specific unit and edition, because the scope and the levels have changed over time and not every transformer class is covered the same way.

Heat and noise from the indoor dry-type

An indoor dry-type rejects its heat into the room, and that heat has to go somewhere. A 75 or 112.5 kVA unit in a closet puts real warmth into the space, and if the room has no ventilation path the room temperature climbs, the transformer runs hotter than its rating assumed, and the insulation ages faster. Treat the electrical room's airflow as part of the install, not an afterthought, and keep the manufacturer's clearances around the case open so the convection actually works.

Transformers hum at twice the line frequency, from magnetostriction in the core. It is steady and not loud, but in a quiet building it carries. Mount the unit on its vibration isolators, keep it off shared walls with offices and conference rooms where you can, and do not bolt the case rigidly to a wall that will act as a sounding board. Liquid-filled units outdoors rarely draw a noise complaint; a dry-type next to an occupied office sometimes does.

The clearance, ventilation, and heat-rejection details for the common indoor dry-type are worked in the companion dry-type guide. The point for the type decision is that a dry-type's heat and sound are the room's problem in a way a liquid-filled unit on an outdoor pad never is.

Maintenance: oil testing vs torque and cleaning

The two types fail differently, so they are maintained differently. A liquid-filled transformer is maintained largely through its fluid. You sample the oil and run dissolved gas analysis (DGA), which reads the gases dissolved in the fluid to catch overheating, arcing, or partial discharge inside the tank long before it becomes a failure. You check the liquid level, the temperature and pressure gauges, the bushings, and the integrity of the containment. The oil is both the coolant and the diagnostic.

A dry-type has no fluid to sample, so its maintenance is mechanical and thermal. You de-energize and clean the dust and debris off the coils and ventilation paths, because a coating of dust insulates the windings and chokes the airflow. You re-torque the connections to the manufacturer's values, since heat cycling backs them off and a loose connection makes heat right where you do not want it. And you scan it under load with an infrared camera to find the hot connection or the blocked ventilation before it finds you. NETA acceptance and maintenance testing specifications give the framework for both types; the intervals and the test values come from NETA, the manufacturer, and the owner's program.

The difference in selection terms: a liquid-filled unit needs an oil-testing program and somewhere to run it; a dry-type needs an outage, a vacuum, a torque wrench, and a thermographer. Neither is maintenance-free, and the unit nobody maintains is the one that surprises you.

The data-center power chain

A data center shows both types doing the job each is built for, in one power path. Outside, the medium-voltage service lands on liquid-filled transformers, pad-mount or substation units, that step the utility voltage down for the building. Liquid-filled because it is outdoors, large, and at medium voltage, exactly its territory.

Inside, the distribution transformers feeding the IT load are dry-type, for the fire and containment reasons that rule occupied buildings, and they are K-rated because the load is about as non-linear as it gets: thousands of switching power supplies behind UPS systems. A standard dry-type at nameplate on that load overheats; the K-rated dry-type is built for it. Many of these also carry electrostatic shielding for the noise sensitivity of the equipment downstream.

So the same facility runs liquid-filled outside and shielded, K-rated dry-type inside, and the type choice at each point follows the same rules this guide has laid out: location, voltage, kVA, fire, and the character of the load. How the redundancy and the distribution downstream of these transformers get built out is the distribution-equipment guide's subject.

Selecting the type

Work the decision in order and the type usually picks itself. Start with location: inside an occupied building points hard to dry-type, or to a less-flammable liquid-filled unit in a vault if the size demands liquid; outdoors, a pad, or a yard opens up liquid-filled. Then the kVA and voltage: low to moderate kVA at low voltage is dry-type's range, while large kVA and medium voltage push you to liquid-filled, because that is how the big ones are built.

Then layer the rest. Fire and environment can override location, and a humid, corrosive, or wet space points to cast resin on the dry side or an ester fluid on the liquid side. Harmonic load means K-rated, usually a dry-type indoors. Sensitive electronic load means shielding and isolation. Efficiency over a long, heavily loaded life favors liquid-filled and a low-loss core, and on a continuously loaded unit that energy difference can drive the call. Budget comes last, not first, because the cheap transformer in the wrong place is the expensive one in two years.

There is no single right answer, only the right answer for the location, the load, and the rules of the space. The most common failure is starting from a habit or a price and backing into a type, instead of starting from where it sits and what it feeds.

What to document

Record the type decision the way you would record a sizing calculation, because the next person who opens the electrical room needs to know why this transformer and not another. Capture the type and build, the cooling class and any fan stage, the fluid and its listing if liquid-filled, the K-rating if any, the kVA and voltages, the tap setting as left, and the reason the type was chosen for this location.

If the unit is liquid-filled, the record also carries the containment, the fluid volume, and the baseline oil test so the maintenance program has a starting point. If it is a K-rated dry-type, the record carries the K-factor and the load it was chosen for, so nobody later loads it with something it was never built to handle.

Common mistakes

• Installing a mineral-oil liquid-filled transformer indoors without a vault or a listed less-flammable fluid.
• Loading a standard dry-type to nameplate on a harmonic-rich load with no K-rating, then blaming the failure on the transformer.
• Setting a liquid-filled unit outdoors with no secondary containment for a leak or rupture.
• Pushing a dry-type past its overload because it looked like it had the room a liquid-filled unit would have had.
• Choosing the type from price or shop habit instead of the location, the load, and the code.
• Counting on a forced-air (ONAF or FA) rating when the fans, controls, or sensors were never installed.
• Leaving every transformer on the nominal tap and chasing a low secondary a tap change would fix.
• Ignoring the DOE efficiency basis on a continuously loaded unit, where the lifetime losses outrun the purchase price.
• Putting an unshielded dry-type on sensitive electronic or medical load and chasing the noise problem later.

Field checklist

Standards and references

NEC Article 450, in NFPA 70, is the transformer installation article, and it governs the type decision wherever fire and location are involved: indoor dry-type allowances, the vault requirement for ordinary liquid-filled units indoors, the less-flammable liquid provisions, and the clearances. The article numbers, kVA thresholds, and vault details change between code cycles, so confirm them against the adopted edition and the local amendments before you rely on them.

The IEEE C57 family is the transformer standard set, covering ratings, the cooling-class designations, testing, and the harmonic-load and K-factor methods at C57.110. UL listings cover the transformer and the less-flammable fluid classification, and a fluid's less-flammable status for code purposes rests on that listing. The DOE efficiency requirements live in 10 CFR Part 431 and set the minimum efficiency for the covered distribution transformer classes. NETA's acceptance and maintenance testing specifications give the field-test framework for both dry-type and liquid-filled units.

Cite the standard that controls the point, and let the project specification and the manufacturer's instructions override a rule of thumb when they are stricter. The efficiency scope, the code edition, and the fluid listing all have to be confirmed for the specific unit, not assumed from this guide.

Units and terms

The transformer trade mixes a few naming systems across nameplates, drawings, and spec sheets, so the same idea reads differently depending on the source.

Rating is in kVA or, for the large units, MVA. Voltage is given as primary/secondary, sometimes with the winding connection, delta or wye. Cooling is the letter code on the nameplate. Fluid and K-rating, when they apply, are called out separately.

kVA / MVA

Apparent power rating, in thousands or millions of volt-amps; the transformer's size

Turns ratio

Ratio of primary to secondary winding turns, which sets the voltage ratio

Dry-type

Transformer cooled by air and solid insulation, with no liquid

Liquid-filled

Transformer with windings immersed in oil or a dielectric fluid

Less-flammable fluid

High-fire-point dielectric liquid, fire point at or above 300 C (ester, silicone)

Cooling class

Nameplate code for the cooling method (ONAN, ONAF, AA, FA)

K-factor

Rating of a transformer's capability to serve harmonic, non-linear load

DGA

Dissolved gas analysis, the oil test that reads internal faults in a liquid-filled unit

No-load tap

Primary voltage-adjustment tap set with the unit de-energized, commonly 2.5 percent steps

Separately derived system

A secondary with no direct connection to the supply, grounded on its own

Related tools

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