Most people rarely think about transformers unless their work depends on them. They sit in the background, unnoticed, until a project needs one — and at that point the question is no longer what a transformer is, but which type to specify.
This page is the starting point for that decision. It maps the main transformer families — liquid-filled and dry-type, power and instrument — against the applications they actually suit, and links out to the detailed guide for each type.
The underlying job is always the same. A transformer adjusts electrical energy from one voltage level to another so it can be used safely, using a primary coil that receives the incoming voltage and a secondary coil that delivers it at a different level. If you want the physics rather than the selection logic, start with the explanation of how power transformers work.
Transformer selection goes wrong when the conversation starts with product names. Start with four questions instead — each answer eliminates whole families before anyone opens a data sheet.
Once those four answers are fixed, the table below settles which family you are actually shopping for.
Every widely used transformer type, the construction that defines it, and where it is the right answer. The last column links to the detailed guide for that type.
| Type | Construction and insulation | Typical voltage class | Indoor / outdoor | Best fit | Learn more |
|---|---|---|---|---|---|
| Padmount transformer | Dielectric fluid in a sealed, compartmentalised, tamper-resistant tank | Distribution, typically up to 35 kV | Outdoor, public space | Airports, campuses and commercial buildings where protection and switching are built in | Oil immersed distribution transformer |
| Substation / power transformer | Fluid-filled tank with external controls and forced cooling | Medium and high voltage | Outdoor, restricted access | Industrial sites and substations requiring bulk transformation | 30kV oil immersed transformer |
| Pole-mount transformer | Fluid-filled, lightweight and sealed | Distribution | Outdoor, pole mounted | Residential and light commercial distribution | Pole mounted distribution transformer |
| Submersible / network transformer | Fluid-filled, sealed against flooding, compact footprint | Distribution | Underground vaults | Dense urban areas supplying apartments and office blocks | Oil immersed power transformers |
| Low voltage general purpose (VPI) | Air-cooled, windings vacuum impregnated with varnish | 600 V class (1 kV IEC) | Indoor; outdoor with weathershields | Lighting and receptacle panels, small commercial step-down | What is a dry type distribution transformer |
| Cast resin dry-type (SCB type) | HV winding vacuum cast in epoxy resin, copper-foil LV winding, Class F insulation | 6–10 kV typical | Indoor | High-rise, data centre and industrial distribution where fire load must be minimal | SCB series 10kV epoxy resin dry type transformer |
| Medium voltage dry-type | Air-cooled, unit substation enclosure | 3–36 kV | Indoor | Line-ups with MV switchgear on the high side and LV switchgear downstream | 10kV dry type transformer |
| Encapsulated (sand and resin) | Windings and often the core sealed in a cured sand-and-epoxy shell | Low and medium voltage | Indoor or outdoor, harsh sites | Washdown, damp, dusty and corrosive locations | Dry type transformer insulation |
| Autotransformer / buck-boost | Single shared winding, no electrical isolation | Low and medium voltage | Indoor | Step-up or step-down within roughly ±5–20% | How to choose a step down power transformer |
| Drive-isolation (K-rated) | Separate windings, electrostatic shield, 200% neutral, K-4 and above | Low voltage | Indoor | VFD and motor-drive loads, non-linear and harmonic-rich systems | Custom dry type transformers |
| Current transformer (CT) | Cast resin or oil insulated, single or multi-ratio | 0.66–36 kV | Indoor or outdoor | Current metering, measurement and protection | Cast resin current transformers |
| Voltage transformer (PT / VT) | Cast resin or oil insulated | 0.66–36 kV | Indoor or outdoor | Voltage metering, measurement and protection | Voltage transformers |
Transformers fall into two main groups, and the choice between them is usually made for you by the installation environment rather than by preference.
Liquid-filled transformers use a dielectric fluid to both cool and insulate their internal windings, circulating by natural convection. Most carry an insulation rating of around 120 °C, though higher-rated designs using enhanced Kraft paper exist. Because fluid transfers heat far more efficiently than air, these units cool better than dry-type designs and tolerate overload conditions with far more resilience. Their sealed tank structure also makes them naturally suited to outdoor installation.
Dry-type transformers are intended mainly for indoor environments and typically carry a higher insulation rating, around 220 °C. They rely on surrounding air rather than fluid, which is why they usually come in ventilated enclosures. They run at higher internal temperatures and, in overload conditions, are generally less resilient unless specifically designed for it — but because no fluid is present, they need no containment systems at all, which makes them the practical choice wherever fire load and oil handling are a concern.
| Feature | Liquid-filled | Dry-type |
|---|---|---|
| Cooling and insulation medium | Dielectric fluid — mineral oil or synthetic — circulated by natural convection | Air, or solid resin encapsulation |
| Typical insulation rating | Around 120 °C | Around 220 °C |
| Fire load and containment | Flammable fluid; bunding or containment required | No fluid, no containment, minimal fire load |
| Thermal behaviour | Cools more effectively; handles overload better | Runs hotter; less resilient to overload unless purpose-built |
| Typical installation | Outdoors by default; sealed tank suits weather | Indoors by default; ventilated enclosure |
| Maintenance | Oil testing and periodic fluid replacement | Dust, connection and ventilation checks |
| Best fit | Utility distribution, industrial sites, high overload duty | Occupied buildings, hospitals, data centres, high-rises |
For a deeper look at where the two diverge in cost and lifetime terms, see the comparison of dry type versus oil immersed transformers.
Liquid-filled transformers come in several designs, each built for a specific environment and duty.
“Padmount” describes a compartmentalised, tamper-resistant transformer installed on a concrete pad. Their secure construction puts them in public and commercial spaces — airports, campuses, commercial buildings — usually painted green to blend into their surroundings. A front cabinet houses the cable connections alongside controls and monitoring, and the unit may include bayonet fuses and under-oil rotary load-break switches. Because protection and switching are built in, they operate without separate switchgear, which often makes them the more cost-effective option.
Substation transformers use a stronger, more durable tank, with control and monitoring equipment mounted outside the tank for access. Many use external cooling fans to support higher load capacity, and they are normally installed in industrial locations that are not open to the public. They rarely work alone — they form part of a larger substation system alongside other electrical equipment, which is where the name comes from. See the 30kV oil immersed transformer for substation projects and the wider switchgear and substation solutions range.
Pole-mount transformers sit on utility poles and are widely used in power distribution. Most are single-phase units, which is why they dominate residential areas where supply is reduced for domestic use. Where larger commercial or industrial demand exists, several units can be grouped to supply three-phase power. In many areas, though, underground systems built around small padmount units are steadily replacing single-phase pole-mounted equipment — more on that in pole mounted distribution transformers.
Submersible units, also called network or vault-type transformers, are designed for underground installation, with construction that accounts for limited space and higher surrounding temperatures. They serve dense urban areas, supplying apartments and office buildings, and are built to withstand harsh conditions — including extended exposure to water during flooding. The full range sits under oil immersed power transformers.
Dry-type designs split mainly by voltage class and by how the windings are protected.
Low voltage dry-type transformers are used when both primary and secondary voltages are 600 V or less. They are commonly installed inside commercial and industrial buildings to raise or lower the service voltage so a specific piece of equipment runs at its required level. Floor-mounted enclosures and flexible application earned them the name general-purpose dry-type transformers. The dry type distribution transformer guide covers how they are specified.
Medium voltage dry-type transformers are usually built in a unit substation-style enclosure and installed in line with other distribution gear — a medium-voltage switch on the high side, low-voltage switchgear at the other end. A single unit can also be installed standalone, with power cables entering the primary and secondary through junction boxes or air terminal chambers. Typical builds at 6 kV and 10 kV are covered under the 10kV dry type transformer and 6kV dry type transformer pages.
Cast coil transformers are the tougher variation of medium voltage dry-type units. Their windings are fully embedded in solid epoxy resin, which seals and protects them, making them a strong choice in damp or harsh environments where open windings on a standard dry-type unit would absorb moisture or suffer damage. See cast resin dry type transformers for construction detail and the SCB series for a worked example.
An autotransformer uses a single coil serving as both primary and secondary. With no separate secondary it provides no electrical isolation between input and output, so it is unsuitable wherever isolation is required. The shared winding reduces copper and core material, which makes these units smaller and more economical for everyday step-up or step-down duty. Where the required voltage change is only about plus or minus 5–20%, a buck-boost style transformer often handles the job well — the selection logic is set out in how to choose a step down power transformer.
Drive-isolation dry-type transformers are built for loads with motor drives such as variable frequency drives. Like standard isolation transformers they have separate primary and secondary windings, but they are additionally designed to withstand heavier non-linear loading and higher harmonic levels. They are K-rated, typically from K-4 upward, and include a 200% rated neutral plus an electrostatic shield between the windings. Specify them whenever harmonic content is above normal and extra robustness is needed; the build options are covered under custom dry type transformers.
Encapsulated transformers are another cast-resin family. Their windings, and often the core, are completely surrounded by a mix of sand and epoxy or a similar resin cured into a solid shell. The sealed construction keeps out moisture, dust and contaminants, suiting tough environments where ordinary ventilated dry-type transformers would be exposed to dirt and humidity. See dry type transformer insulation and the dry type transformer insulation series.
Whichever type you settle on, the same information has to be fixed before anyone can quote accurately. A supplier cannot price a transformer from a kVA figure alone, and a quotation that arrives without losses and impedance stated is not yet a comparison.
| Item to confirm | Why the supplier needs it |
|---|---|
| Rated capacity (kVA) | Sets the frame size, the losses and the physical footprint |
| Voltage ratio and connection group | A 10/0.4 kV Dyn11 unit is not interchangeable with a 10/0.4 kV Yyn0 unit even at the same capacity |
| Impedance voltage (%) | Governs voltage drop, short-circuit current and whether units can be paralleled |
| Enclosure rating and installation location | Indoor, outdoor, washdown or corrosive duty decides the enclosure and the insulation system |
| Cooling method (AN or AF) | Air natural versus air forced changes the usable capacity and the maintenance regime |
| Temperature control and monitoring | Determines whether RTDs, a controller or fans are required, and how the unit is protected |
| Efficiency standard for the destination market | Failure to state it produces a quotation you cannot legally install in the target market |
| Accessories and test scope | Fans, shields, surge protection and witnessed testing are all separate line items |
Send those eight items and the quotation comes back with losses, impedance and test scope stated explicitly rather than left to assumption. If the selection is still open, contact our engineering team with your voltage, capacity and site conditions.
They divide first into liquid-filled and dry-type. Liquid-filled covers padmount, substation, pole-mount and submersible or network designs. Dry-type covers low voltage general purpose, medium voltage, cast coil and cast resin, encapsulated, autotransformers and drive-isolation units. Instrument transformers (current and voltage transformers) form a separate family used for metering and protection rather than power transfer.
Let the installation decide. Dry-type is the default indoors in occupied buildings because it has no flammable fluid and needs no containment, which is why hospitals, high-rises and data centres specify it. Liquid-filled is the default outdoors and where high overload capacity or the highest efficiency at large ratings matters, because the fluid transfers heat more effectively than air.
A dry-type unit, in almost every case. Ventilated low voltage general purpose units suit electrical rooms; cast resin and encapsulated units suit damp, dusty or washdown areas where open windings would degrade. Where an indoor installation still needs liquid-filled construction, it requires a vault with containment and fire protection, which is normally the more expensive route.
A K-rated or drive-isolation dry-type transformer. These use separate primary and secondary windings, an electrostatic shield between them, a neutral rated at 200% and a K-factor of K-4 or higher, so they tolerate the non-linear loading and extra heating that drives, rectifiers and switched-mode power supplies create. A standard general-purpose unit in the same position will run hot and fail early.
Both are sealed resin designs, but the construction differs. Cast coil units have the windings fully embedded in solid epoxy resin, which suits damp or harsh environments. Encapsulated units surround the windings, and often the core, with a cured sand-and-epoxy shell, giving a heavier but very robust seal for contaminated or corrosive sites.
It is a matter of where the unit sits in the network and how it is loaded. Distribution transformers step voltage down to the level a building or neighbourhood uses, usually at lower ratings and with a load that varies through the day. Power and substation transformers sit further upstream at higher ratings and higher voltage, working alongside switchgear as part of a substation.
Outdoors, in public or commercial areas where the unit has to be tamper-resistant and low-profile. They are installed on a concrete pad, normally painted green to blend into their surroundings, and often include bayonet fuses and under-oil load-break switches so they need no separate switchgear.
When the unit goes indoors or where fluid containment is not acceptable, and fire safety and low maintenance matter more than the last few percent of efficiency at very large ratings. Dry-type units are common in commercial and industrial buildings, and in public and medical facilities with strict fire requirements.
Work through four answers: voltage class, installation location, load profile and whether isolation is genuinely required. Those four eliminate whole families before you look at a data sheet. After that, confirm capacity, voltage ratio and connection group, impedance, enclosure rating and cooling method, and then ask an engineer to check the selection against your site conditions.
If you are unsure which transformer type is right for your project, work through the four questions above and then talk the answers through with an engineer who can match a unit to your application. Kampa manufactures liquid-filled and dry-type transformers, including instrument transformers, with full OEM and ODM customisation. Send your voltage, capacity and site conditions and we will come back with the type, the losses and a firm lead time.
