Most transformer problems are not failures — they are mismatches between what a buyer assumed and what the rating plate actually says. Understanding dry type transformer ratings means being able to read five numbers off the plate and know exactly what they commit you to: rated capacity in kVA, percentage impedance (%Z), temperature rise, vector group, and cooling class. Everything else on the plate either describes those five or tells the service team what to buy later.
This guide decodes the standard nameplate field by field, using the IEC 60076-1 rating-plate requirements that most international dry-type units follow. By the end you should be able to check a delivered unit against its specification in about five minutes, and spot the two or three fields that most often disagree.
A specification sheet is a pre-production document. It states design intent. A nameplate is fixed to the unit only after it passes its routine tests, so it records as-built reality: the impedance that was actually measured, the losses that were actually recorded, the tapping range that was actually fitted. When the two disagree, the plate governs, and your protection settings, cable sizing and paralleling decisions all follow the plate.
IEC 60076-1 makes the rating plate mandatory for power transformers above 50 kVA. The reference below lists what has to be on it.
| Group | Fields | Why you care |
|---|---|---|
| Identity | Kind of transformer; standard and year; manufacturer name and country; serial number; year of manufacture; number of phases | Warranty tracking, spare-parts matching, customs and inspection |
| Power and frequency | Rated power per winding; rated frequency | Sets the continuous load the unit may carry at the declared cooling and ambient conditions |
| Voltage and current | Rated voltages and tapping range; rated currents per side | Confirms the ratio, and gives you the LV current that sizes cables and breakers |
| Impedance | Short-circuit impedance as a percentage, with its reference power | Determines fault current and how the unit shares load in parallel |
| Connection | Connection and phase-displacement symbol, for example Dyn11 | The first thing to check before paralleling two units |
| Cooling and thermal | Cooling class code; guaranteed temperature rises if non-standard | Explains any dual rating, and defines how hard the insulation is being worked |
| Insulation | Insulation levels where the highest system voltage is 3.6 kV or above | Impulse and power-frequency withstand for coordination with your switchgear |
| Mass | Total mass; transportation mass if different; untanking mass for heavy units | Crane selection, floor loading, rigging plan |
| Conditional | Tap-change table; connection diagram; winding-temperature-indicator settings; internal CT data; minimum cooling-medium temperature | Commissioning and protection setup |
On a dry-type unit the liquid fields disappear and three dry-type specifics appear instead: the insulation system temperature class (usually F or H), the IEC 60076-11 climatic, environmental and fire classes — commonly written as C2 E2 F1 — and an IP rating where an enclosure is fitted. Our dry type transformer specification guide covers those classes in full detail.
Rated power is the apparent power the transformer can carry continuously without exceeding its permitted temperature rise. The important word is continuously, and it is always conditional. A plate reading 1000 kVA is only 1000 kVA under the conditions printed beside it: the cooling class in service, the reference ambient temperature, and the installation altitude.
IEC assumes a yearly average ambient of 20 °C and a maximum of 40 °C. Put that same unit in a sealed substation room sitting at 45 °C, or above 1,000 m altitude where air density drops, and the honest capacity is lower. This is one of the most common sources of “the transformer is running hot” complaints that turn out to be a specification problem rather than a manufacturing one.
Units with forced cooling carry two ratings, written as a pair — for example 1,600 / 2,000 kVA AN/AF. The lower figure applies with natural air cooling only. The higher applies once fans run. Both are legitimate plate values; they simply describe two operating states. Plan your normal load around the natural-cooling figure unless you are willing to depend on fan availability.
Percentage impedance, printed as something like Uk = 6%, is the voltage drop across the transformer at rated current, expressed as a percentage of rated voltage. Its practical role is twofold: it sets the voltage regulation you experience under load, and it sets the prospective short-circuit current your downstream equipment must survive.
Lower impedance means better voltage regulation and higher fault current. Higher impedance means the opposite. Typical dry-type values are 4% up to about 630 kVA and 6% from roughly 630–800 kVA upward, with 6% common across the 1000–2500 kVA range. Those values are a design convention, not a rule of nature, so the plate figure is what your protection study must use.
Impedance is also the reason two transformers of identical kVA and identical vector group can still refuse to share load evenly. Where units are paralleled, impedance must match within a few tenths of a percent, or the stiffer unit takes more than its share.
Temperature rise is the permitted temperature increase above ambient, measured by the resistance method during a temperature-rise test. It is not the same as the insulation class. A Class F (155 °C) insulation system is normally applied with a 100 K average winding rise allowance in IEC 60076-11 terms, but the guaranteed figure stamped on your plate may be lower — 80 K, for instance — if the buyer specified extra thermal margin.
A lower guaranteed rise is not free. It requires more conductor and more core material to reduce losses, so the unit is physically larger and more expensive for the same kVA. This is a deliberate trade: pay in size and cost now, gain insulation life and overload headroom later. Our dry type transformer insulation guide explains how the class letters A to H map onto those rises.
The vector group records the winding connections and the phase displacement between primary and secondary. In Dyn11, the HV winding is delta, the LV winding is star with a neutral brought out, and the LV phasor lags the HV by 30°. In Yyn0 both windings are star and the displacement is zero.
This matters for exactly one reason in most projects, and it is a serious one: paralleling two transformers with different vector groups produces circulating currents that can destroy both units. Matching kVA is not enough. Vector group, impedance and tap position must all line up.
| Code | Meaning | Applies to |
|---|---|---|
| ONAN | Oil natural, air natural | Oil-immersed baseline |
| ONAF | Oil natural, air forced — fans fitted | Oil-immersed with dual rating |
| AN | Air natural, convection only | Dry-type baseline |
| AF | Air forced, fans assist cooling | Dry-type with dual rating |
Dry-type plates usually shorten the code to two letters because there is no liquid circuit. If a plate shows AN/AF with two kVA figures, the cooling code and the rating pair must be read together — one explains the other.
No-load loss and load loss, both in watts, are printed on most plates and are the basis for any fair commercial comparison between offers. No-load loss is present whenever the unit is energised; load loss scales roughly with the square of the current. Where energy prices are high, the twenty-year cost of those two figures frequently exceeds the purchase price, which is why loss guarantees and their verification method deserve the same scrutiny as the kVA figure.
Insulation level appears as a pair such as LI 170 / AC 70 — a lightning impulse withstand value and a power-frequency withstand value in kV. It tells your protection engineer what surge environment the winding insulation can absorb, and it must be coordinated with the arresters and switchgear around it.
For units in the 10 kV class and similar distribution duties, the plate is usually read alongside the technical data in our 10 kV dry type transformer overview, which sets out the ratings, losses and impedance values typical for that voltage level.
If you are still comparing suppliers rather than checking a delivery, the practical starting points are the cast resin range in our SCB series dry type transformer page and the dry type transformer insulation series, both of which publish the rating fields discussed here. The dry type transformer types comparison is a useful cross-check if you have not yet settled on VPI versus cast resin.
Five fields do the real work on a dry-type nameplate: kVA, %Z, temperature rise, vector group and cooling class. Read them together, because each one constrains the others — a dual rating only means something when you know the cooling state, and a low impedance only looks attractive until you calculate the fault current. Check the delivered plate against the specification, keep the photograph, and send the whole plate rather than the kVA figure alone when you need a like-for-like replacement.
It is the short-circuit impedance: the voltage needed to circulate rated current through the short-circuited winding, expressed as a percentage of rated voltage. It sets the prospective fault current and how well the unit shares load when paralleled.
Neither is universally better. 4% gives tighter voltage regulation and higher fault current; 6% limits fault current and reduces mechanical stress on the windings. Typical dry-type practice is 4% up to about 630 kVA and 6% above that.
The common commercial steps are 30, 50, 100, 160, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000 and 2500 kVA. Ratings above 2500 kVA are usually engineered to order. For the physical footprint at each step, see our dry type transformer sizes and dimensions chart.
The unit has forced cooling. The lower figure applies in natural cooling mode (AN) and the higher figure applies with fans running (AF). Plan normal operation around the lower figure unless fan availability is guaranteed.
No. Different vector groups mean a phase displacement between secondaries, which drives large circulating currents. Vector group, impedance and tap position must all match before paralleling.
Insulation class is the temperature the insulation system can survive, such as 155 °C for Class F. Temperature rise is the permitted increase above ambient during operation, expressed in kelvin. The rise must always leave headroom below the class limit.
Start with rated power and cooling class, then voltage ratio and vector group, then %Z, temperature rise and losses. Those six values are enough to judge whether a unit suits a given project.
