The answer first: a nameplate is your unit's engineering ID card — read it in this order
Every field on a distribution transformer nameplate answers one commissioning or ordering question, and the ones that matter most are rated power (kVA), rated voltage, impedance voltage (Uk%), vector group, cooling class, insulation level and temperature rise. In thirty seconds you can confirm the unit on the dock matches your purchase order: the kVA and HV/LV voltages must be exact, the vector group (usually Dyn11) and impedance (about 4–6% for distribution units) must match your system study, and the temperature-rise class must suit your climate. Miss any one of these and you either de-rate the transformer in service or get stopped at pre-shipment inspection — which is exactly why the nameplate, not the brochure, is the document that wins disputes.
Key takeaways
- kVA is a thermal rating, not a load guarantee — it is the continuous apparent power at rated voltage and stated temperature rise, corrected for altitude, ambient temperature and harmonics.
- Impedance voltage (Uk%) sets both voltage regulation and the maximum short-circuit current; distribution units are commonly 4%, 4.5% or 6%.
- Vector group (Dyn11 vs Yyn0) determines phase shift, neutral handling and harmonic behaviour — get it wrong and paralleling or protection fails.
- Cooling class (ONAN/ONAF/KNAN) and temperature rise (for example 60 K oil / 65 K winding) set how hot the unit runs and how much load it can actually carry.
- Insulation level (LI/BIL) tells you the surge it can withstand — a 75 kV BIL unit on an 11 kV system needs proper surge protection.
Why the nameplate beats the brochure every time
The brochure tells you what the factory can build; the nameplate tells you what this specific serial number is. It is the auditable record the manufacturer commits to under IEC 60076 / IEEE C57.12 / GB 1094, and it is what an insurer, a utility inspector or your commissioning engineer will read. If a field on the nameplate disagrees with your order, the nameplate is the document that carries weight in a claim — so read it before you accept delivery, not after.
Field by field: what every line means
Rated power (kVA / MVA) — and what it does not mean
Rated power is the continuous apparent power the transformer can deliver at rated voltage, rated frequency and the stated temperature rise, at the stated altitude. It is not the maximum you can draw, and it is not transferable across conditions: a 1,000 kVA unit at 40 °C ambient or above 1,000 m altitude is de-rated, and a unit feeding harmonic loads must be sized with a de-rating factor. Compare real cost per kVA across ratings in our price breakdown guide.
Rated voltages (HV / LV) and tapping
The nameplate lists the high-voltage and low-voltage windings, for example 11/0.4 kV (or 6.6/0.4, 20/0.4, 33/0.4 depending on the grid), plus the tap range — typically ±2×2.5% for an off-circuit tap changer and ±10% for an on-load unit. Confirm the no-load ratio matches your grid voltage class before you energize; a mismatch is one of the fastest ways to damage a new unit.
Impedance voltage (Uk%) — the number that limits fault current
The impedance voltage is the percentage of rated voltage that produces rated current with the LV winding shorted. Distribution transformers commonly carry 4% (smaller ratings), 4.5% or 6% (larger ratings) per GB/T 6451 (industry practice, as of 2026). It matters twice: higher Uk% means better fault-current limiting but worse voltage regulation, and it is the starting input for any short-circuit calculation.
Vector group (Dyn11 vs Yyn0) — the symbol that controls phase shift
The vector-group symbol states the winding connections and phase displacement. Dyn11 (delta HV, star LV with neutral, 30° lead) is the modern default for distribution transformers in China and Europe; Yyn0 (star/star) is the older alternative with different neutral and harmonic handling. Paralleling two units with different groups, or connecting a group your protection was not set for, is a classic commissioning failure — the full decoding is in our vector group guide.
Cooling class (ONAN / ONAF / KNAN) — how the heat gets out
The cooling code tells you the medium and circulation: ONAN is oil-natural, air-natural (the standard for small distribution units); ONAF adds fans for a higher rating; KNAN uses a non-mineral liquid. A unit with dual ratings (ONAN/ONAF) has a base self-cooled rating and a higher fan-cooled rating — the nameplate carries both. See the full breakdown in our cooling methods guide.
Insulation level (LI / AC / BIL) — the surge it can survive
This field states the lightning impulse and power-frequency withstand voltages, for example LI 75 kV / AC 28 kV for an 11 kV class unit (typical values per IEC 60076-3). The insulation class letter (A/B/F/H) on the same plate fixes the hot-spot limit, which drives the insulation life. Choose the insulation level to match your system's exposure, then protect it with coordinated arresters.
Temperature rise, frequency and the remaining fields
Temperature-rise limits (commonly 60 K oil / 65 K winding for mineral-oil units per IEC 60076-2) define the thermal envelope; the rated frequency (50 or 60 Hz) must match the grid; and the oil mass, total mass, serial number and year of manufacture complete the identification for transport, lifting and warranty purposes. The oil type (mineral, ester or silicone) is stated separately and matters for fire safety — compare them in our oil types guide.
How to use the nameplate to verify your order
Before you sign the acceptance documents, walk the nameplate line by line against your purchase order and technical specification: kVA, voltages, vector group, impedance, cooling class, insulation level, temperature rise and frequency must all match. Photograph the plate and attach it to your pre-shipment inspection records — it is your strongest evidence if anything is wrong. Not sure which unit your load needs? Work the numbers in our engineering toolbox calculator first, then specify the nameplate fields deliberately.
Sources / 资料来源
- Source: IEC 60076-1 — Power transformers — Part 1: General (rating and nameplate requirements).
- Source: IEC 60076-2 — Power transformers — Part 2: Temperature rise for liquid-immersed transformers.
- Source: IEC 60076-3 — Power transformers — Part 3: Insulation levels, dielectric tests and external clearances in air.
- Source: IEEE C57.12.00 — IEEE Standard for General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers.
- Source: GB 1094 series — Power transformers (Chinese national equivalent of IEC 60076).
- Source: GB/T 6451 — Oil-immersed power transformers technical parameters and requirements (typical short-circuit impedance values).
- Source: Industry experience — typical Uk% (4/4.5/6%), 60 K/65 K temperature-rise limits and 75 kV BIL for the 11 kV class (QDTB engineering, as of 2026).