A transformer nameplate is required by IEEE/ANSI C57.12.00 to carry a specific set of ratings and markings — but the standard only requires that the information be present, not that everyone reads it the same way. Here's what each field controls, in the order you'll usually find it.
The headline numbers are kVA (or MVA) rating, primary and secondary voltage, and a cooling class code like ONAN, ONAF, or OFAF. The cooling code isn't decorative — it's a four-letter description of how the unit is cooled, and it often carries multiple ratings on one nameplate because forced cooling stages raise capacity above the base natural-cooling number.
| Position | Meaning | Common values |
|---|---|---|
| 1st letter | Internal cooling medium | O = mineral oil, K = less-flammable liquid, A = dry-type (air) |
| 2nd letter | Internal circulation | N = natural, F = forced (pumped) |
| 3rd letter | External cooling medium | A = air, W = water |
| 4th letter | External circulation | N = natural, F = forced (fans/pumps) |
So a nameplate reading 21/28/35 MVA ONAN/ONAF/ONAF2 means: 21 MVA on natural oil and air cooling alone, 28 MVA once the first stage of cooling fans kicks on, 35 MVA with a second fan stage running. The base ONAN number is what the unit can carry indefinitely with no fans running at all — the number procurement should size against if fan failure is a real operating scenario.
Percent impedance shows up as a single number, typically 4–8% for distribution-class units, but it does two jobs: it sets how much the secondary voltage sags under load, and it caps how much fault current the transformer will pass through to a downstream fault. Lower %Z means less voltage drop under load but higher available fault current — which drives the interrupting rating required of every breaker downstream. It's the direct input to a fault current estimate; see the fault current calculator for the formula that uses it.
BIL, given in kV, is the surge/lightning-impulse withstand rating of the transformer's insulation system. It has to be coordinated with everything else at that voltage class on the system — bushings, switchgear, surge arresters — not evaluated on the transformer alone. A transformer with a higher BIL than the switchgear feeding it doesn't buy any extra protection; the weakest link in the chain sets the real withstand level.
A code like Dyn11 describes the winding connection and phase relationship: the first letter is the primary connection (D = delta, Y = wye), the second is the secondary (lowercase, same letters, with "n" if the neutral is brought out and grounded), and the number is the phase displacement between primary and secondary in multiples of 30 degrees. Dyn11 is delta primary, grounded-wye secondary, secondary leading by 330 degrees. This matters most when paralleling transformers or matching an existing installation — units with different vector groups generally can't be paralleled, full stop.
High-voltage terminals are marked H1, H2, H3 (and H0 for a grounded neutral); low-voltage terminals are marked X1, X2, X3 (and X0). These markings, combined with the vector group, are what a field crew uses to confirm phasing before energizing — getting this wrong is one of the more expensive mistakes possible during commissioning.
No-load loss (core loss, present continuously) and load loss (winding loss at rated load) are sometimes printed on the nameplate and sometimes only on the factory test report — either way, they're the two figures needed for a lifetime total-cost-of-ownership estimate. Temperature rise (commonly 55°C or 65°C rise above ambient) sets how hard the unit can be loaded before insulation life is affected; total and oil weight matter for foundation design and rigging, not electrical performance.
Every configurable transformer on the Data Centers and Renewables desks exposes cooling class, voltage class, and lead time at the configuration stage — no need to wait for a nameplate to know what you're ordering. Run the numbers once you have a real nameplate through the engineering calculators: fault current from %Z, or lifetime loss cost from no-load/load loss — then see the short-circuit studies guide for how that number feeds into a full study. Unfamiliar with a related term? Check the glossary, or see current transformer lead times across cooling classes and voltage tiers.
Codes like ONAN, ONAF, and OFAF describe the cooling method under IEEE C57.12.00: the first letter pair describes the internal cooling medium and circulation (O = mineral oil, N = natural circulation, F = forced circulation), and the second pair describes the external cooling medium and circulation (A = air, F = forced air, W = water). A transformer often carries multiple ratings — for example 21/28/35 MVA at ONAN/ONAF/ONAF2 — where forced cooling stages raise the effective capacity above the base natural-cooling rating.
Percent impedance sets how much the output voltage sags under load and, more importantly for procurement, how much available fault current a transformer will let through on a downstream fault — higher %Z means lower fault current but more voltage drop under load. It's a required input for any fault current or protective device coordination calculation.
BIL (Basic Insulation Level) is the withstand voltage rating, in kV, that the transformer's insulation is designed to survive during a lightning-impulse or switching-surge event. It has to be matched or exceeded by the insulation levels of everything else connected to that voltage class — switchgear, bushings, surge arresters — not just the transformer itself.
The vector group describes the winding connections and phase-shift relationship between primary and secondary: the first letter is the primary winding connection (D = delta, Y = wye), the second is the secondary (lowercase y or d), and the number is the phase displacement in multiples of 30 degrees. Dyn11 means a delta primary, grounded wye secondary, with the secondary leading the primary by 330 degrees (11 x 30) — critical for paralleling transformers or matching an existing system.
Configure by voltage class, cooling type, and BIL on the catalog, and get a quote with real lead time in 24 hours.