How to Read a Transformer Nameplate

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.

Rating plate: kVA, voltage, and cooling class

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.

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.

Impedance (%Z) — the number that sets fault current

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, and Transformer Impedance (%Z) Explained for tolerance, regulation and how to choose a value.

BIL — Basic Insulation Level

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.

Vector group — how the windings are connected

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.

Terminal markings and polarity

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.

Losses, temperature rise, and weight

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.

Frequently asked questions