GuideTransformers10 min read
How to read a transformer nameplate
A transformer nameplate is the specification of the unit, stamped on steel and fixed to the tank, and IEEE C57.12.00 says what has to be on it: the maker and serial number, the kVA with its cooling class, the voltages and connection of each winding, the tap positions, the impedance, the insulation level (BIL), the temperature rise, the fluid and its volume, the weights, and a vector diagram showing how the windings are connected. Read in order, those fields tell you whether a transformer will fit your supply and your load before anyone opens a compartment.
The short version
- kVA is the continuous rating at the rated temperature rise; two figures (1,500/2,000) are without and with fans (ONAN/ONAF, or AA/FA on a dry-type).
- “12470GrdY/7200 – 480Y/277” gives both windings’ voltages and connections: a grounded-wye primary at 12,470 V line-to-line, a wye secondary at 480 V with 277 V to neutral.
- Taps are listed as a table of positions and voltages (A to E, or 1 to 5), usually ±2 × 2.5% on the primary, moved only de-energized.
- %Z, BIL and temperature rise set the fault current, the surge withstand and how hot it runs: 5.75%, 95 kV and 65 °C are the standard pad-mount figures.
- The fluid line (mineral oil, FR3, silicone) and the gallons decide where it can go and what the freight weighs; a non-PCB statement matters on anything built before the mid-1980s.
- A listing that gives these fields is a listing you can act on; one that gives “1000 kVA transformer” and a photo isn’t.
The fields, in the order they matter
IEEE C57.12.00 lists what a liquid-filled transformer’s nameplate must carry, and C57.12.01 does the same for dry-types. The order on the plate varies by maker; the order a buyer reads them in doesn’t.
| Field | What it says | Why it matters when buying |
|---|---|---|
| kVA rating | Continuous capacity, with cooling classes (ONAN/ONAF, KNAN, AA/FA) | Whether it carries your load |
| Phases and frequency | 3-phase or 1-phase, 60 Hz (50 Hz on imported units) | A 50 Hz unit on 60 Hz runs cooler; the reverse saturates the core |
| High-voltage rating and connection | e.g. 12470 Δ, or 12470GrdY/7200 | Must match your supply, including the neutral |
| Low-voltage rating and connection | e.g. 480Y/277, 208Y/120, 240 Δ | Must match your loads, including line-to-neutral ones |
| Taps | Positions and voltages, and which winding | Whether the supply voltage can be corrected |
| Impedance (%Z) | At a reference temperature, on the rated tap | Fault current, regulation, paralleling |
| BIL | Impulse level of each winding, kV | Surge withstand; must suit the system and arresters |
| Temperature rise | 65 °C liquid; 150/115/80 °C dry | Overload capacity and life |
| Insulating fluid | Mineral oil, FR3, silicone; gallons; non-PCB statement | Where it can be installed; containment; disposal |
| Weights | Core and coils, tank and fittings, fluid, total | Rigging and freight |
| Vector diagram and terminal markings | How the windings connect; H1–H3, X0–X3; the 30° displacement | Paralleling and phasing |
| Polarity (single-phase) | Additive or subtractive | Banking and paralleling |
| Serial number, year, maker, standard | Identity, age, IEEE C57.12.00 or IEC | Records, test reports, parts |
| Efficiency | DOE efficiency class on units built since 2016 | Losses over its life |
Voltages, connections and the vector diagram
The voltage line is the one most buyers read first and the one most often misread. The convention (IEEE C57.12.70): the line-to-line voltage first, then the connection, then after a slash the line-to-neutral voltage if the winding has a neutral brought out.
- 12470 Δ
- Delta, 12,470 V between lines, no neutral. The usual industrial primary.
- 12470GrdY/7200
- Grounded wye, 12,470 V between lines, 7,200 V to the grounded neutral. The usual utility primary; needs the system neutral at the transformer.
- 12470Y/7200
- Wye with the neutral brought out but not necessarily grounded; the plate and the diagram say whether H0 is available.
- 480Y/277
- Wye secondary, 480 V between lines, 277 V to neutral (X0). The standard plant secondary.
- 208Y/120
- Wye secondary for receptacles and lighting.
- 480 Δ or 240 Δ
- Delta secondary, three-wire, no neutral. Some older plants; also a 240 V delta with a center tap on one winding for 120 V (the “high leg” is marked).
- 2400 × 4160Y/2400
- A dual-voltage winding: can be connected for 2,400 V delta or 4,160Y/2,400 V wye, by relinking.
- 12470 × 24940GrdY
- Dual primary voltage, series-parallel, selected with a switch or links. The listing should say which it’s set to.
The vector diagram beside the voltages shows the windings as a triangle and a star with the terminals numbered, and the angular displacement between them, 30° on a delta-wye with the low side lagging by the ANSI convention. The delta-wye guide explains it; for buying, the point is that two transformers on one bus must show the same diagram.
Taps
The tap table lists each position of the de-energized tap changer and the primary voltage it suits: on a 12,470 V unit with ±2 × 2.5% taps, positions A to E at 13,094, 12,782, 12,470, 12,158 and 11,846 V. The transformer delivers rated secondary voltage when the supply matches the tap selected. Moving to a lower tap raises the secondary for a low supply; the changer is operated only with the unit de-energized (a nameplate warning says so), and a listing should say which position it’s on. A unit with a load tap changer has a second, longer table, usually 33 positions over ±10%, on the secondary (the LTC guide).
Impedance, BIL and temperature rise
- Impedance
- Written “%Z 5.75 @ 85 °C” or “%IZ”. The reference temperature is 85 °C for liquid-filled units and 75, 100 or 120 °C for dry-types depending on their rise. The impedance guide covers what the number does.
- BIL
- Given per winding: “HV 95 kV, LV 30 kV”. Standard values by class: 600 V windings 30 kV (dry-type 10 kV); 5 kV class 60 kV; 15 kV class 95 kV (dry-type 60 kV standard, 95 optional); 25 kV class 125 kV (or 150); 35 kV class 150 kV (or 200). A unit with a lower-than-standard BIL needs arresters and a closer look.
- Temperature rise
- “65 °C rise” is standard for liquid-filled units since the 1960s; “55/65 °C” means a unit rated at 55 °C rise with a 12% higher rating allowed at 65. Dry-types: 150 °C standard, 115 or 80 °C for cooler-running units with overload capacity (the dry-type guide).
- Cooling class
- ONAN (oil natural, air natural; older plates say OA), ONAF with fans (FA), KNAN for a less-flammable K-class fluid such as FR3, and AA/FA for dry-types. Each kVA figure on the plate goes with one class.
- Altitude
- Rated for 3,300 ft (1,000 m) unless the plate says otherwise; thinner air cools less and insulates less.
Fluid, weights and the rest
The fluid line names the liquid and gives the volume: “Mineral oil, 310 gal”, “Natural ester (FR3), 290 gal”, “Silicone”. A statement that the fluid contained no PCBs when shipped (“Non-PCB, less than 1 ppm at time of manufacture”) appears on units built since about 1980; its absence on an older unit means a test is in order (the oil guide). The weights follow: core and coils, tank and fittings, fluid, and total, which is the number the rigger wants, along with the untanking weight if the core ever has to come out. Then the maker, the serial number, the year (sometimes only in the serial), the standard (“IEEE C57.12.00-2015”), instruction-book references, and on units built since 2016 the DOE efficiency class. Pad-mount plates add the fusing (bayonet type and link size, backup fuse rating) and the switch arrangement; dry-type plates add the insulation class (220 °C) and enclosure type.
A 1,000 kVA pad-mount nameplate, read through
A typical plate on a unit like the ones listed here, with what each line settles:
| Nameplate line | Reading |
|---|---|
| 1000 kVA, 65 °C rise, ONAN, 3 phase, 60 Hz | Carries 1,000 kVA continuously; no fans; 1,203 A at 480 V |
| HV 12470 Δ, LV 480Y/277 | Delta primary on a 12.47 kV three-wire feeder; wye secondary with a neutral for 277 V lighting |
| Taps: A 13094, B 12782, C 12470, D 12158, E 11846; set on C | Two 2.5% taps above and below rated; left on the rated tap |
| %Z 5.75 @ 85 °C | About 20,900 A available at the 480 V terminals; 22 kA switchgear is marginal, 42 kA is safe |
| BIL: HV 95 kV, LV 30 kV | Standard for 15 kV class; arresters optional on the primary |
| Mineral oil, 318 gal; non-PCB at time of manufacture | Containment needed indoors or near drains; no PCB test needed for a unit of this age |
| Weights: core and coils 4,100 lb; tank and fittings 2,900 lb; oil 2,350 lb; total 9,350 lb | A 10,000 lb lift; a flatbed, not a box truck |
| Loop feed, 200 A load-break bushings; Bay-O-Net fuses 80 A with ELSP backup; 2-position switch | Fits a looped campus feeder; fused as the maker intends |
| Serial 12345678, 2024; IEEE C57.12.00 / C57.12.34; DOE 2016 | Two years old; meets the current efficiency rule |
That plate answers the questions a buyer has before the price does: supply (12.47 kV delta, three-wire), loads (480Y/277), size (1,000 kVA, no fans), protection (fuses as fitted), rigging (9,350 lb), and environment (mineral oil, 318 gallons). The listings here are written from the nameplate for that reason, and Magic Search reads sellers’ pages for the same fields.
A pole-mount nameplate
A single-phase pole-mount’s plate is shorter and has two lines the three-phase plate doesn’t. “25 kVA, 7200/12470Y – 120/240” means a 25 kVA unit whose primary connects from a 12,470 V system’s line to its neutral at 7,200 V (one high-voltage bushing, with the tank as the other terminal, or two bushings for a line-to-line connection), and whose secondary gives 240 V across the outer terminals and 120 V from each to the center tap. The polarity line says additive or subtractive, which fixes where X1 sits relative to H1 and matters the moment two units are banked for three-phase or paralleled. The rest is the same: taps (often ±2 × 2.5% or a single 5% tap), impedance (1.5 to 3%), BIL (95 kV), fluid and gallons, weight, and a “non-PCB” statement. Utility units also carry the utility’s own asset number, which is not the serial.
Checking a used transformer against its nameplate
The nameplate is a claim; the tests confirm it. On a used unit, the turns-ratio test checks the voltages and taps, the short-circuit impedance test checks %Z, the oil tests check the fluid, and the weight on the scale checks that the oil is all there. Three things to look at on the plate itself:
- That it’s the original. A rebuilt or rewound unit should have a rebuilder’s plate beside the maker’s, with the new ratings; a unit rewound to a different voltage sometimes carries only the old plate.
- That the taps and dual-voltage links are set as the listing says. A 2400 × 4160 unit connected for 2,400 V is a different transformer from one connected for 4,160 V.
- That the serial matches the test report and any PCB certificate, since reports travel between units more often than they should.
Questions people ask
What information is on a transformer nameplate?
The manufacturer, serial number and year; the kVA rating and cooling class; the rated voltage and connection of each winding; the number of phases and the frequency; the tap voltages and positions; the percent impedance and its reference temperature; the basic impulse level (BIL) of each winding; the temperature rise; the insulating fluid and its volume; the weights of the core and coils, the tank, the fluid and the whole unit; the vector (connection) diagram and terminal markings; the applicable standard; and on newer units the efficiency class. Single-phase units also state polarity.
What does 12470GrdY/7200 mean on a transformer?
A wye-connected winding with its neutral grounded (GrdY), rated 12,470 V between lines and 7,200 V from each line to the neutral (12,470 ÷ 1.732 = 7,200). On a three-phase unit it’s the primary connection for a multi-grounded-neutral utility system; on a single-phase pole-mount it means the primary is connected from one line to the neutral at 7,200 V, on a 12,470 V system.
What does kVA mean on a transformer nameplate?
The apparent power the transformer can carry continuously at its rated temperature rise in a 30 °C average ambient: the volts times the amps, whatever the load’s power factor. Two figures separated by a slash (1,000/1,333) are the self-cooled rating and the rating with fans running; three figures (1,500/2,000/2,500) add a second stage of cooling.
What is BIL on a transformer nameplate?
Basic impulse insulation level: the peak of the standard lightning impulse (1.2 × 50 microseconds) the winding’s insulation withstands, in kV. It’s set by the voltage class: 95 kV is standard for 15 kV-class liquid-filled windings, 125 kV for 25 kV class, 150 kV for 35 kV class, 30 kV for a 600 V-class winding. Dry-type transformers have lower standard BILs for the same class, which is why they get surge arresters.
What is the temperature rise on a nameplate?
The average rise of the winding temperature above the ambient at full load, measured by resistance. Liquid-filled units are rated 65 °C rise (older ones 55 °C, or 55/65 with a 12% higher rating at 65); dry-types 150, 115 or 80 °C. Together with the 30 °C design ambient and a hot-spot allowance, it sets the insulation temperature the unit is designed to run at and so its expected life.
How do I know if a transformer is additive or subtractive polarity?
The nameplate says, and the bushing layout shows it: on a single-phase unit, additive polarity has H1 and X1 diagonally opposite (H1 top left, X1 bottom right); subtractive has them on the same side. The standard is additive for single-phase units up to 200 kVA with a high-voltage winding of 8,660 V or less, and subtractive for everything else. It matters when units are banked or paralleled.
Sources: IEEE C57.12.00 (nameplate requirements, clause on nameplates); IEEE C57.12.70 (terminal markings and connections); IEEE C57.12.34 (pad-mount transformers); IEEE C57.12.01 (dry-type transformers).