How to read a transformer nameplate.
Fifteen fields, six that set the price. This is the page to open while you are standing at the unit with your phone.
Send a nameplate photoOne photo of the plate is worth twenty minutes on the phone.
The nameplate is the spec sheet. Everything a buyer needs to price a unit is stamped on a piece of steel bolted to the tank, and a clear photo of it replaces every question we would otherwise have to ask you. If you read nothing else on this page: find the plate, get the whole plate in frame, and make sure the small print is legible.
On a padmount the plate is inside the cabinet, usually on the inside wall of the low voltage compartment. On a polemount it is on the side of the tank, often facing the pole, which is why the photo usually happens after the unit is down. On a substation unit it is at eye level on the tank wall near the control cabinet. Dry types carry it on the enclosure, sometimes behind a removable panel.
Do not open an energized unit to get a photo. If the unit is still in service, send us the exterior and the ZIP code and we will work with that until it is safe to get closer.
What each field means.
Plates vary by manufacturer and decade. These are the fields that show up on almost all of them.
| kVA | The rating. The single biggest driver of what a unit is worth. On a three phase unit it is the total, not per phase. |
|---|---|
| Primary volts (HV) | What feeds the unit. 12470GrdY/7200 means a grounded wye system, 12470 line to line, 7200 line to neutral. |
| Secondary volts (LV) | What comes out. 480Y/277 and 208Y/120 are the two you will see most on padmounts. |
| Phase | 1 or 3. A single phase 500 kVA and a three phase 500 kVA are different units with different buyers. |
| Hz | 60 in North America. A 50 Hz plate means an export unit and a narrower market. |
| BIL | Basic impulse level in kV. Tells you the insulation class and whether it suits a given system. |
| %Z or IZ | Impedance. Sets available fault current on the secondary. Engineers filter on this. |
| Temp rise | 55C, 65C, or 55/65C. Dual rating means extra capacity and a slightly better number. |
| Cooling class | ONAN, ONAF, KNAN. The K means a less flammable fluid, which changes the oil story. |
| Liquid | Mineral oil, FR3 natural ester, silicone, or none on a dry type. Drives PCB questions and freight. |
| Taps | Usually four 2.5 percent taps, two above and two below nominal. Full capacity taps are worth more. |
| Connection | Delta wye, wye wye, and the vector group. Dyn1 and Dyn11 are common. |
| Serial and year | Traceability. Year sets the PCB question at 1979 and older. |
| Manufacturer | Tier one brands hold value. See who we buy from. |
| Weight and gallons | Total weight decides the truck. Gallons decides the oil handling. |
Which fields actually set the price.
| Sets the number | kVA, primary, secondary, phase, manufacturer, year, condition |
|---|---|
| Moves the number | Impedance, BIL, taps, temp rise, cooling class, fluid type |
| Sets the logistics cost | Total weight, gallons of liquid, dimensions, pad or pole mounting |
kVA and the voltage pair do most of the work. A 1000 kVA padmount at 12470GrdY/7200 to 480Y/277 is a unit with a deep resale market, because that combination is installed everywhere in the country. The same 1000 kVA at an unusual primary is worth less to us, not because the copper weighs less, but because the pool of buyers who can use it without a rewind is smaller.
Year is the field people forget. On a liquid filled unit, 1979 and older changes the process: we sample the oil before we commit to a firm number. That is not a penalty, it is a step. Read the PCB page before you call if your plate shows a pre 1980 build date.
Impedance and BIL rarely change our offer much, but they decide whether a specific buyer can use the unit. Send them anyway. They cost you nothing to photograph and they shorten the resale cycle, which is part of why we can pay what we pay.
The voltage notation, decoded.
This is where most miscommunication happens, so it is worth being precise.
12470GrdY/7200. A grounded wye system. 12470 volts measured between any two phases, 7200 volts measured from one phase to neutral. The two numbers describe the same system.
13200V. No slash, no Y. A delta primary at 13200 volts. Different animal from 13200GrdY/7620, and not interchangeable.
480Y/277. A wye secondary. 480 between phases, 277 phase to neutral. The standard commercial building service in North America.
208Y/120. The other common secondary. Lighting and receptacle loads.
120/240. Single phase secondary, residential and light commercial, usually on polemounts and single phase padmounts.
Copy the string exactly as printed, including the letters. GrdY, Y, and a bare voltage all mean different things to the engineer who ends up buying the unit.
When the nameplate is gone.
It happens. Plates get sandblasted, painted, corroded off, or removed by a previous owner. A unit without a plate is still worth money, it just takes longer to price and the first number is a range rather than a firm offer.
Send these instead and we can usually get close:
- The full exterior, from far enough back to see the whole tank and the bushings
- The open cabinet or the bushing arrangement, which narrows the voltage class quickly
- Any casting numbers on the tank, the radiator, or the cabinet door
- Tags on the bushings, the tap changer, or the pressure relief device
- A tape measure or a person in one shot for scale
- Anything from your own records: purchase order, one line diagram, asset tag
Bushing count and spacing alone tell an experienced buyer a great deal. Six bushings on a padmount with a load break elbow arrangement puts you in a narrow band before we have read a single number.
How to shoot the plate so it reads.
- Square to the plate, not at an angle. Angles blur the small print.
- Fill the frame with the plate. Do not include the whole tank in the same shot.
- Turn the flash off and use a flashlight from the side. Direct flash blows out stamped steel.
- If the plate is painted, shoot with light raking across it. The stamping still casts a shadow.
- Take two: one of the whole plate, one close on the kVA and voltage block.
- Send the original file, not a screenshot of it. Screenshots lose the resolution we need.
Common questions.
Do not scrape it. Take a raking light photo from the side and send it anyway. Paint fills the stamped characters and a low angle often reads them. If it is truly gone, send the exterior, the open cabinet, and any tag on the bushings or the tank wall.
Both. 12470GrdY/7200 is one system described two ways: 12470 volts between phases, 7200 volts from a phase to neutral. Send the whole string exactly as printed. The slash matters.
Grounded wye. The neutral point of the winding is tied to ground. It is not the same as a delta connection at the same voltage, and it changes which systems the unit can serve. Write it exactly as the plate has it.
Yes. A dual rated unit can run at a higher load at the 65C rise. It is a small premium but it is real, and it tells us the unit is a later design.
Yes. It gives us the manufacturer build record, which confirms the year and often the original oil spec. For utilities it also goes on the manifest we hand back for your asset file.
Trust the plate. Yard records drift over decades. The plate is what we price from, and if testing disagrees with the plate we tell you before anything moves.