How to Read a Transformer Nameplate

The fields that decide a replacement

kVA rating, primary voltage, secondary voltage, phase, impedance and frequency. Match those six and a replacement will function. Miss impedance in particular and you can have a unit that works alone but will not parallel with an existing one, or that changes the available fault current enough to invalidate the protection study downstream.

What kVA actually means

Kilovolt amperes, which is apparent power, not real power. It is voltage times current, without accounting for power factor. A 300 kVA transformer will deliver 300 kVA of apparent power, which at a power factor of 0.9 is 270 kW of real power. Transformers are rated in kVA rather than kW because heating depends on current regardless of power factor, and heating is what limits the machine.

Impedance, and why it is on the plate

Usually shown as a percentage, for example 5.75 percent. It is the voltage needed on the primary to drive full load current through a shorted secondary, expressed as a fraction of nominal. Low impedance means better voltage regulation but higher fault current. High impedance means the opposite. It governs whether two units can be paralleled and it sets the fault current available to everything downstream, which is why the protection engineer wants it.

Temperature rise and insulation class

65 degrees Celsius rise is the modern standard for liquid filled distribution transformers. It states the average winding temperature rise above ambient at full load, not the operating temperature. Insulation class is the temperature the insulation system is designed for. A 55 degree rise unit run at 65 has more thermal margin and can carry overload better, which is occasionally specified deliberately.

Connection and tap information

The plate shows the winding configuration, commonly delta primary to grounded wye secondary on a pad mount, usually with a small diagram. Taps are shown as a table of positions and the voltage each produces, typically two above and two below nominal at 2.5 percent each. Full capacity taps carry rated kVA at every position, reduced capacity taps do not, and the plate says which.

What does kVA mean on a transformer

Kilovolt amperes, which is apparent power. It is voltage times current without accounting for power factor. A 300 kVA transformer delivers 300 kVA of apparent power, which at 0.9 power factor is about 270 kW of real power. Transformers are rated this way because heating depends on current regardless of power factor.

How do you read a transformer nameplate

Start with kVA, primary voltage, secondary voltage, phase, impedance and frequency, which are the fields that determine whether a replacement will work. Then the connection diagram, the tap table, temperature rise, insulation class and the serial and model numbers for parts and support.

What does impedance percent mean on a transformer

The primary voltage needed to drive full load current through a shorted secondary, as a percentage of nominal. It sets voltage regulation and available fault current, and it determines whether two transformers can be paralleled.

What does 65 degree rise mean

The average winding temperature rise above ambient at full load, in degrees Celsius. It is the modern standard for liquid filled distribution transformers. It is a rise above ambient, not an absolute operating temperature.