Electric Transformer Rating (Explained for Beginners)

Electric transformer rating makes or breaks your electrical project. This is because under-rating the transformer can cause it to fail, break down, or be completely damaged. It happened in my work, one of the most critical transformers had serious damage, and we had to replace it ASAP to reconnect loads.

In this article, I will discuss, how to rate a transformer, with examples for beginners. Let’s dive into the details.

what is an electric transformer rating?

Electric transformer ratings refer to the specifications and characteristics that define the performance and capabilities of an electrical transformer.

Transformers are essential components in electrical power systems, used to transfer electrical energy between different voltage levels.

Key transformer ratings include:

  1. Voltage Rating:
    • Primary Voltage (Vp): The voltage on the high-voltage side (input side) of the transformer.
    • Secondary Voltage (Vs): The voltage on the low-voltage side (output side) of the transformer.
  2. Power Rating:
    • Apparent Power (S): The product of the voltage and current in a circuit, measured in volt-amperes (VA). It represents the total power in an AC circuit, both real (active) and reactive power.
    • Real Power (P): The actual power consumed or delivered by a device, measured in watts (W).
    • Reactive Power (Q): The non-working power component due to the phase difference between voltage and current, measured in volt-amperes reactive (VAR).
  3. Frequency:
    • The frequency of the alternating current (AC) for which the transformer is designed, is typically 50 or 60 Hertz.
  4. Temperature Rise:
    • The maximum allowable temperature rise of the transformer’s winding and oil insulation above the ambient temperature.
  5. Impedance:
    • The percentage impedance (%Z) indicates the internal resistance to the flow of current within the transformer. It is expressed as a percentage of the rated voltage.
  6. Insulation Class:
    • Transformers are assigned insulation classes based on the temperature rise of the winding. Common insulation classes include A, B, F, and H.
  7. Cooling Method:
    • Transformers are designed for different cooling methods, such as oil-immersed (liquid-filled) or dry-type (air-cooled).
  8. Vector Group:
    • Describes the phase relationship between the primary and secondary winding voltages. Common vector groups include Yyn0, Dyn11, etc.
  9. Efficiency:
    • The efficiency of a transformer indicates the ratio of output power to input power, usually expressed as a percentage.
  10. Tap Changer:
    • Some transformers have tap changers that allow for adjusting the turns ratio and, consequently, the output voltage.

Transformer ratings are crucial for ensuring proper operation, efficiency, and safety within an electrical power system.

They are specified by standards and regulations to facilitate compatibility and reliability in the grid.

Standard KVA Rating of Transformers

We use KVA for transformer rating, (KVA means Kilo volt Ampere).  To determine the size of the transformer, it is important to determine the load by KVA.

However, Transformers are mostly used for the purpose of power distribution, that’s why common standard KVA transformers are commonly used.

Two types of transformer KVA ratings are mentioned below.

For the Phase Delta Wye transformer 408 to 120/208 common standard KVA ratings are 15 KVA, 30 KVA, 45 KVA, 75 KVA, 112.5 KVA, 225 KVA, 300 KVA, and 500 KVA.

For a Single-phase transformer with the size of 277- or 480-volt transformer common standards are 5 KVA, 7.5 KVA, 10 KVA, 15 KVA, 25 KVA, 37.5 KVA, 50 KVA, 75 KVA, and 100 KVA.

Please note, Besides the mentioned standard ratings there are other sizes of transformer available. However, the most common size is mentioned above.

Transformer KVA Rating Formula

The transformer KVA rating formula is derived from the power rating of the transformer. In this article, we will include a formula for both single-phase and three-phase transformers. So, let’s start with the formula of a single-phase Transformer.

Single Phase Transformer:

Single-phase Transformer Power Rating P = V x I

While : P is transformer power in VA, V is transformer voltage, I is Transformer current.

Note that transformer power is fixed for low and high-voltage sides. As you know, the transformer affects voltage and current only. In the formula, we use voltage and current of the same side (low or high voltage).

Now, KVA Rating for Single Phase Transformer: P = (V x I)/1000

Hence putting the value of Current and voltage we will get the required rating of a single-phase transformer.

KVA Rating Formula for Three-phase Transformer:

The power rating of the three-phase transformer:

P = √3. V x I

Hence the rating of the phase transformer in KVA is given below

P = (√3. V x I)/1000

How to Increase Transformer Rating?

Increasing the transformer rating is possible by increasing the cooling method. Adding new fans or oil pumps increases the transformer rating up to 66%, Some transformers have more than one rated power on the nameplate depending on the cooling stage.

Most people think that rating depends upon the power rating formula. I.e. From the formula of power, they can change the rating. But this will need some extra work.

By increasing the value of current or voltage different types of losses such as eddy current losses and hysteresis losses occur.

Similarly, the same situation occurs for increasing the frequency. And the transformer becomes hot. Hence to increase the rating of the transformer we must try to cool down the transformer.

And for this purpose, we will need additional cooling fans to install on the transformer. Also, an automatic system will be required which automatically starts cooling the fan when the core temperature exceeds the specified temperature limit.

The more the transformer is cooled, the more power will be transferred from the supply to the load side.

Transformer Rating Example

We already discussed the KVA rating formula for both single-phase and three-phase transformers. Now let’s take an example to get a better understanding of it. Here we will calculate the KVA rating of both single and three-phase transformers.

Single Phase Transformer Rating calculation

For a single-phase transformer, the rating formula is given by: P = (V x I)/1000

Assume the voltage and current values as

Voltage (V)= 120 V

Current (I)= 50A

Now put the values in the rating formula of the single-phase transformer to find the KVA rating of the transformer.

P = (V x I)/1000

(120 V x 50 A) / 1,000 =6 KVA

Hence the rating of a single-phase transformer with 12o Volt and 50 A current is 6 KVA.

  • 3 Phase Transformer Rating calculation formula

To calculate the rating of the transformer we will need to know about the primary and secondary voltages as well as current. Besides this, we will also need to assume a power factor value. Consider the following values on the nameplate of the 100KVA transformer.

Primary Voltages or High Voltages (H.V) is 11000 V = 11kV.

The primary Current on the High Voltage side is 5.25 Amperes.

Secondary voltages or Low Voltages (L.V) is 415 Volts

The secondary Current (Current on the Low voltage side) is 139.1 Amperes.

Now using the 3 Phase transformer KVA rating formula= P = (√3. V x I) /100

Putting the values of either primary or secondary size we get the rating of the transformer.

P = (√3. 11000 x 5.25)/1000 =100 KVA