Transformer Load Calculation Formula: Things you Should Know!

Transformers supply you with the required energy from the generation source. But, the need depends on the number of electrical appliances at your place. Hence, understanding the transformer load calculation becomes very important.

Hospitals and industries have different output requirements in comparison to schools, colleges, and government buildings. The energy is received in the form of power.

Transformers receive power from the source. The amount of power required depends on the number of appliances in the concerned building and the time of their operation. 

The needs vary depending on the type of business or purpose of building. So, the calculation of power requirements is the base of understanding the type of transformer needed. 

Go on reading for assistance in mentioned calculations. 

What is a Transformer Load?

We can define a transformer load in two ways. Simply, it is the quantity of power that moves through the transformer. However, the transformer load can also be a regulator modifying its output. 

The modifications can be in the form of power received by the appliances or the altered voltage flow to the receiving equipment.

So, during the transformer load calculation, you are calculating the current that can be transmitted. The full load capacity and safe load capacity are considered during the transformer load calculation.

  • – The full load is the maximum amount of current that can be transmitted by the transformer. 
  • – Safe load is 75-80% of the maximum amount of current that can be transmitted by the transformer. It is the amount of load that can be connected to the transformer.

What are the power ratings in Transformer Load Calculation?

Let’s understand the power ratings now!

The transformer load calculation is based on the output voltage and current of the electrical load of equipment to be used. The resultant will represent the power of the transformer with the unit in kilovolt amperes. To understand simply, 1 kVA can handle 100 Volts and 10 Amps of current.

How to calculate the apparent power ratings of equipment?

In transformer load calculation, the apparent power (kVA) or power (kW) of the equipment is based on its input needs which are voltage and current. 

  • – V represents voltage and is mentioned on the equipment. 
  • – The letter A represents Ampere, which is generally mentioned with the specifications of the equipment. 
  • – In case of the absence of Ampere or current ratings, the value stating resistance or R can be used for current calculation. 

Ohm’s law can be used for calculation by the formula: V = IR, where V stands for Voltage, I stands for Current, and R stands for Resistance.

The Transformer Load Calculation Formula 

P = VI/1000. The obtained apparent power will be in kVA, which is to be divided by 0.8

If the result is in decimal, values need to be rounded off, to calculate the power rating relating to the transformer power. (The division includes extra current required on turning the device on). 

Single-phase and three-phase transformers

The mentioned calculations are aimed at single-phase transformers. If the transformer has power with a phase difference of 120 degrees, it is known as a three-phase transformer. The formula slightly varies in the case.

Thus, the transformer load calculation of a three-phase transformer will use the formula – P = VI√3/1000. 

Other than multiplication with √3 or 1.732, all the steps remain the same including the division of the start-up current required by the device. 

A few examples stating available sizes of transformers used commercially are 75 kVA, 112.5 kVA and 750 kVA. Typically, the range is between 3kVA to 1000 kVA.


The transformer load calculation can also be performed in terms of current ratings. The apparent power will be divided by the supply voltage followed by multiplication with 1000. 

The end step will be multiplication with 25 (taking extra current on the start-up of the device into consideration). 

We can also utilize the transformer load calculation method to find the voltage requirements of our equipment. For this, the formula will be (apparent power (kVA) × 1000) ÷ (current ratings). 

So, now you have the safe voltage amount to flow from your equipment from the transformer load calculation formula.


The transformer is an important device required for the transmission and provision of power from the source. It provides power at the required voltage. 

Transformer load calculation is the most significant and easiest method that can be used for the selection of a transformer. The method of calculation varies for single-phase transformers and three-phase transformers.

Transformer load calculation can be used for the selection of a transformer. The selection of a transformer also depends on other factors:

  • – Voltage taps
  • – Voltage ratio
  • – Method of connection
  • – Harmonic content of load
  • – Impedance values
  • Transformer failure predictability
  • Insulation temperatures
  • Paralleling transformers
  • Insulation classes
  • Sound levels. 


1. Is full load capacity the final transformer load value?

The transformer load calculation involves the usage of a safe amount of current load. This current load is 75% to 80% of the full current capacity of the transformer. 

2. What is 0.8 in transformer load calculation?

Division by 0.8 or multiplication with 1.25 in transformer load calculation indicates the inclusion of excess current required by the device when it is switched on.

3. Does transformer load calculation use the same formula for both single-phase transformers and three-phase transformers?

The transformer load calculation uses a different formula for each type of transformer. 
The single-phase transformer is the product of voltage and current divided by 1000. The formula for transformer load calculation of a three-phase transformer differs by simple multiplication of the previous formula with √3 or 1.732.