Transformers are always rated in kVA and not kW As the name suggests, a transformer only transfers power from one circuit to another without changing the values of power and frequency. In other words, it can only raise or lower the values of current and voltage while keeping power and frequency constant. General data is printed on the transformer nameplate for more detailed information.

Examples include VA rating, single phase/three phase (power or distribution transformer), step-up/step-down, connections, etc. In simple terms, transformers have two types of losses;

#### 1. copper loss

#### 2. iron or ferrous losses or insulation losses

Copper loss (I²R) depends on the current passing through the transformer windings, while iron loss or core loss or insulation loss depends on the voltage. That is, the total losses depend on the voltage (V) and current (I) expressed in volt-amperes (VA) and not on the load power factor (PF). This is why a transformer rating may be expressed in VA or kVA instead of W or kW.

Let’s explain in more detail why a transformer is rated in VA and not kW.

When manufacturers design a transformer, they do not know what kind of load the transformer will be connected to, e.g. they are not sure of the exact application of the transformer in different scenarios. Loads can be resistive (R), inductive (L), capacitive (C) or mixed (R, L and C). This means that there will be different power factors (PF) on the secondary (load) side of the different types of connected loads, depending also on R, L and C. Thus, the rating of the transformer is expressed in volts-amperes (VA) instead of watts (W) in the case of the Transformer.

Let’s use a solved example to clear the transformer rating in VA instead of W. As long as the magnitude of the current/voltage is the same, the transformer losses remain the same. Regardless of the power factor of the load current/voltage.

Example:

Assume that for a single-phase step-up transformer

Transformer rating in kVA = 11kVA

Primary voltage = 110 V

Primary current = 100 A

Secondary voltage = 220V

Secondary current = 50 A.

Secondary equivalent resistance = 5 Ω

Iron loss = 30 W

In the first case, if we connect a resistive load to the secondary of the transformer at unit power factor Φ = 1, then the total losses in the transformer are copper losses + iron losses, i.e.: I²R + iron losses

Put value: (50 2 x 5 ) + 30W = 12.53kW

i.e. the losses in the primary and secondary transmission remain the same. (See also example of secondary losses below)

The transformer output will be: P = V x I x Cosϕ

Again, putting in the secondary values (if we put in the same values for the primary) P = 220 x 50 x 1 = 11kW.

Now the transformer rating: kVA = Volt-amperes ÷ 1000

kVA = (220 x 50) ÷ 1000 = 11 kVA.

Now, in the second case, a capacitive or inductive load is connected to the secondary of the transformer with power factor Φ = 0.6. Again, the total losses in the transformer will be copper losses + iron losses, i.e.: I²R + iron losses

Put value: (50 2 x 5 ) + 30W = 12.53kW

Thus it is proved that the primary and secondary losses are the same. But the output of the transformer will be: P = V x I x Cos Φ

Again putting in the secondary values (if we put in the primary, the values are the same) P = 220 x 50 x 0.6 = 6.6kW.

Now the rating of the transformer: kVA = Volt-amperes ÷ 1000

kVA = 220 x 50 ÷ (1000) = 11 kVA

This means that the 11kVA transformer rating means that it can handle 11kVA. it’s our turn to convert 11kVA to 11kW (which we can do by increasing the power factor to 1 for purely resistive loads), which is unpredictable, and it’s difficult to even get a power factor that will be a different value in the case of inductive and capacitive loads.

It is clear from the above example that the transformers have the same rating (11kVA) but different outputs (11kW and 6.6kW), this is due to the fact that the power factor values are different when connecting different types of loads, which is unpredictable for the transformer manufacturer the losses are the same in both cases.

These are therefore the exact reasons why transformer ratings are given in kVA instead of kW.