How to Calculate the Rating of Single Phase & Three Phase Transformers in kVA
We know that a transformer is always rated in kVA. Below are the two simple formulas which can be used to find and calculate the rating of Single Phase and Three Phase Transformers.
In any case, both the voltage and currents should be form an individual side either primary or secondary respectively ( e.g. Primary voltage x primary current or secondary voltage x secondary current). To property size a transformer for a required home appliance, you should select the supply voltage and secondary load current in amperes.
Sizing a Single Phase Transformer
Rating of Single Phase Transformer:
P = V x I
Where:
V = Primary or Secondary Voltage
I = Primary or Secondary Current
P = Power of transformer in VA (Volt-Amps)
Rating of a single phase transformer in kVA
kVA= (V x I) / 1000
Example:
Suppose a 1-phase transformer having the secondary voltage and current of 240V and 62.5A respectively. Calculate the size of the single-phase transformer.
Solution:
Secondary Voltage: 240V
Load Current: 62.5A
Transformer Rating = P = V x I
Putting the values:
P = 240V x 62.5A
P = 15000 VA = 15 kVA
Sizing a Three Phase Transformer
Rating of a Three Phase Transformer:
P = √3 x V x I
Where:
V = Primary or Secondary Voltage
I = Primary or Secondary Current
√3 = 1.732
P = Power of transformer in VA (Volt-Amps)
Rating of a Three phase transformer in kVA
kVA = (√3 x V x I) / 1000
Example:
A 3-phase transformer having the primary voltage and current of 7200V and 4A respectively. Calculate the size of the three phase transformer.
Solution:
Primary Voltage: 7200V
Primary Current: 4A
Transformer Rating = P = √3 x V x I
Putting the values:
P = 1.732 x 7200V x 4A
P = 49,881 VA ≈ 50 kVA

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