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Transformer Calculation Formulas
This free online transformer calculator allows you to calculate the full load current in the primary and secondary transformer windings. The inputs are the transformer kVA (power), and the voltage in the primary and secondary windings. You can use this calculator for both single-phase and 3 phase transformer calculations, for calculating your turns ratio (windings ratio), as well as whether it is a step down transformer or a step up transformer.
Note that all calculations below are for an ideal transformer, i.e. where the power factor is equal to 1.
Number of Phases
You can choose from a 3 phase transformer or a single-phase transformer. Note that this will affect the resulting calculation, as different equations are used. The formula for both three-phase and single phase transformers is given below.
3 phase transformer current is equal to:
I3ph = P3ph / (√3 × V3ph)
Where:
I3ph [kA]= the current flowing through the windingsP3ph [kVA]= the rated 3 phase power of the transformerV3ph [kV]= the 3 phase voltage at the windings
And single phase transformer current is equal to:
I = P / V
Where:
I [kA]= the current flowing through the windingsP [kVA]= the rated single phase power of the transformerV [kV]= the single phase voltage at the windings
Note that both these formulas apply to both the primary and secondary sides respectively, but not combined. Do not mix the voltage/current on the primary side with the voltage/current on the secondary side.
Transformer Rating
The transformer rating is the rated power of the transformer. This is usually given in kVA, but can equally be given in VA or MVA.
Primary Transformer Voltage
The primary transformer voltage is the voltage in the primary windings of the transformer. This is usually given in kV, but can equivalently be given in V or MV.
Secondary Transformer Voltage
The secondary transformer voltage is the voltage in the secondary windings of the transformer. This is usually given in kV, but can equivalently be given in V or MV.
Primary Full-Load Current
The primary full load current is the current flowing through the primary windings of the transformer. This is usually given in Amperes (A), but can equivalently be given in kA or MA.
For 3 phase transformers, the primary full load current (i.e. the current in the primary windings) is equal to:
Ip = P / (√3 × Vp)
Where
Ip [kA]= the current flowing through the primary windingsP [kVA]= the rated 3 phase power of the transformerVp [kV]= the 3 phase voltage at the primary windings
For single phase transformers, the primary full load current (i.e. the current in the primary windings) is equal to:
Ip = P / Vp
Where
Ip [kA]= the current flowing through the primary windingsP [kVA]= the rated single-phase power of the transformerVp [kV]= the single-phase voltage at the primary windings
Secondary Full-Load Current
The secondary full load current is the current flowing through the secondary windings of the transformer. This is usually given in Amperes (A), but can equivalently be given in kA or MA.
For 3 phase transformers, the secondary full load current (i.e. the current in the secondary windings) is equal to:
Is = P / (√3 × Vs)
Where
Is [kA]= the current flowing through the secondary windingsP [kVA]= the rated 3 phase power of the transformerVs [kV]= the 3 phase voltage at the secondary windings
For single phase transformers, the secondary full load current (i.e. the current in the secondary windings) is equal to:
Is = P / Vs
Where
Is [kA]= the current flowing through the secondary windingsP [kVA]= the rated single phase power of the transformerVs [kV]= the single-phase voltage at the secondary windings
Transformer Turns Ratio
The transformer turns ratio (also known as the transformer windings ratio) represents the ratio between the primary and secondary windings of a transformer. This is important as it is directly proportional to the amount of voltage that will be stepped down or stepped up between the primary and secondary windings.
The formula for the transformer turns ratio is:
n = Vp / Vs = Np / Ns
Where
n= the transformer turns ratioVp= the voltage at the primary windingsVs= the voltage at the secondary windingsNp= the number of windings on the primary side of the transformerNs= the number of windings of the secondary side of the transformer
Transformer Type
The type of transformer can either be a step-down transformer or a step-up transformer.
A step down transformer converts the high voltage and low current from the primary windings of the transformer to a low voltage and high current value in the secondary windings of the transformer. Hence a step-down transformer will have a primary transformer voltage that is greater than its secondary transformer voltage.
A step up transformer converts the low voltage and high current from the primary windings of the transformer to a high voltage and low current value in the secondary windings of the transformer. Hence a step-up transformer will have a primary transformer voltage that is lower than its secondary transformer voltage.
