# Transformer Sizing Calculator (3 phase, kVA & Windings Calculations)

## 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 windings
• `P3ph [kVA]` = the rated 3 phase power of the transformer
• `V3ph [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 windings
• `P [kVA]` = the rated single phase power of the transformer
• `V [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.

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 windings
• `P [kVA]` = the rated 3 phase power of the transformer
• `Vp [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 windings
• `P [kVA]` = the rated single-phase power of the transformer
• `Vp [kV]` = the single-phase voltage at the primary windings

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 windings
• `P [kVA]` = the rated 3 phase power of the transformer
• `Vs [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 windings
• `P [kVA]` = the rated single phase power of the transformer
• `Vs [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 ratio
• `Vp` = the voltage at the primary windings
• `Vs` = the voltage at the secondary windings
• `Np` = the number of windings on the primary side of the transformer
• `Ns` = 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. 