**THREE PHASE POWER / VOLTAGE / CURRENT RELATIONSHIPS**

The standard method of specifying the parameters of three phase transformers is

1. Line to Line Voltages

2. Line Currents

3. Total Power Rating

However often other information is provided

4. Line to Neutral Voltages

5. Total Current

6. Power Rating per Phase

The following mathematical formulae need to be applied to convert between the various formats for specifying transformers.

**Delta connected systems**

Only the Line to line voltage is defined

*V*_{LL}The Line current (

*I*) or the Total current (_{L}*I*) may be defined_{T}The Power Rating may be defined either per phase (

*P*) or total (_{Ph}*P*)_{R}The Power Rating is calculated from the voltages and currents as

**Star connected systems**

Either the Line to line voltage (

*V*) or the Line to Neutral voltage (_{LL}*V*) may be defined._{LN}The relationship between these voltages is

The Line (or Phase) current (

*I*) or the Total current (_{L}*I*) may be defined_{T}The Power Rating may be defined either per phase (

*P*) or total (_{Ph}*P*)_{R}The power rating is calculated from the voltages and currents as

Or

Note: Some three phase transformers are used to supply three single phase systems. In instances where the loads on the different phases are not equal the total power rating (

*P*) should be calculated using the power rating or line current of the largest load._{R}