Determining the size of a **transformer** & the **voltage** drop caused by starting a large-sized motor includes various factors and necessitates specific details about the motor & the system.

The following is a general summary of all the stages involved

**Transformer Sizing Considerations**

The **transformer**‘s size should allow it to start a motor with a higher current without experiencing an excessive voltage drop. It is typical practice to size the transformer to withstand a voltage drop of no more than 5 to 10 percent while starting a motor.

Following are the steps to calculate the transformer rating:

Full Load Current of a Transformer = KVA/(√3 X Secondary Voltage)

**I _{LX}=KVA/(1.732xV_{2})**

Short Circuit Current of a Transformer = Transformer Full Load Current / Impedance

**I _{sc}=I_{LX}/z**

Analysing Motor Parameters

- Identify the motor’s rated power (in kW or HP).
- Find the rated voltage (in V) for the motor.
- Find the motor’s power factor (typically listed on the nameplate or datasheet of the motor).
- Take note of the starting method used by the motor (Star-Delta, Direct-on-Line, Soft Starter, VFD, etc.).

Motor Full Load Current = (Motor Size x 1000)/( √3 x Motor Voltage x Power Factor)

**I _{LM}=(KW x 1000)/(1.732 x V_{M} x P.F)**

**Voltage Drop of a Transformer (in Secondary)**

The following formula can be used to estimate the voltage drop of a transformer produced by the inrush current of a motor & the maximum KVA rating of the transformer:

**V _{D}=I_{RS}M/Q_{1}**

Where,

V_{D} = Voltage Drop

I_{RS}M = Inrush Current

Q_{1}= Maximum KVA of Transformer at Short Circuit

**Motor Code**

An electric motor’s type and features are identified using motor codes. They are often made up of a series of letters & numbers, each representing a distinct aspect of information.

Motor codes are a useful tool in identifying & selecting the appropriate electric motor for a given application.