Calculate Short Circuit Current of any Transformer in just 3 steps

In this tutorial I’ll explain three simple steps to calculate short circuit current of any transformer. It will also help you in deciding circuit breaker rating. So, let’s start !

Step 1

Get the following details

• KVA/MVA rating of a transformer (for understanding purpose assume it is 100KVA)

• Secondary Voltage (Assue 440 volts)

• % Impedance (You’ll get it from the name plate of transformer, for our example assume 5%)

Step 2

Calculate Full Load Current

For 3 phase transformer, use following formula

[latexpage]

$I_{secondary}={KVA \times 1000\over \sqrt3\times V_{secondary}}$

For 1 phase transformer, use following formula

[latexpage]

$I_{secondary}={KVA \times 1000\over \ V_{secondary}}$

Let’s calculate full load current in our example.

[latexpage]

$I_{secondary}={100 \times 1000\over \sqrt3\times 440}=131.21A$

Step 3

Calculate the short circuit current

We’ll now calculate the actual short circuit current using the following formula.

[latexpage]

$ I_{SC}={I_{Secondary}\over Impedance or Z}$

$ I_{SC}={131.21\over 0.05}$

$ I_{SC}=2624.2A$

So, this is our Short circuit current. This will help you to decide the rating of circuit breaker. In this case you need a breaker who’s fault current breaking capacity is more than 2624.1A or 2.6kA.

You can also calculate primary current of any transformer in just 2 steps, to know more about it, click here.