**A **student of electrical engineering or an electrical engineer working in the industry may have to come across a situation, where he/she have to calculate short circuit current of a transformer (single phase or three phase). And I personally found calculating a short circuit current of a transformer interesting, and of course it is…!

Design engineer, or the engineers working in the protection field they generally came across this situation. So here are your simple three steps to calculate short circuit current of any transformer. Believe me it’s easy, very easy.

## Step 1

Before you start calculating, gather following data.

- KVA/MVA rating of the transformer
- Secondary voltage of the transformer
- % impedance of the transformer (you will find this on the name plate of the transformer).

(By the way, if you want to know what is impedance, you can watch my video on it, just click the link – Resistance Vs Reactance Vs Impedance)

**Step 2**

Now as you have gathered the data, calculate the full load secondary current by using following formula-

**For 3 phase ***transformers*

Vsec = secondary voltage

**Step 3**

Now calculate short circuit current, by using following formula-

Isec = secondary current calculated in step 2.

Z = Impedance

That’s all, you have your short circuit current.

To make things more clear also have a look at following example,

Let’s say we have following data for our 3-phase transformer,

- KVA rating – 1000 kva
- Secondary voltage – 11 Kv OR 11000 Volts
- % impedance – 5%

Now as per step 2, we’ll calculate the full load secondary current-

Let’s put the values,

**Isec = 5.25 amps**

This is our full load secondary current.

Now as per step 3, calculate the short circuit current

**I****sc = ****105 amps**

This is our short circuit current.

*You can see short circuit current is almost 20% greater than the full load normal secondary current.*

Short circuit current also indicates what rating of circuit breaker do we need. In our case, as the short circuit current is 105 amps, of course we need to select circuit breaker with the breaking capacity more than 105 amps.

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(680)

Good explanation