What is the let-through short circuit current of a 500kVA, 120/208v three-phase transformer?

To find the let-through short circuit current of a transformer, it's important to understand the relationship between the transformer’s capacity (in kVA), the voltage levels, and the resulting current during a short circuit condition.

For a 500 kVA, 120/208V three-phase transformer, we begin by determining the full load current using the formula:

[

I = \frac{P}{\sqrt{3} \times V}

]

Where:

• P is the power in VA (500 kVA = 500,000 VA)

• V is the voltage (208V in a three-phase system)

First, substituting in the values:

[

I = \frac{500,000}{\sqrt{3} \times 208} \approx 1,384.1 A

]

Now, to calculate the let-through short circuit current, we typically use a short circuit ratio or manufacturer data that references the transformer’s impedance. The let-through current represents the maximum current that will flow when a fault occurs before protective devices operate.

The significant factor to determine the short circuit current can be simplified using the formula:

[

I_{sc} = \frac{I_{full load}}{Z}