For a 100-kVA, 3-phase transformer, what is the maximum size of the primary overcurrent device?

To determine the maximum size of the primary overcurrent device for a 100-kVA, 3-phase transformer, we begin with the formula to calculate the full-load primary current. This is essential because the primary overcurrent device is usually rated based on the transformer's full-load primary current.

The full-load current for a 3-phase transformer is given by the formula:

[ I_{primary} = \frac{KVA \times 1000}{\sqrt{3} \times V_{primary}} ]

Where:

• KVA is the rating of the transformer (100 kVA in this case).

• ( V_{primary} ) is the line-to-line voltage of the primary side.

Assuming a commonly used primary voltage of 480V, the calculation would look like this:

[ I_{primary} = \frac{100 \times 1000}{\sqrt{3} \times 480} \approx 120.2 A ]

For protection devices, the National Electrical Code (NEC) allows an overcurrent protection device to be sized at 125% of the full-load current for transformers. Thus, we can calculate:

[ Maximum\ Overcurrent\ Protection = I_{primary} \times