The maximum rating of the primary overcurrent protection device in a 300 KVA, 480-240-volt transformer system delivering 100 amps to 3Ø loads is:

To determine the maximum rating of the primary overcurrent protection device for a transformer system, it is essential to understand how to calculate the correct size based on the transformer's capacity and voltage levels.

In this case, the transformer has a capacity of 300 KVA and operates at a primary voltage of 480 volts. When calculating the rated secondary current, you can use the formula:

[ \text{I} = \frac{\text{KVA} \times 1000}{\text{Voltage}} ]

For the secondary side:

[ \text{I} = \frac{300 \times 1000}{240} = 1250 , \text{amps} ]

However, we are focused on the primary side. For the primary side calculations, we use the full capacity of the transformer:

[ \text{I}_{primary} = \frac{300 \times 1000}{480} \approx 625 , \text{amps} ]

Now, to size the overcurrent protection device, we take into account the National Electrical Code (NEC) guidelines. According to NEC, the maximum rating of the primary overcurrent protection for a transformer is typically set at 125% of