What is the maximum standard trip setting for a circuit breaker protecting a 1500kVA transformer with a 3.2% impedance?

To determine the maximum standard trip setting for a circuit breaker protecting a 1500kVA transformer with a 3.2% impedance, it's essential to first calculate the full-load current of the transformer. The full-load current can be calculated using the formula:

[ I = \frac{S}{\sqrt{3} \times V} ]

where ( S ) is the apparent power in kVA and ( V ) is the line voltage.

For a 1500kVA transformer, under a standard line-to-line voltage of 480V (common in industrial applications), the calculation would be:

[ I_{FL} = \frac{1500}{\sqrt{3} \times 480} = \frac{1500}{831.4} \approx 1.80 A ]

Next, you calculate the maximum standard trip setting using the formula:

[ \text{Maximum Trip Setting} = \text{Full Load Current} \times \text{Multiplier (based on Impedance)} ]

For transformers, it's common to use a factor based on the impedance percentage. With the given 3.2% impedance, the trip setting is typically around 125% of the full-load current to account