Which formula determines the maximum fuse size for a transformer with a 31.25-ampere full load current rating using primary only protection?

The formula that determines the maximum fuse size for a transformer using primary-only protection is based on the concept of overcurrent protective devices (OCPD), which are critical for preventing equipment damage due to overload or fault conditions. In this context, the calculation uses the full load current (FLA) and applies a multiplier to ensure that the OCPD can handle the load while also providing adequate protection.

The correct formula states that the maximum size of the overcurrent protective device should be less than or equal to the full load current multiplied by 1.25. This ensures that the OCPD is capable of protecting the transformer by allowing for some margin above the normal operating current of 31.25 amperes. The 1.25 factor is an industry-standard guideline that accommodates temporary inrush currents and other operational variances that may occur, while still protecting against overcurrent situations that could damage the transformer.

Using this formula, the maximum OCPD would be calculated as follows:

OCPD ≤ 31.25 A × 1.25 = 39.0625 A.

Therefore, the OCPD should be able to handle a maximum current of approximately 39.06 amps, which means in practice, you would