Enter the total number of amps and select the transformer size to determine the maximum overcurrent protection.

Fusing Transformer Formula

The following formula is used to calculate the maximum over protection current a fusing transformer should support.

FS = TA * OF

  • Where FS is the fusing overcurrent protection amps
  • TA is the transformer primary current (amps)
  • OF is the overcurrent protection factor
    • = 125 % for 9 amps or more
    • = 167% for 2-8.99 amps
    • = 500% for less than 2 amps


What is fusing transformer?

Fusing transformer is the process of protecting a transformer with fusing so that it can handle larger amounts of current.

Example Problem

how to calculate overprotection for fusing transformer?

First, determine the total amperage the transformer will see. In this example, the total amperage is 7 amps.

Next, reference the list above for how much of an overprotection factor the transformer should have. In this case, it is 167%

Finally, calculate the fusing transformer amperage required:

FS = TA * OF

FS = 7 * 1.67

FS = 11.69 amps