Fusing Transformer Calculator

| Added in Physics

What is Fusing Transformer and Why Should You Care?

Fusing transformers properly is essential for protecting them from overcurrent damage. The NEC (National Electrical Code) provides guidelines for maximum overcurrent protection based on transformer current ratings.

Understanding these calculations ensures your transformers are protected while avoiding nuisance tripping during normal operation.

How to Calculate Fusing Transformer

The formula for maximum fusing amps is:

[\text{Fusing Amps} = \text{Total Amperage} \times \text{Overcurrent Factor}]

Overcurrent Protection Factors:

  • 9+ amps: 125% (factor of 1.25)
  • 2-8.99 amps: 167% (factor of 1.67)
  • Less than 2 amps: 500% (factor of 5.00)

Calculation Example

Given:

  • Total Amperage: 5 amps
  • Current Range: 2-8.99 amps (factor = 1.67)

Calculation:

[\text{Fusing Amps} = 5 \times 1.67 = 8.35 \text{ amps}]

For a 5-amp transformer, use a fuse rated up to 8.35 amps.

Overcurrent Protection Reference

Total Amperage (A) Factor Maximum Fusing (A)
1.5 5.00 7.50
5.0 1.67 8.35
10.0 1.25 12.50
20.0 1.25 25.00

Frequently Asked Questions

Smaller transformers have higher inrush currents relative to their rating. The higher protection factor prevents nuisance tripping during normal startup while still providing overcurrent protection.

The NEC specifies 125 percent for transformers 9 amps and above, 167 percent for 2-8.99 amps, and 500 percent for transformers under 2 amps on the primary side.

Proper fusing prevents transformer damage from short circuits and overloads by interrupting excessive current before it can cause overheating or fire.

Not necessarily. Use the smallest fuse that does not trip during normal operation including inrush current. This provides better protection while meeting code requirements.