Transformer Primary Fuse Size Calculator

Introduction: The Transformer Primary Fuse Size Calculator is a practical tool for electrical professionals and individuals responsible for specifying fuses for transformers. This calculator simplifies the process of determining the appropriate primary fuse size based on the transformer’s primary voltage and VA (Volt-Ampere) rating, ensuring safety and reliability in electrical systems.

Formula: The recommended primary fuse size is calculated using the formula: (VA Rating / Primary Voltage) * 1.56. The result is rounded up to the nearest whole number to provide a practical fuse size.

How to Use:

  1. Enter the primary voltage of the transformer in volts.
  2. Input the VA rating of the transformer.
  3. Click the “Calculate” button to obtain the recommended primary fuse size in amps.

Example: If the primary voltage is 240 volts and the transformer has a VA rating of 1000, enter these values into the calculator, click “Calculate,” and the result will provide the recommended primary fuse size in amps.


  1. Q: Why is it important to use the correct primary fuse size for a transformer? A: Using the correct fuse size protects the transformer and ensures safe and reliable operation in electrical systems.
  2. Q: Can I use this calculator for different types of transformers? A: This calculator is designed for general use with transformers. Ensure that the specific transformer’s characteristics are considered for accurate results.
  3. Q: What is the significance of the 1.56 multiplier in the formula? A: The multiplier accounts for inrush current and other factors, ensuring the fuse size provides adequate protection.
  4. Q: Should I round the calculated fuse size up or down? A: It’s recommended to round up to the nearest whole number to ensure sufficient protection.
  5. Q: Can I use a fuse with a higher amp rating than the calculated value? A: While it’s crucial not to use a fuse with a lower rating, using a slightly higher-rated fuse may be acceptable in some cases. Consult electrical codes and standards.

Conclusion: The Transformer Primary Fuse Size Calculator is a valuable tool for ensuring the safe and efficient operation of transformers in electrical systems. Use this calculator to determine the recommended primary fuse size, providing peace of mind in electrical installations and enhancing system reliability.

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