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Choosing the Right Fuses: Transformers 101

April 23, 2024

Transformers 101: Choosing the Right Fuses

Types of Fuses

Fuses come with a variety of options, each type designed for a specific application. While these fuses may share the same current rating, they have different clearing curves which means some can handle large inrush currents for up to 10 seconds, while some blow right away.

Selection Criteria

When Selecting a fuse, you must consider the following:
  1. Current (Amps)
  2. Voltage
  3. Available Fault Current (Generally measured in kA)
  4. Application (What are you protecting?)

Choosing the Right Size for a Transformer

Determine where you will fuse primary only or primary and secondary and calculate the primary current (Amps).

Use the table below to guide you further:

Primary Fuse

Primary AmpsMax Primary Fuse % Rating
9 or more125*
2 to less than 9167
less than 2300

Primary & Secondary Fuse

Secondary AmpsMax Primary Fuse % Rating
9 or more250125*
less than 9250167
*If 125% does not correspond to a standard ampere rating, the next higher standard rating shall be permitted.

Placement of Fuses

  • Primary Fuse (Input)
    • essential to protect against short circuit and overloads.
  • Secondary Fuse (Output–optional)
    • safeguards against overloading the transformer.

Consider the Starting Current

Primary fuses must be able to handle the high inrush current of a transformer. This can be as much as 30 x calculated current for .01 seconds. A time delay dual-element fuse is recommended.

Check this fuse selection table for control and 3-phase transformer:



Let’s suppose we have a transformer with the following specifications:

  • Transformer Rating: 45 kVA
  • Primary Voltage: 600V (3-phase)
  • Secondary Voltage: 208/120V (3-phase)

Calculate Primary Current:

Primary Current (A) = kVA x 1000 / √3 x Primary Voltage (V)

Primary Current (A) = 45 kVA x 1000 / √3 x 600 (V)

Primary Current (A) ≈ 45000 /  √3 x 600 (V) ≈ 45000/1039.2 ≈ 43.35 A

Choose Fuse Rating (125% of the calculated primary current)

Fuse Rating (A) = 1.25 x Primary Current (A)

Fuse Rating (A) = 1.25 x 43.35 A

Fuse Rating (A) ≈ 1.25 x 43.35 A ≈ 54.19 A

So, we’d select a fuse close to a rating of 54.19 A, which is 60 A.


More Information

Vanco Electrical Supply Ltd.

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