Transformers 101: Choosing the Right Fuse

    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 Amps Max Primary Fuse % Rating
    9 or more 125*
    2 to less than 9 167
    less than 2 300

     

    Primary & Secondary Fuse

    Secondary Amps Max Primary Fuse % Rating
    Primary Secondary
    9 or more 250 125*
    less than 9 250 167

    *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:

     

    vancoelectrical.com/fuse-table

    Example

    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.