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Six steps to properly protect step-up transformers

Step 1: Get proper documentation for your transformer

Once you select the transformer, obtain documentation from the transformer manufacturer including the inrush data. It is essential to work with the transformer manufacturer to get the inrush current rating information as it is not included on the nameplate, nor can you perform the needed calculations without this information. 

Step 2: Refer to the applicable NEC code

Size the primary overcurrent protective device in accordance with NEC Article 450 requirements.

   Properly protect step-up transformers

  Designing step-up transformer applications

  Understanding transformer inrush current

 

Step 3: Protect for maximum inrush current

Choose an overcurrent protective device that’ rated for riding through the maximum inrush current the transformer can pull. 

Guide to overcurrent protective device ratings

Overcurrent protective device   Applicable listing  Make rating    Break rating  
Molded case circuit breakers  UL 489 6x 6x
Safety switch  UL 67 6x 6x
Bolted pressure switch (including Pringle® switch)  UL 977 12x 6x
Power circuit breakers  UL 1066 12x 12x
High-pressure contact fusible switch  UL 977 12x 12x
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Step 4: Multiply

Multiply the amperage frame rating of the primary overcurrent protective device by the lower make or break rating value. 

Step 5: Compare

Compare the value in Step 4 with the inrush current value in Step 1. If Step 5 is greater than Step 1, this OCPD is adequate for the application.

Step 6: Confirm all needed codes are met

Ensure the use of the devices meets all other applicable national and local code requirements.

Understand what it takes to protect step-up transformers

Our white paper provides technical recommendations to advance safety and understand the related electrical codes to adequately size overcurrent protective devices.

Download the whitepaper

  • Communicate with an expert

    Do you have additional questions or want to connect with someone well-versed in how to protect step-up transformers? We have experts ready to answer any questions you may have.
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