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What type of transformers does the NEC 2017 require a 1 hour fire seperation barrier?

A 1 hour fire separation barrier is required for any dry type transformer over 112.5 kVA with less than class 155C insulation. As a practical matter, few dry type transformers in this size range utilize insulation systems below 155C. The most common insulation type is 220C.

The 1 hour fire separation requirement would also apply to liquid-filled transformers specified in Article 450 based on the type of liquid used and other ratings.

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