The door sill or curb around a liquid-cooled transformer installed to protect a transformer vault doorway shall not be less than 4 inches high.
This minimum requirement is mentioned in the article 430.43 issued by National Electrical Code. It provides Effluent limitations guidelines representing the degree of effluent reduction attainable by the application of the best conventional pollutant control technology (BCT).
The article is contained within subpart D - Dissolving Sulfite Subcategory, which is contained in Part 430 - the pulp, paper and paperboard point source category. The title is 40 - Protection of Environment.
Therefore, the door sill or curb around a liquid-cooled transformer installed to protect a transformer vault doorway shall not be less than an approved height sufficient to confine the oil from the largest transformer in the vault, and shall in no case be less than 4 inches height
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