RES Technical Corner by Brett Eliasz, PE

Technical Corner

For the article this month we will look at an approved method to determine clearances for oil filled transformers from a building and other transformers using FM Global requirements and the first letter of the 4-letter cooling class of the transformer. Note: transformers to be installed outside.

The following table shows the required clearances:

1. Determine if you have a FM approved transformer by getting the nameplate data and contacting the transformer manufacturer. (May be difficult-and forced to assume non-approved transformer).

2. Determine what type of oil you have by looking at the 4-letter cooling class code on the nameplate-

First Letterusually an “O” indicating an oil flash point less than 300 C-not a FM approved oil-will catch on fire faster. Otherwise, a “K” indicating an oil flash point of greater than 300 C-FM approved oil-will catch on fire slower.

3. Determine the gallons of oil in the transformer by looking at the nameplate-

4. Determine if the building exterior walls are Non-combustible, combustible or 2 hour fire rated: A. By asking an Architect (recommended) OR B. Use Informative Annex E of the 2017 NEC OR C. See footnote #1 in the table above.

The following table shows the required clearances between oil filled transformers:

Hopefully this article finds you well and can be used as a reference for your project needs. If anyone would like to contribute to the Rochester Engineer and add an article or would like to request information on a specific topic (not limited to Electrical) just email me at beliasz@bergmannpc.com. As always, any comments are appreciated…! Thank you for reading.

Brett Eliasz, P.E., LEED AP BD+C , RES Director