A question has arisen related to the installation of a new 112.5kVA dry-type transformer, which is equipped with ventilated openings. The transformer will be located in an electrical equipment room, directly across from a 480V switchboard. The rear of the transformer will be pushed tight against a concrete wall. The distance between the front of the transformer and the front of the switchboard will be 48 in. Why was this proposed installation rejected by the AHJ?

  1. It was turned down because a 112.5kVA dry-type transformer must be installed in a vault or a room of fire-rated construction with a 3-hour fire rating.
  2. It was turned down because the distance between the transformer and the switchboard should have been 60 in.
  3. It was turned down because the transformer was pushed tight to the concrete wall, which will reduce air flow needed to cool the transformer, as required by the manufacturer and the NEC.
  4. The AHJ is wrong. There was no need to reject the installation as it is compliant with the requirements of the NEC.

Answer: C

As noted in the second paragraph of 110.13(B), "Electrical equipment provided with ventilating openings shall be installed so that walls or other obstructions do not prevent the free circulation of air through the equipment."

The last paragraph in 450.9 has wording very similar to 110.13(B). It states, "Transformers with ventilating openings shall be installed so that the ventilating openings are not blocked by walls or other obstructions. The required clearances shall be clearly marked on the transformer."

The working space required by 110.26(A)(1) is a non-issue here, as 48 in. between the switchboard and the transformer is clearly NEC-compliant.

Owen is the owner and president of National Code Seminars and the holder of master electrician certifications in 46 states. He can be reached at necexpert@aol.com.