Q. On a 3-phase, 4-wire, 480/277V service, a question has arisen as to the required rating of the service disconnect. The calculated load is 780A. The fusible disconnecting means is rated at 1,000A, based on the maximum fuse size that can be installed. What is the minimum rating of the service disconnect permitted for this application?

  1. 800A
  2. 900A
  3. 1,000A
  4. All of the above

Answer: C

As per the second paragraph of 230.95, “The rating of the service disconnect shall be considered to be the rating of the largest fuse that can be installed or the highest continuous current trip setting for which the actual overcurrent device installed in a circuit breaker is rated or can be adjusted.”

The calculated value in this example would have permitted an 800A rated disconnecting means, however, because of the possibility of a 1,000A rated fuse being installed in this disconnect, the rating of this disconnect shall be a minimum of 1,000A.

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.