NEC Quiz: Article 310, Part 4 Answers

by Mark Lamendola

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  1. That it does not exist. Article 310 jumps from 310.15 to 315.60. In between there, you find the Table 310.15x tables.
     

  2. Actually, you can use that column. But only if every component (including connectors) is rated for a 90 degree application [110.14(C)]. Connectors typically don't come rated for 90 degree usage. So to get a connector of adequate ampacity, you must use the larger conductors in the 60 DegrC column. If you use a connector that fits the conductor in the 90 DegrC column, the connector will be too small to handle the expected current.

    But what if a portion of the cable runs through a space thatís at 85 degrees because steam pipes run in that space? You have no splices or connectors in that portion, so there's no reason you can't use the 90 column for the conductor. To understand this better, read Annex D, Example D3(a).
     

  3. [Table 310.15(B)(16)]. No. The limit for this table is three conductors. See Annex B for information on how to solve this problem.

  4. Each Table has its particular scope defined, and you can read that in the header space above each table. Regarding these two tables, Table 310.15(B)(16) is for three conductors and Table 310.15(B)(17) is for a single conductor. Be sure to always read the header carefully before selecting a particular table. It's also a good idea to mark the page (e.g., with a stick on note or clip-on bookmark) so you don't inadvertently use the wrong table.

  5. I assume you're looking at Table 310.15(B)(16). Your first mistake is you are using the 90 degree column, just because THHN is listed there. You have to use the 60 degree column, unless you aren't going to have any connectors in the run where these conductors are going.

    This means the table goes up to only 555A. You could do the calculations described in Annex B to determine just how ungainly and huge a conductor you would need. But, it's probably not available and you probably don't want to hear the electricians complain about how bad the cable pull was. Not that you'd have the connector crimpers and dies readily available, either....

    So what you do is run parallel conductors. You can run two 750 MCMs in parallel for each phase and solve this problem nicely. This size conductor is big, but run often enough that you won't need exotic equipment to install it. Nor will you need special panel terminations and other "gotchas" that an "off table" installation would entail.