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Thursday, September 29, 2016
Question:
Thank you for keeping the Code Question of the Day going. I enjoy reading it.
2014 NEC 352.20(B) says "PVC conduit larger than metric designator 155 (trade size 6) shall not be used". I am designing an underground duct bank for 24kV feeder cables, 2000A capacity. Originally I was planning on using parallel sets of 3-1/C #500kCM, so 6" PVC conduit was OK. It was suggested to switch to parallel sets of 3-1/C #1000kCM to reduce the number of parallel sets required. However that means I need 8" PVC conduit.
Several manufacturers offer 8" PVC conduit, but the NEC does not allow it. This size limit is also stated in the NEC for other conduits, like RMC, HDPE and RTRC.
Why does the NEC limits these conduits to 6" max.?
Thanks, Raul Dominguez, P.E.
A
Answer:
Hey Raul thanks for your question and the compliment. PCV conduit is required to be
listed as stated in 352.6. UL 651, the product standard for PVC conduit and fittings also limits the size to 6 inch so they are consistent with the rule you shared in 352.20(B). The 6 inch maxim size was added in the 1996 NEC based on the substantiation that listed 6 inch PVC conduit was available, even though the 1993 NEC only provided a minimum size of 1/2 inch in 347-10.
More information on the product listing is available at:
http://productspec.ul.com/document.php?id=DZYR.GuideInfo
Don't forget about the temperature limitations in 352.10(I) and 110.40 that can affect the conductor ampacity, and the challenge of installing and terminating 1000 kcmil conductors.