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    Permitted uses of NM cables

    Permitted uses of NM cables

    New postby bigdog on Fri Jan 02, 2015 11:38 am

    Hi Jerry
    My question relates to NM cable and allowed uses. Came across an exterior service panel at an old (40's) house where all the house NM wiring had been run in sch 40 conduit out from under the house and into the panel. I know NM can not be used outside of the house but the NEC only states it has to be used in "normally dry conditions" I know this work wasn't permitted as I checked with the local building department.
    1) I guess since its not buried the inside of this conduit could be considered normally dry? Is there leeway with the AHJ?
    2 I always see NM run under old houses and sometimes its even fastened to the floor joist. Is in a crawlspace considered normally dry?

    Just a little confused because I know it can't be used in a whip to a condensing unit. Can you point to any sections in the NEC that might clarify. Thanks

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    Joined: Thu Apr 12, 2012 10:54 pm

    Re: Permitted uses of NM cables

    New postby Jerry Peck - Codeman on Fri Jan 02, 2015 11:08 pm


    This should answer your question quite well:
    - From the 2008 NEC: (bold and underlining are mine)

    - - 300.9 Raceways in Wet Locations Above Grade.
    - - - Where raceways are installed in wet locations abovegrade, the interior of these raceways shall be considered to be a wet location. Insulated conductors and cables installed in raceways in wet locations abovegrade shall comply with 310.8(C).

    Outdoors, and the location you described, is a "wet location", thus the interior of the raceway is considered a "wet location" and the conductors in the raceway are required to be suitable for use in wet locations.

    - - 310.8 Locations.
    - - - (A) Dry Locations. Insulated conductors and cables used in dry locations shall be any of the types identified in this Code.
    - - - (B) Dry and Damp Locations. Insulated conductors and cables used in dry and damp locations shall be Types FEP, FEPB, MTW, PFA, RHH, RHW, RHW-2, SA, THHN, THW, THW-2, THHW, THWN, THWN-2, TW, XHH, XHHW, XHHW-2, Z, or ZW.
    - - - (C) Wet Locations. Insulated conductors and cables used in wet locations shall comply with one of the following:
    - - - - (1) Be moisture-impervious metal-sheathed
    - - - - (2) Be types MTW, RHW, RHW-2, TW, THW, THW-2, THHW, THWN, THWN-2, XHHW, XHHW-2, ZW
    - - - - (3) Be of a type listed for use in wet locations

    - - - (D) Locations Exposed to Direct Sunlight. Insulated conductors or cables used where exposed to direct rays of the sun shall comply with (D)(1) or (D)(2):
    - - - - (1) Conductors and cables shall be listed, or listed and marked, as being sunlight resistant
    - - - - (2) Conductors and cables shall be covered with insulating material, such as tape or sleeving, that is listed, or listed and marked, as being sunlight resistant

    NM cable does not meet the requirements for use in wet locations - thus the NM cable needs to be removed and replaced with conductors suitable for use in wet locations.

    One option is to install a raintight (NEMA 3R enclosure) wire gutter where the NM cables/conduit comes through the wall, terminate the NM cable to THHW/THWN/etc in that wire gutter, then run the THHW/THWN/etc in conduit from that wire gutter to the panel where it now goes (saves more holes in that exterior panel if they reuse the existing holes).

    If they were to run THHW/THWN/etc back to where the NM cable goes to, they would need to replace the NM cable runs with conduit runs, and they would need the same number of conduits as NM cables to avoid derating (depending on various factors, the derating may work out for 4 current carrying conductors in each conduit - basically one conduit for two NM cables - but - it might not work out and then they would have derating issues to deal with and resolve (increase wire size may be one solution, etc).
    Jerry Peck - CodeMan

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