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Thread: Using MTW-THHN-THWN-2 for direct burial

  1. #1
    Join Date
    Aug 2003

    Using MTW-THHN-THWN-2 for direct burial

    Can wire marked MTW-THHN-THWN-2 be used for the grounding conductor with URD wire from the utility meter/disconnect to the panel?

  2. #2
    Join Date
    Sep 2004

    Re: Using MTW-THHN-THWN-2 for direct burial

    Normally you would not use these wire types for direct burial.

    However, if it is okay to use a direct-buried bare copper wire in your situation, (which I assume it is). - then what would be wrong with using a green-insulated wire. If the dirt ruins the insulation & the insulation falls off, you then have a bare copper wire.

    What do others think?

  3. #3
    Join Date
    Mar 2005

    Re: Using MTW-THHN-THWN-2 for direct burial

    Bare copper wire can only be used in soils that will not corrode it. Some soils have a source of sulfur and sulfur metabolizing bacteria that generate sulfuric acid. I have not lived in such an area where the topsoil was affected but there are some well waters here in Ohio that have this problem. In these instances all water heater warranties are void. If you want metal plumbing to not be eaten you need an acid neutralizer which is a tank that is filled with crushed marble and your well water goes through it.

    You can also have the same problem on farms because of bacteria that metabolize urea to form carbon dioxide and ammonia. Ammonia readily corrodes copper and steel hence the requirement for insulated equipment grounding conductors and neutrals on farms.

    However. the bare copper wire might actually be a good idea under other circumstances where procuring a good ground is difficult. To be efrective as a supplementary grounding electrode the raceway that supplies the underground wiring needs to be nonmetallic.

    [ April 17, 2005, 03:38 AM: Message edited by: mc5w ]

  4. #4
    Join Date
    Dec 2004

    Re: Using MTW-THHN-THWN-2 for direct burial

    You can only use it for direct burial if it is listed for Direct burial. THWN is not listed for direct burial. It is listed for damp area locations. Hope this helps alittle.
    Chris Hill, EE,ME,GA-CR
    Power Factor Correction

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