What type of wire in conduit?

New_Mexico_WillNew_Mexico_Will Registered Users Posts: 107 ✭✭
I need to run wire from my main panel in my powerhouse to a sub panel in my cabin.  It will be a 50 amp breaker, so I'm going to use 6/2.  What kind of wire should I use?  I'm getting a lot of conflicting information online, and NEC doesn't seem to be clear on this topic.  I'd like to use NM-B since it's the cheapest.  It will be in 2" PVC conduit 24" underground and will be the only wire in it.  Is that ok?  Would UF be better?  Or do I need to run individual THHN wire$?  Thanks in advance.  Great forum you guys have here.


  • Wheelman55Wheelman55 Registered Users Posts: 140 ✭✭✭
    My electrician gurus have always said individual wires inside conduit.  
    Building Off-Grid in Terlingua, TX
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  • roadglideroadglide Registered Users Posts: 6 ✭✭
    my vote  THHN.,.,just bought some today at Home D...for .56 cents per foot...8 awg
  • BB.BB. Super Moderators, Administrators Posts: 31,685 admin
    The NM-B is going to be a pain to pull--If you have a longer run and/or some corners. THHN is going to be much easier to pull.

    If you don't get colored wire (black/red/white/green)--At least buy some colored electrician tape so you can wrap the last 6 inches (or whatever code is) with the correct colors.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mcgivormcgivor Solar Expert Posts: 3,817 ✭✭✭✭✭✭
    NMB is not permitted to be installed in conduit 
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  • Raj174Raj174 Solar Expert Posts: 794 ✭✭✭✭
    edited September 2020 #6
    Looks like 6/2UF-B direct burial is slightly more $ than the three THHN wires needed but will save the expense and installation of the PVC. 

    4200W PV, MNE175DR-TR, MN Classic 150, Outback Radian GS4048A, Mate3, 51.2V 360AH nominal LiFePO4, Kohler Pro 5.2E genset.
  • RCinFLARCinFLA Solar Expert Posts: 1,351 ✭✭✭
    THHN if inside conduit.  It's all about heat dissipation for given power loss per foot due to wire resistance..  NEC for 14-10 gauge is in range of 2 watts per foot of heating for gauge current limit ratings.  The more blankets of insulation the lower the current has to be to keep the temp rise down.  The more current carrying wires in a given conduit the lower the max current needs to be to keep the total heat generated per foot down within the confines of the conduit..

    Thick SOOH cord used for portable generator hookup cords is a good example of current rating reduction required for given gauge size. SOOH has a lot of multiple layers of insulation blankets that reduces heat dissipation to outside surface of cable

    Heat degrades the insulation integity over time, making it brittle and prone to cracking.  THHN has 90 degsC insulation rating.  THWN is a bit cheaper but only 75 degC insulation temp rating.  They will have corresponding max amperage rating based on maximum insulation temp rating.   Most all you can buy these days is 90 degs C rated insulation as better insulation is not as expensive as thicker copper to keep temp down.

  • New_Mexico_WillNew_Mexico_Will Registered Users Posts: 107 ✭✭
    Many thanks for all of the detailed responses.  It clarified a lot for me.  I'll go ahead and use individual strands of THHN.
  • DennisMDennisM Registered Users Posts: 4
    Pretty sure to be in compliance with code, you still need to run a ground wire in the conduit beside your L1 and L2 wires.
  • New_Mexico_WillNew_Mexico_Will Registered Users Posts: 107 ✭✭
    Yes, but if I interpret the code correctly,  the sub panel must also have its own ground rod.
  • DennisMDennisM Registered Users Posts: 4
  • New_Mexico_WillNew_Mexico_Will Registered Users Posts: 107 ✭✭
    Thanks for the clarification Dennis 👍
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