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Thread: NM cable in PVC conduit

  1. #1
    DIY Junior Member Uncle_D's Avatar
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    Sep 2011

    Default NM cable in PVC conduit

    I am building a 10x20 ft produce shed. Will clean and sort produce in there. One end will have a 6x10 ft walk-in cooler. Shed will be 40 feet from the house. I plan on running 6-3 NM-B cable inside PVC conduit. 35ft in crawlspace under house. 40 ft underground to shed. The total run panel to subpanel will be 90ft or less. I will install a 60amp breaker in the house main panel (200amp service panel) to feed this circuit. In the shed I want to install a 12 circuit main lug panel.

    1st circuit 110V 20amp for A/C cooler unit. 2nd circuit 110V 20amp for shed A/C unit. Then two or three 110V utility circuits. Then another 110V circuit for outlets on the outside of the produce shed. I will put in two grounding rods at the shed for the main lug panel grounding. I will keep the ground and neutral in the shed subpanel separate.

    I’ve read that I can NOT install NM cable inside conduit in the ground. I must use separate runs of 6ga wire (for 60amp) for the two hots and neutral. Then I can use 1 strand of 10 ga for the ground. I can purchase the 6-3 NM-B (with ground) for $2.00 per foot. The single strands of 6ga cost 75cents per foot (each strand). Plus 55 cents per foot for the 10ga bare copper ground. That is $2.80 per foot to run the single strands.

    1st question: Why can’t I use the cheaper 6-3 NM-B cable which actually has more insulation than the single strands of 6ga wires? Can I get these single strands in black, red and white?

    2nd question: Any code problem with using the 12 circuit main lug box in the shed?
    I live in Tennessee. In the very rural country. No city inspectors. But, I want to be safe and up to NEC code.

    Thanks for any help.

  2. #2
    Test, Don't Guess! cacher_chick's Avatar
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    Jan 2008
    Land of Cheese


    NM cable is not rated for wet use, which includes anything underground even though it might be in conduit.

    The proper wire will be marked THWN, which is NOT the same insulation as what is used in the NM cable. The "W" is the important part, meaning it is wet rated.

    Most of the wire sold in stores here now carries the duel rating THHN/THWN.
    Last edited by cacher_chick; 09-14-2011 at 03:23 PM.

  3. #3
    Engineer Furd's Avatar
    Join Date
    Jul 2007
    Wet side of Washington State


    As Catcher_chick noted type NM cable is not rated for wet areas so that is OUT for that reason alone. Second reason is that you would find 6-3 w/g to be almost impossible to pull through anything less than 1-1/2 inch conduit and maybe even the 1-1/2 inch conduit. Remember that you can have no more than four 90 degree bends (or equivalent) between "access points" (conduit body or J&P box) in your conduit run.

    Yes, you CAN get the #6 THHN/THWN in multiple colors.

    Just because you live away from the city does NOT mean that you are exempt from permits and/or inspections. Your county or state may be the enforcing political subdivision where you live. Yes, there IS a problem using the MLO (main lugs only) panel in an outbuilding because the NEC requires that all power MUST be able to be turned off with no more than six hand movements. This has been defined as no more than six circuit breakers, either single pole or double pole with tied handles. Since the MLO panel can handle 12 circuits a main breaker is required. If you have already installed the panel you may be able to purchase a 60 ampere breaker AND a "hold-down bracket" and use that for the main breaker.

  4. #4
    DIY Junior Member Uncle_D's Avatar
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    Sep 2011


    That information helps even if it is not what I wanted to hear. Sometimes the truth hurts.
    Okay. So I have to run the separate strands of wire rated for wet conditions even if it is enclosed in gray, PVC conduit.

    I haven't purchased any wire or subpanel yet. Sounds like I could use a main lug subpanel in the shed if I limit it to a maximum of 6 possible circuits. That should be adequate anyway.

    I've checked on the price of direct bury 6-3 w/ground. Ouch! I've also been reading some electrical books. Sounds like I cannot use a 10ga bare ground wire with the 6ga feeds. It reads like I can only go 1 size smaller. 8ga ground. Both hot feeds and neutral are required to be the same ga.

    One local supplier of wire only has one color of #6 wire. He said to wrap the ends of each wire with colored tape to mark them as black/red/white. I don't like doing it that way. Plus, I thought I read that all wire that size and smaller must be the appropriate color the full length.

    Inspectors. I'm 63 y.o. and just haven't had very good experience with building inspectors over the years even when I am super nice and bow and show humility.

    One time I installed a 100amp main service panel (inside) with meter base on outside shared wall with panel on the inside. Installed according to a sheet of instructions I received from my local electric company. After installation I had the electric company inspect and verify. They said "Yep. That is correct." and put their inspection tag on the meter base to show they approved it.

    I then called for the county electrical inspector. He saw the electric company approval tag but still said no. I nicely told him the electric company said it was okay and also showed him the electrical company instruction sheet that matched my install. He said he didn't care what the electric company or their sheet said. He said "Do what I say or I won't pass it." Just one example. I am just gun-shy of inspectors after years of dealing with them. They just seem more intent on showing their authority then helping. Just my nickels worth.

    Anyway, thanks for the info. Now I will check on wire availability and cost along with a smaller main lug panel.


    Last edited by Uncle_D; 09-15-2011 at 07:19 AM.

  5. #5
    Electrical Contractor/Instructor jwelectric's Avatar
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    Jun 2007
    North Carolina


    The information about the use of NM-B or romex if you prefer is correct but the information about the panel might not be correct enough.

    225.31 Disconnecting Means. Means shall be provided for disconnecting all ungrounded conductors that supply or pass through the building or structure.

    225.36 Suitable for Service Equipment. The disconnecting means specified in 225.31 shall be suitable for use as service equipment.

    There must be a means provided to disconnect all the hot conductors at the remote building and this disconnect must be suitable as service equipment. You will be hard pressed to find a main lug single phase panel that is suitable as service equipment. Do yourself a favor and get a panel that has a main breaker and not waste time having to get another inspection.

    A #10 copper equipment grounding conductor is good for up to a 60 amp overcurrent device (breaker). It is the size EGC that will be in #6 UF cable and the cable can be protected by a 50 amp breaker. It can also land in a 100 amp breaker in a 100 amp main breaker panel that I would use in the remote building. The cost at Lowe’s will be around $65 and less than the $75 re-inspection fee that I would be charged if I required a re-inspection.

    You will be required it install a grounding electrode system at this building also.

    250.32 Buildings or Structures Supplied by a Feeder(s) or Branch Circuit(s).
    (A) Grounding Electrode.
    Building(s) or structure(s) supplied by feeder(s) or branch circuit(s) shall have a grounding electrode or grounding electrode system installed in accordance with Part III of Article 250. The grounding electrode conductor(s) shall be connected in accordance with 250.32(B) or (C). Where there is no existing grounding electrode, the grounding electrode(s) required in 250.50 shall be installed.

    It doesn’t matter what kind of instructions you receive from the power company, another electrician, one of your friends or even the advice you get on this site, the only thing the electrical inspector can enforce is the codes as adopted in your area. You will have a lot better luck with your inspector if you contact him/her before work starts instead of waiting until done to ask if it will pass.

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