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Thread: Conduit Fill Question

  1. #1
    DIY Member arfeller's Avatar
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    Default Conduit Fill Question

    Hi, i have a conduit coming out of the bottom of my service panel that currently has some 12/2 romex ran through it. I would like to know how many 12/2 wires i can run through it.The outside of the conduit is stamped -

    "EXTRA HEAVY PVC USE NEC TABLE FOR WIRE FILL CAPACITY 2.87 SQ IN"

    Soo... i looked up the NEC fill tables and it really did not make sense to me. I'm not running single wires, i'm running 12/2 romex.

    Hopefully i've explained this adequately. Please let me know if you need more information.

    Thank you!



    And yes, i know i need to address the wires coming down from the top, and the side. I rewired all the knob and tube in the attack. So am still working through that.

    Rob
    Last edited by arfeller; 02-20-2008 at 09:57 PM. Reason: Added Picture

  2. #2
    Moderator & Master Plumber hj's Avatar
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    Default Pvc

    It would help if you specified the size PVC. Without something to compare it to, we don't know if it is 1 1/4", 1 1/2", or 2", which would change our answer, although technically you should not run Romex inside conduit.

  3. #3
    Aspiring Old Fart, EE, computer & networking geek Mikey's Avatar
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    If it claims 2.87 sq in, it works out to be real close to 2". 334.15(B) allows Sch 80 PVC rigid nonmetallic conduit for protection of NMC in exposed work.

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    Electrical Contractor/Instructor jwelectric's Avatar
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    312.5 Cabinets, Cutout Boxes, and Meter Socket Enclosures.
    Conductors entering enclosures within the scope of this article shall be protected from abrasion and shall comply with 312.5(A) through (C).

    (A) Openings to Be Closed. Openings through which conductors enter shall be adequately closed.

    (B) Metal Cabinets, Cutout Boxes, and Meter Socket Enclosures. Where metal enclosures within the scope of this article are installed with messenger supported wiring, open wiring on insulators, or concealed knob-and-tube wiring, conductors shall enter through insulating bushings or, in dry locations, through flexible tubing extending from the last insulating support and firmly secured to the enclosure.

    (C) Cables. Where cable is used, each cable shall be secured to the cabinet, cutout box, or meter socket enclosure.

    Exception: Cables with entirely nonmetallic sheaths shall be permitted to enter the top of a surface-mounted enclosure through one or more nonflexible raceways not less than 450 mm (18 in.) and not more than 3.0 m (10 ft) in length, provided all of the following conditions are met:
    (a) Each cable is fastened within 300 mm (12 in.), measured along the sheath, of the outer end of the raceway.
    (b) The raceway extends directly above the enclosure and does not penetrate a structural ceiling.
    (c) A fitting is provided on each end of the raceway to protect the cable(s) from abrasion and the fittings remain accessible after installation.
    (d) The raceway is sealed or plugged at the outer end using approved means so as to prevent access to the enclosure through the raceway.
    (e) The cable sheath is continuous through the raceway and extends into the enclosure beyond the fitting not less than 6 mm ( in.).
    (f) The raceway is fastened at its outer end and at other points in accordance with the applicable article.
    (g) Where installed as conduit or tubing, the allowable cable fill does not exceed that permitted for complete conduit or tubing systems by Table 1 of Chapter 9 of this Code and all applicable notes thereto.
    FPN: See Table 1 in Chapter 9, including Note 9, for allowable cable fill in circular raceways. See 310.15(B)(2)(a) for required ampacity reductions for multiple cables installed in a common raceway.

  5. #5
    DIY Member arfeller's Avatar
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    Ok,

    So if i understand correctly.

    1. It is not appropriate to use the conduit into the bottom of the box.
    2. You can use conduit if you come in the top of the box and follow the rules outlined.

    Soo... I guess it does not matter how many wires i can put in this conduit.

    I'm still curious, but I will look into alternate means to run cable from my box to my basement.

    Thank you.

    The local inspector signed off on the service change as shown in the picture.

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    DIY Junior Member msehler's Avatar
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    I don't know too much about the NEC, but I do substation design and one of the areas is conduit fill for control cables. The NEC probably has a table for 1 conductor (1 cable), 2 conductor, and 3+ conductor. This is because when pulling the cable 2 binds up more than 3, etc, so you can only fill a certain percentage of the available cross section. So figuire out how many cables you are pulling, and how much space you have available (square inches minus the cross section of the existing ROMEX in the conduit) keeping in mind that you can only use your percentage of available space given to you from the NEC. I can also tell you that most utilities run new conduit instead of pulling through existing because it is a pain in the butt to pull cable through conduit that has existing runs in it, and they have to grease the heck out of it (the cable).

    This is also for complete conduit runs with less than 360 degrees of bends and a cable puller. I'm assuming your run is far shorter, and violating fill requirements won't hurt anything EXCEPT...except I believe romex has a problem with heat compared to other types of cable...so someone more knowledgable than I will have to explain how many is too many for ROMEX inside of conduit. (When you increase heat you change the resistivity of the wire but on this small of a scale I don't think that it will have any noticable affect.)

  7. #7
    In the Trades Bob NH's Avatar
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    Quote Originally Posted by arfeller View Post
    Ok, So if i understand correctly.

    1. It is not appropriate to use the conduit into the bottom of the box.
    2. You can use conduit if you come in the top of the box and follow the rules outlined.

    I'm still curious, but I will look into alternate means to run cable from my box to my basement.
    The area of 2" Schedule 80 PVC conduit is 2.87 square inches.

    Note that the exception of 312.5(C) is an exception to the requirement to fasten each cable to the box or cabinet. It is not a general requirement.

    You can run NM cable from the box using cable clamps that allow up to two cables per cable clamp without any conduit. If you want to collect the cables in a conduit for neatness you can run those clamped cables through a short piece of conduit (not exceeding 24" and not attached to the cabinet) without regard to conduit fill requirements or derating. You should have a fitting to protect the cables from abrasion at the inside edge of the conduit.

    There is another provison (334.15(B)) for protecting cables "passing through a floor" in exposed work that involves running cables through a piece of conduit. The construction at the intersection of the wall and the floor IS NOT the floor and doesn't require additional protection.

    If you are putting cable in a conduit then the area of the cable should be calculated by using the WIDTH of the cable as if it were diameter to calculate the area, with allowable fill of 40% for 3 or more cables.

    Another way to get the power to the basement would be to run a conduit from the service panel to a subpanel in the basement with THHN conductors (two hot, neutral, and ground) or a short length of SE R cable. It is convneient to have a subpanel in you have a lot of circuits on another floor.

  8. #8
    DIY Member arfeller's Avatar
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    Hmmm... I just got a new breaker box as shown in the picture that has more then enough capacity for my home. I can't justify adding a subpanel. I'm just trying to get the wire downstairs to my unfinished basement so i can run the wires along the ceiling joints to replace circuits through out the house.

    The wires exit my box as seen in the picture, then go down through the floor into the basement.



    The box is surface mounted on the wall.

    I think the NEC manual should have more pictures
    Last edited by arfeller; 02-29-2008 at 11:41 PM.

  9. #9
    Electrical Contractor/Instructor jwelectric's Avatar
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    90.1 (C) Intention. This Code is not intended as a design specification or an instruction manual for untrained persons.

  10. #10
    Licensed Electrical Contractor Speedy Petey's Avatar
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    Quote Originally Posted by jwelectric View Post
    90.1 (C) Intention. This Code is not intended as a design specification or an instruction manual for untrained persons.
    90.1(C) is my favorite code section!!!

  11. #11
    In the Trades Bob NH's Avatar
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    You can run the cables through the floor to the basement as follows:

    1. Disconnect the 2" conduit from the box and mount a piece of 3/4" plywood on the wall below the box, extending to about 12" from the floor, or down to what appears to be a baseboard. Fasten the conduit to the wall or plywood with conduit clamps. Chamfer the inside of the conduit at both ends so the corners won't damage the insulation.

    2. Run the cables from the box, using clamps rated for 1 or 2 cables per clamp, and staple or clamp them to the 3/4" plywood, and beyond that run them through the 2" conduit down to the basement.

    As long as that conduit doesn't exceed 24" long you can put in as many cables as you can stuff into it.

  12. #12
    I&C Engineer (mostly WWTP) Lakee911's Avatar
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    If you strip the outter shieth of the romex then you can run them up to a 40% fill. Chances are it's THHN. You'll need to derate the wiring though depending on the length of the conduit, as Bob said. BTW, I'm pretty sure you can run nearly 100#12 THHN/THWN in a 2" conduit.

    In that last picture, I see some wiring running unprotected in the picture and then entering a conduit. It looks as the outter shieth has been stripped of the wires entering the conduit. That's ok, but the conduit needs to have a bushing inserted in it to protect the wiring from sharp edges.

  13. #13
    Licensed Electrical Contractor Speedy Petey's Avatar
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    Quote Originally Posted by Lakee911 View Post
    If you strip the outter shieth of the romex then you can run them up to a 40% fill.
    Um....no. This is not a legal solution, especially if you are doing it to gain fill space.

  14. #14

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    It is Code legal to install romex in conduit. Chapter 9 Tables tell us that for over 2 cables, use 40% fill. Measure the major diameter of the ellipse of the cable.
    #12 NM is about .126 sq. inches fill. The section quoted talks about conduit running into the top of the box--but it does not say that you cannot run conduits into the sides and bottom...and these can have NM in them. Use lots of pulling lube.

  15. #15
    Electrical Contractor/Instructor jwelectric's Avatar
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    Quote Originally Posted by ked View Post
    The section quoted talks about conduit running into the top of the box--but it does not say that you cannot run conduits into the sides and bottom...and these can have NM in them. Use lots of pulling lube.
    I beg to differ;

    (C) Cables. Where cable is used, each cable shall be secured to the cabinet, cutout box, or meter socket enclosure.

    Exception: Cables with entirely nonmetallic sheaths shall be permitted to enter the top of a surface-mounted enclosure through one or more nonflexible raceways not less than 450 mm (18 in.) and not more than 3.0 m (10 ft) in length, provided all of the following conditions are met: (a) Each cable is fastened within 300 mm (12 in.), measured along the sheath, of the outer end of the raceway.
    (b) The raceway extends directly above the enclosure and does not penetrate a structural ceiling.
    (e) The cable sheath is continuous through the raceway and extends into the enclosure beyond the fitting not less than 6 mm ( in.).
    (g) Where installed as conduit or tubing, the allowable cable fill does not exceed that permitted for complete conduit or tubing systems by Table 1 of Chapter 9 of this Code and all applicable notes thereto.

    Now show me something that differs with this and I will recant my post.

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