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Thread: Same phase conductors in conduit

  1. #1
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    Same phase conductors in conduit

    What is the guidance for installing mulitple 600V or less power conductors of the same phase in the same conduit. For example, putting 2 or 3 phase "A" conductors in the same homerun conduit. Is this practice discouraged or accepted? Are there conductor heating issues that discourage this practice. I have not seen anything in the Code that prohibits this. Thanks.

  2. #2
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    There is a basic prohibition of this type of installation with a few exceptions. Look at 300.3(B).
    Rob

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    All responses based on the 2011 NEC unless otherwise noted

  3. #3
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    Quote Originally Posted by jcccd View Post
    Are there conductor heating issues that discourage this practice?

    It's not so much of a conductor heating problem, but rather a conduit heating problem. A conductor carrying current will create around itself a magnetic field. If you have all phases in the same conduit (e.g., Phases A and B in a 120/240V single phase system, or Phases A, B, and C in a three phase system), then the magnetic fields created by each of the conductors tends to cancel out the fields created by the others. As seen by the conduit itself, then, the field is essentially zero.

    But with only one phase in a given conduit, presuming the conduit is made of a magnetically permeable material, then there will be a significant field seen by the conduit. Therefore, current will flow in the conduit itself, and that is one bad thing. The other bad thing is that the conduit will get hot. That is why the rules limit the use of "iso-phase" installations to non-magnetic raceways.

    Welcome to the forum.
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

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    Quote Originally Posted by infinity View Post
    There is a basic prohibition of this type of installation with a few exceptions. Look at 300.3(B).
    300.3(B)(1)Ex installations that have a solid metal enclosure on one or both ends have to follow 300.20(B) and are very rarely done. . There's a big down side and almost never an up side to going isolated phase on solid metal emclosures.

    The time that isolated phase is more commonly considered is bottom supplying large open bottom, concrete pad mounted gear that has very limited wire deflection space at the bottom. . 2008 added new wording to expressly allow what we've done for years. . 300.12Ex2 allows you to layout PVC conduit bells in the pad pour directly under each phase lug group and pull isolated phases [and an equipment ground, 250.122(F)] in each conduit. . This only works if you have precise measurement to each lug location on the gear or you risk missing your lineup and having to deflect the wires anyway.
    David
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  5. #5
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    Quote Originally Posted by charlie b View Post
    That is why the rules limit the use of "iso-phase" installations to non-magnetic raceways.
    And not only nonmet raceway, but no metal "pulling" elbows allowed anywhere in the run. . 250.86Ex3 allows them to be used in regular runs, but 300.3(B)(1)Ex says NO for isolated phase runs.
    David
    Inspector
    Medina County Ohio
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  6. #6
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    JCCCD - Is your question about multiple conductors of the same phase in a single conduit that also contains the other phases and neutral?

    Most of us assumed you were asking about putting just "A" phase conductors in the conduit with no return conductors. That is not allowed except in special conditions.

    Putting multiple circuits or parallel cables in the same conduit is not a problem as long as the return conductors are in the same conduit and all of the normal ampacity derating and conduit fill requirements are met.

    Another way of looking at it - any current going out in a wire through a conduit has to return in another wire in the same conduit. If it goes out on A phase and comes back on the neutral or B phase, the neutral and B phase wires have to be in the same raceway.
    Bob Wilson

  7. #7
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    I should have been more clear, (written communication is not easy) but more specifically, I was wondering about grouping branch circuits homeruns, i.e. lighting loads or receptacles loads, of the same phase in the same conduit. If a homerun conduit has up to three current carrying conductors, from three different branch circuits, but all on the same phase, will that cause problems associated with the conductor's magnetic fields, especially if it is just 20A circuits. Typically, I like to see multiple branch circuits from different phases in the same conduit, but I couldn't locate anything in the Code that enforced that practice. Thank you.

  8. #8
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    Quote Originally Posted by jcccd View Post
    If a homerun conduit has up to three current carrying conductors, from three different branch circuits, but all on the same phase, will that cause problems associated with the conductor's magnetic fields, especially if it is just 20A circuits.
    No problem at all as far as magnetic fields, the only issue with more than three current carrying conductors in a single raceway or cable is derating and with 20 amp circuits that is generally not an issue until you pass nine current carrying conductors.

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    You could also have numerous circuits of the same phase in a metallic conduit feeding switches in a single box and the coresponding switch legs (without grounded conductors present) would negate any inductive heating.

    Roger
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  10. #10
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    Quote Originally Posted by jcccd View Post
    If a homerun conduit has up to three current carrying conductors, from three different branch circuits, but all on the same phase, will that cause problems associated with the conductor's magnetic fields, especially if it is just 20A circuits. Typically, I like to see multiple branch circuits from different phases in the same conduit, but I couldn't locate anything in the Code that enforced that practice. Thank you.
    "three different branch circuits, but all on the same phase"
    That's different from what I pictured, but my answer doesn't change much. . This is still a question about isolation. . 300.20 applies equally to single or multiwire branch circuits, feeders, or services. . In all cases, all conductors must be in the same raceway [300.3(B)] or "close proximity" if single conductors buried or in midair [300.5(I)]. . You get a problem when you don't group properly.

    Any neutral must be accompanied by any phase conductor[s] grouped with that neutral. . The accompanying phase conductor must remain with the neutral except when provided with a counterbalancing switch leg.

    Circuits with loads using multiple "hot" ungrounded conductors, feeders, and services must all keep all "hot" conductors grouped.

    "three different branch circuits, but all on the same phase"
    If you're running multiple phase conductors from the same phase, you obviously have the same number of neutral conductors. . The neutral for each circuit balances any magnetic problem for that circuit. . You have 3 separate circuits that just happen to share the same conduit.

    If you keep each hot grouped with its associated neutral or switch leg, you won't have a magnetic problem.
    David
    Inspector
    Medina County Ohio
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    PSECI

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