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Mains conduit fill question (N. America)

Discussion in 'Electrical Engineering' started by Bob E., Dec 21, 2011.

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  1. Bob E.

    Bob E. Guest

    Have three AWG 6, one AWG 8 (insulated ground) and two AWG 14 conductors (all
    THHN or THWN) that need to run in conduit for < 20 feet.

    I calculate the cross-section of the conductors and see that a 3/4" trade
    size EMT conduit is too small and that a 1" size will carry these conductors
    within code limitations.

    Must the AWG 8 ground conductor be used in the fill calculation? Or only the
    current-carrying conductors?

    Just looking for confirmation.


  2. Bob E.

    Bob E. Guest

    Actually need answer only to this Q:
  3. Ecnerwal

    Ecnerwal Guest

    It's presumably there in the NEC somewhere (I don't know one way or the
    other off the top of my head), but the practical answer is, use 1-1/2"
    or 2" or greater. Extra room in conduit is cheap, and terribly nice to
    have later on. It also makes pulling a lot easier than fighting a
    maximum fill load.

    If you really want to hate yourself, you could run 1/2" for the #14s and
    3/4 for the 3 #6 & #8 in parallel. The 3/4 will be fun to pull, but is
    legit if you don't have the #14s in there.

    I do have some 1" conduit - it carries 2 #10 wires out to my well (the
    bare 2/0 ground wire is external so as to help the overall grounding
    situation, as the well casing is the end of my ground network and there
    are several ground rods driven into the bottom of the trench and
    attached to the 2/0.) 1/2" is "more than adequate" for that much wire,
    but I won't go smaller than 1" in a buried application, and then only
    when I know the anticipated wiring for any anticipated use is well below
    the fill for 1" conduit. I might add 3 more #10 or #12 to put an outlet
    or two out by the well head - that's still 1/2" for fill.

    Anything the least bit unknown just gets 2", so I don't have to chew
    myself out later, and if a trench is involved, usually a completely
    empty 2" run goes in as well as whatever is actually in the trench, just
    in case. Conduit is cheap, trenches are expensive (not that you
    mentioned a trench, but explaining my conduit philosophy as it applies
    to trenches.)
  4. Bob E.

    Bob E. Guest

    Thanks people. 1" is required, larger desirable.

  5. Jamie

    Jamie Guest

    yes, the ground is also included in the math, why? because the ground
    also carries current when things go wrong and you don't want it to over
    heat in the pipe before the protection kicks in.

    Besides, it don't hurt to have some extra room.. Just go to the
    next size, you won't be sorry. You'll be able to pull the wire much
    easier now and later if needed.

  6. ehsjr

    ehsjr Guest

    You must include the AWG 8 (or whatever size wire you use)
    equipment grounding conductor or bonding conductor in the
    conduit fill calculation.
    From the NEC: "Equipment grounding or bonding conductors, where
    installed, shall be included when calculating conduit or tubing
    fill. The actual dimensions of the equipment grounding or bonding
    conductor (insulated or bare) shall be used in the calculation."

  7. Bob E.

    Bob E. Guest

    What is the rule about Grounding conductor in the US?
    I believe the National Electrical Code (NEC)


    allows for reduction in grounding conductor of one AWG wire size. For
    example, AWG 8 grounding conductor can be used with AWG 6 current-carrying

    That's a *general* rule, for which there are always many exceptions in the
    minutia of the NEC.
  8. Bob E.

    Bob E. Guest

    The more I read about the NEC the more I do not understand. ;-)
    Are you sure you're not a Yank? ;-)

    Wire size is enumerated by 2's (AWG 14, 12, 10, 8, 6, etc.) yet we call steps
    from one size to another "one wire size". What ever happened to AWG 13? Or 7?
    Inquiring minds want to know!

    Bob (the OP)
  9. Bill Gill

    Bill Gill Guest

    The NEC is not to be understood. It is to be followed, if you
    can figure out what it says.

  10. Jamie

    Jamie Guest

    I am sorry but the ground wire is also included in the calculations
    because it too, can carry constant current in cases where protection
    does not engage. Get your facts correct. The ground wire is just as
    important of a conductor as any in a race way and is assumed to be able
    to handle the full current load at 100% duty. If what you say is true,
    which I know it isn't, we could put pissy small ground wire in the race
    way, why waste copper. But that isn't the case because it isn't true.

    You see, we know a lot about this subject because one of our sister
    plants once got a citation for insufficient ground wire size and space
    in a race way due to an accident investigation. The EMT had a short in
    it but didn't trip the protection down stream. Some one got electrically
    hurt, not killed but hurt. #6 AWG wires with #8 Ground wire in the pipe
    with only enough clearance to account for the #6 wires and the ground
    wire not being of said gauge by code. They got a fine even though it
    wasn't the cause of the accident. The problem was physical damage caused
    by a fork truck. Yes, the the fine also included improper guarding of
    electrical areas and spacing.

    The law suit put into place by the person that got injured never fully
    follow through because about a mouth later, while said injured person
    was suppose to be home recovering, was driving around and got side
    railed and killed in an intersection accident. Of course, the person
    having a alcohol problem I am sure didn't contribute to it.

    Now if you want to talk about drain wires we can get on that subject too!

    Btw, we manufacture high power wires and cables for a variety of
    things. However, the power transmission systems in use in our facility
    isn't any better then the average hack systems you see in most places.
    The only good wiring that i've seen in house is what our own
    electricians do for the production machines and offices. THey do follow

  11. Guest

    I do not know about the US, butt in the rest of the world, the reason
    for "wasting" copper in the PE connector is not for carrying nominal
    current for 100 % of the time, but rather to carry the fault current,
    which typically is 4-10 times the nominal current. The idea is to
    insure that the fault current is sufficiently large, in order to
    _quickly_ burn the fuse or trip other safety device, while keeping the
    ground potential rise at the faulty device at a safe level.

    Even if the phase and neutral wires are thick enough to dissipate the
    losses at nominal load, at least in Europe, you also have to consider
    the total loop resistance from the fuse, through the most distant
    outlet with some extension cords developing a short or ground fault.
    The fault current must be sufficiently large to rapidly and reliably
    blow the fuse.
  12. ehsjr

    ehsjr Guest

    Nope. For conduit fill, the reason for including the equipment
    grounding conductor and/or bonding conductor is because they
    take up space. The computation does not consider current. It is
    strictly a physical size computation. For example, if you used
    #10 awg on a 20 amp circuit, your computation has to be based on
    the 10 awg conductor size, not on the 20 amp breaker.

    For computing conductor ampacity derating, the ground or bonding
    conductors and the neutral conductor that carries only unbalanced
    current are not required to be counted.

    See the NEC code: 310.15(B) (2) (a) (4) and
    310.15(B) (2) (a) (5)

    (4) Neutral Conductor.
    (a) A neutral conductor that carries only the unbalanced current from
    other conductors of the same circuit shall not be required to be counted
    when applying the provisions of 310.15(B)(2)(a).

    (5) Grounding or Bonding Conductor. A grounding or bonding conductor
    shall not be counted when applying the provisions of 310.15(B)(2)(a).

    Table 310.15(B)(2)(a) Adjustment Factors for More Than Three
    Current-Carrying Conductors in a Raceway or Cable
    He has his facts correct. See the NEC code article cited above.
    What Bud said is true.
    You're wrong.
    I suspect that this reveals that you think "derating" as Bud
    used it means using smaller diameter conductors. That is NOT
    what derating means.

    You must use at least whatever the code requires for the
    particular circuit - and the code doesn't allow "pissy small"
    equipment grounding conductors or bonding conductors. I suspect
    you are using the term "pissy small" to mean too small to safely
    carry a fault current.

    The equipment grounding conductor or bonding conductor is sized
    to whatever the code requires for the particular circuit. If the
    conductors will be run in conduit, then the size of the conduit
    is computed based on the physical size and number of conductors
    that will be in the conduit.


  13. josephkk

    josephkk Guest

    Per NEC all conductors must be included in the fill calculation.

  14. josephkk

    josephkk Guest

    Not required to be that large by the NEC but may be required to be so by
    local code. Personnally i consider it to be proper design.

  15. josephkk

    josephkk Guest

    Often true, but always part of conduit fill calculations.

  16. josephkk

    josephkk Guest

    Thank you Ed. Jamie should pay attention to Article 250.

  17. Jamie

    Jamie Guest

    Guess you've never had to deal with those passing out citations to
    violations.. They don't care how you, the violator interpret the rules.

    I've seen the section that was used to make the citation valid, it was
    clear and to the point!. You can read all the other side articles you
    want and hope the one that counts does not get viewed, while your
    defending yourself. Our lawyers can tell you all about how that one
    plays out.

    I can say with out a doubt, you would never get hired at one of our
    locations if you claimed to hold an E1 and practice like that.

    I'll say no more on the subject, I've said too much already. It's
    obvious you gamble, and if you do hold a E1, go a head and sign off on
    those questionable jobs. Just hope no one tells you to start opening up
    race ways for inspection.

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