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Shared Neutral

Discussion in 'Electrical Engineering' started by Maintech, Nov 8, 2003.

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  1. Maintech

    Maintech Guest

    Where in the NEC does it say that a single neutral can be "shared" between
    two or more circuits? My question is this, that I've seen where someone has
    ran up to seven 120v single phase circuits for HID lighting, yet there was
    only two neutral wires, the same gage wire as the hot lines, i.e. 12 AWG,
    20amp. I've also seen where a neutral is shared between two circuits for
    120v receptacles. It has always been my practice that when running a circuit
    off of a single phase breaker to whatever load that I run a separate neutral
    wire for that circuit and keep it separated.
  2. Brad R.

    Brad R. Guest

  3. Nukie Poo

    Nukie Poo Guest

    210.4 Multiwire Branch Circuits.
    (A) General. Branch circuits recognized by this article shall be permitted
    as multiwire circuits. A multiwire branch circuit shall be permitted to be
    considered as multiple circuits. All conductors shall originate from the
    same panelboard.
    FPN: A 3-phase, 4-wire, wye-connected power system used to supply power to
    nonlinear loads may necessitate that the power system design allow for the
    possibility of high harmonic neutral currents.
    (B) Dwelling Units. In dwelling units, a multiwire branch circuit supplying
    more than one device or equipment on the same yoke shall be provided with a
    means to disconnect simultaneously all ungrounded conductors at the
    panelboard where the branch circuit originated.
    (C) Line-to-Neutral Loads. Multiwire branch circuits shall supply only
    line-to-neutral loads.
    Exception No. 1: A multiwire branch circuit that supplies only one
    utilization equipment.
    Exception No. 2: Where all ungrounded conductors of the multiwire branch
    circuit are opened simultaneously by the branch-circuit overcurrent device.
    FPN: See 300.13(B) for continuity of grounded conductor on multiwire
    (D) Identification of Ungrounded Conductors. Where more than one nominal
    voltage system exists in a building, each ungrounded conductor of a
    multiwire branch circuit, where accessible, shall be identified by phase and
    system. This means of identification shall be permitted to be by separate
    color coding, marking tape, tagging, or other approved means and shall be
    permanently posted at each branch-circuit panelboard.
  4. SQLit

    SQLit Guest

    Yours is a good practice especially now days with switching power supplies.
    A very simple answer for your question is that in single phase services the
    phases are 180 degrees from each other. A 3 phase system is 120 degrees from
    each other. Reducing this to a very simple analogy, you get a pulse of
    electric from one phase, it dies then the next one comes along and dies.
    Never using the wire at the same time.

    Remember the NEC is the minimum standard nothing more. There are lots of
    installations that require more than the minimum

    I saw several years ago a new building that the engineer allowed 3 120
    volt circuits shared by a 10 awg neutral. It was a computer facility and we
    ended up repulling most of the building with separate circuits and neutrals.

    I hope this helps,
  5. jim

    jim Guest

    How would switch mode power supplies effect a single phase (120/240) service
    by overloading the neutral? In a 3 phase service, switch mode power
    supplies can overload neutrals due to the third harmonic (and odd multiples
    of the third harmonic) being additive since the phases are 120 degrees
    apart. This doesn't occur in single phase systems.
    Why? If your neutral currents were this high (greater than 30A) due to
    harmonics, you would have certainly damaged the transformer, unless it was
    oversized and designed for high harmonic service.

    The biggest concern for shared neutrals is a safety issue. The electrical
    system must only be worked on by individuals who understand the
    implications of the shared neutrals and will protect themselves
  6. John

    John Guest

    This is not a simple analogy, it is simply wrong. Nothing comes back on a
    pure 240v (or 3 phase) neutral; they don't take turns!
  7. John

    John Guest

    You have to be careful that the current on the neutral does not exceed the
    wire capacity, since there is no protection on the neutral. You also have
    to be sure that the current through any metal opening is zero; either by
    having hots and neutrals carrying the same current, or by having two hots
    from opposite legs carrying the same current, or by some combination of
    these two. Plastic isn't bothered by this.
  8. daestrom

    daestrom Guest

    On a pure 240V single phase setup, you're absolutely right.

    On three phase, with non-linear loads (such as electronic power supplies),
    the three phase currents do *not* add up to zero thoughout the entire cycle
    and the non-zero result flows in the neutral.

    For an example, take three power supplies that uses a full-wave bridge to
    feed some capacitors, each connected to one phase and neutral of a
    three-phase supply. The diode in the power supply connected to 'A' and
    neutral only conducts for a brief portion of the cycle when the
    instantaneous voltage is higher than the capacitor charge. While phase 'A'
    is that high, either phase 'B' or 'C' is *not* necessarily high enough to
    forward bias the diodes in those power supplies. So the 'return' current
    for the 'A' power supply flows in the neutral. Similarly, when 'B' voltage
    is high enough to cause its power supply diodes to conduct, phase 'A'
    voltage may not be high enough to continue conduction through its power
    supply's diodes. Result is 'pulses' of current on the neutral with a
    fundamental frequency of 180 Hz.

  9. Maintech

    Maintech Guest

    Thank You SQLit

  10. Zzzap

    Zzzap Guest

    You can share neutrals on two or more different phases. You
    can't share it on the same phase.

    I have never used this technique on more than two phases, for no
    other reason than having a 12-4 wire handy is unusual. The logic
    is that sharing the neutral on the same phase the unbalanced
    load will add to one another, and the same can be said on the

    However, there are new problems with modern electrical systems,
    such as places with electronic equipment. What building anymore
    doesn't have computers or electronic balasts? Don't ever share a
    neutral in such a case even in your own house if a neutral the
    circuit might be used for an electronic device. Whole buildings
    have had to be upgraded in the past 20 years because of
    Harmonics. And harmonics indeed is almost an entirely new
    problem of the modern age of computers.

    Other than this go ahead and share a neutral. However, you will
    find out that the wire is a ton cheaper when buying 12-2 than
    buying 12-3 or 12-4. That alone makes it prohobitive in such a
    situation where every dollar counts like residential wiring.

    To go one step further, even though you can share neutrals still
    and stay in code, why do it? Electronic devices are coming in
    our lives daily by the minute. Do you really want to upgrade in
    another 10 years when the NEC no longer allows it and shared
    neutrals are causing you problems?

    Where your friend shared so many neutrals I would never do such
    a thing. I am sorry to say I can't find it in the code, and
    therefore cannot say for sure if what he did wasn't up to par.
    also indicate he did wrong.

    More references:
  11. Maintech

    Maintech Guest

    Thanks CBEEBE1!!
    Your post prompted me to go back and reread National Electric Code 210.4 C.
    The problem I have with the "multiwire branch circuit" is that in 210.4 C is
    the exception #2 that requires that "all ungrounded conductors, HOT, of the
    multiwire branch circuit are opened simultaneously by the branch-circuit
    overcurrent device".
    Even in the facility I'm working in now, which is less then one year old,
    the contractors shared one single neutral with three circuits off of a
    208/120 panel, feeding 120 single phase receptacles. BUT the circuit
    breakers are all single pole, i.e. I go to replace a damaged receptacle
    which comes from breaker #1, which is single pole, I test to insure my
    safety and lock it out, and yet that neutral is shared with #3 and #5 which
    may have loads on them, and I'm unaware that the neutral is shared because
    they where to "cheap" to install a three pole breaker that would alert me
    that this is a multiwire circuit. What happens to me when I go and remove
    that receptacle from it's box? ZAP!
    This was the reason I posted this, I learned from you guys and I thank you!
    I've only delt with single pole which prompted me to run a neutral for each
    circuit, or two pole 240 volt.
    This is the third facility I've worked in that I've seen crap like this and
    I hate it. It almost prompts me to open up any panel and check the number of
    hots and neutrals and tracing them before attempting any repair, or I'll
    have to trace the conduit and turn off all the circuits contained within.
    My second big gripe with this is that these installations where done by a
    "UNION" contractor who claim to be the "professionals" in their field, but
    yet with a little bit of study, a simple maintenance mechanic like me can
    see where they SCREWED UP, but yet who pays the price, ME!!!
  12. Thanks CBEEBE1!!
    If you really know the code you know that the above citation has nothing to
    do with 3 branch circuits that supply receptacle or lighting circuits. 210.4

    Also, the requirement to disconnect all ungrounded conductors applies to
    (a)dwelling units (not commercial/industrial) and (b) devices mounted on the
    same 'yoke'. 210.4 B
    This is key to not being zapped if you work on an outlet on the circuit.
    The neutral on this type of circuit should be pigtailed to allow removal of
    the device without opening the neutral.

    It can be hard to deal with, but it may very well Not be a code violation.
    (see above)
    Remember, that job was done by the "Lowest Bidder" (probably) and those guys
    had a boss who got a bonus for finishing the job early and below budget. In
    the final analysis you have to give part of the blame to the AHJ. In
    essence, the AHJ makes the electrical code 'on site', meaning that he can
    enforce or make exception to a code rule at his whim. While large
    exceptions are very rare, small things are done all the time.
  13. Maintech

    Maintech Guest

    Yea and you know what? If everyone followed the rules it would be a better
    place, but they don't and I'm the one to suffer when they do use a recept as
    a splice point. My big gripe is that in a dwelling unit that it's required
    that the "hot" side be two pole breakers when there is a shared neutral, but
    yet in an industrial arena where the electrical is worked on much more, this
    is not a requirement. You know I hate it when I see any kind of spark from
    any wires after I've followed the "Lock Out Tag Out" rules. Why can't this
    requirement be extended to the non-dwelling side as well to tie the
    breakers together.
    Maybe I'm just being a bitch.
  14. Maintech

    Maintech Guest

    This is Illinois Farm country, What inspector???
  15. Ben Miller

    Ben Miller Guest

    OSHA and the NEC assume that in an industrial environment the service will
    be performed by qualified people. Hence the differing requirements from
    If you see a spark after you lock out a circuit, then you did not perform
    lockout/tagout properly. The correct procedure requires you to verify that
    power is off on the device before you start work. You would then discover
    that it was still energized on one side. I agree that this is time wasted,
    but at least you are safe. It would be easier for maintenance if there was a
    marking stating that it was a multi-wire circuit. I have been in facilities
    where every receptacle is marked with the breaker(s), panel location, etc.
    that feed it, but these are few and far between. One that comes to mind was
    a power plant.

    Ben Miller
  16. Maintech

    Maintech Guest

    Thanks Ben for your quick responce! I do check the hot side of single phase
    circuits to insure that I've shut down the correct circuit, but how do you
    check the nuetral side, if shared, and unaware that it is, with a
    multimeter? Your statement about "marking circuits as multi-wire" is my big
    bitch here. They don't do it in the industrial side, where as in a dwelling
    you should install two pole breakers. They don't do it in the industrial
    side. I have come across where circuits are shared with ballast lighting,
    i.e. sparks.
  17. John Gilmer

    John Gilmer Guest

    Well, even if there is a shared neutral AND the neutral is wired through a
    outlet the other HOT wire will be nearby. The "extra" hot wire should be a
    "clue!" AND your run of the mill sensitive DIGITAL multimeter will
    register a few volts when the probe is held next to a hot wire. You can
    also invest $20 (or MUCH less) in a pocket voltage tester that beeps and
    lights when near a hot wire.
    If you only have one pole loads it is "excessive" to require two pole
    breakers. In the case of lighting loads it is common to reduce lighting
    levels by switching out 1/2 or 1/3 of the fixtures on a string. The
    electrician with any real experience and common sense should be aware of
    "funny" things that can happen when you break the neutral.
  18. Maintech

    Maintech Guest

    Sorry guys! I guess I opened up a can of worms with my itty bitty gripe.
    I'm just a maintenance guy that at least looks at the NEC when doing
    electrical work unlike a many of my counterparts who use a cheap reference
    guide like "Uglies" and when the shit hits the fan they'll use the excuse of
    "I didn't know!". One thing I have learned from all this is to just pay
    attention to the work at hand. I've worked in a place before where one side
    of a 240v single phase and the ground where used to supply a nearby 120v
    receptacle, hence the ground became the neutral. I still work in an
    environment where some think that ground is just a neutral.
    All my bitching and griping in here isn't going to get me anywhere in life,
    but it does let me vent off some steam I get from my so-called counterparts
    who don't know the difference between three phase delta or wye.
    If you want to hound me more about my ignorance just remove 98wowsa from
    the e-mail.
    And hey........Happy Holidays!
  19. Any inspector who makes up the
    Here the inspectors are intimidated by well experienced contractors and AE
    A new hospital addition is a case in point. The AE desiged the incoming
    underground service like this. 4-3" conduits enter an occupied basement and
    run approx. 80' (unprotected) across the ceiling and turn up into the main
    switchgear on the first floor.

    Same hospital, different building, the secondary conduits turn up out of the
    first floor (buried) in the center of the building and run vertically into
    the bottom of the main switchgear on the second floor. The only protection,
    other than the metal conduit, is metal studs and sheetrock forming a chase
    around the conduits.

    This was made known up the line....but the influence of the hospital and AE
    firm resulted in no correction.

    Having said that, I agree that most AHJ's are honest and do their best to
    hold to the NEC. It can be hard tho, when you see major violations like
    these, to swallow a correction notice over some trivial problem.
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