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14-3 shared neutral

Discussion in 'Electronic Basics' started by Beeper, Feb 23, 2005.

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  1. Beeper

    Beeper Guest

    I'd like some feed back on this one. When I built my house, I had a very
    honest Father and Son contractor who were jack of all trades. They did the
    wiring also. Sloppy but functional. A few years later, while making some
    changes, I noticed they used 14-3 wiring to run 2 seperate circuits. These 2
    circuits shared the neutral. I questioned them on it and they said that is
    perfectly ok. I shared my concerns with them so they asked an electrical
    inspector and he confirmed their beliefs. It's ok to do that. I understand
    the load is such that it doesn't tax the neutral but what happens years down
    the road when the homeowner throws an extra load here and an extra load
    there on these circuits until the neutral is carrying more amperage than it
    is designed for? The breakers don't trip because the hots are carrying just
    under 15 amps each. 14.5 + 14.5 = 29. The 15 amp neutral can't carry 29
    amps. I know it's probably a little far fetched but it is possible. I
    have...no had 2 circuits like this in my house. Do I have OCD or what?
    Residential electricians...what do you think?
     
  2. Guest

    The neutral carries the DIFFERENCE of those currents, not the SUM.
    If the phase currents are equal, the neutral has I=0.
    Wade
     
  3. John Larkin

    John Larkin Guest


    Right, so long as the two branches are on different phases. And in
    office buildings where there are three separate phases of 120, that
    used to be true, too. But PCs and monitors have switching power
    supplies that pull a high current spike at the peak of the AC line
    cycle, and the spikes from the three phases don't align in time, so
    they don't cancel, and fires have resulted from the huge neutral
    currents.

    New directives regarding line harmonics (requiring unity-power-factor
    power supplies) fix this issue.

    John
     
  4. spudnuty

    spudnuty Guest

    This is true so long as someone competent does the work. I have seen
    situations where someone added so much stuff on to a circuit that they
    tied another breaker on to the kluged end of their mess probably
    because the smaller gauge and resistive connections caused so much line
    drop downstream. It took two breakers to turn off this mess! Was I
    swearing. I saw this twice last week.
    I also saw one where someone had replaced a 220 AC outlet with a
    standard outlet. The lady wondered why her Christmas lights were the
    brightest on the block and why they kept blowing out so much!

    Richard
     
  5. Terry

    Terry Guest

    Err? Different 'Legs' might be a better term?
    The point about peak currents and being close to unity power factor seem
    very valid comments.

    In an industrial setting (or in a large apartment building) they MAY happen
    be two 'phases'. But in most individual residence situations the two wires
    which have 230 volts between them are, sort of, plus 115 volts and minus 115
    volts to neutral. They are (Not quite true because we are talking AC here)
    usually the two outer ends of a 230 volt 'single phase' which has a centre
    tapped neutral to create the two 115 volt 'legs'. These legs are often
    mistakenly called 'Phases'. They are typically Leg A (Say Black) and Leg B
    (Say Red).
    Thus if there is load on both 'legs' it will tend to cancel out any load in
    the neutral middle conductor. However; think about this ................;
    if there is load on only one side (leg) of the circuit, the neutral will
    carry the same current as the hot lead on that side; and the other hot lead
    will be carrying nothing; right?
    It is a common mistake to think of the neutral as NOT carrying current; but
    all our basic circuit training tells us that current has to flow from a
    supply, through a load and return!
    That return IS the neutral conductor and it better be intact and in good
    shape! The fact that a good neutral is almost or only a volt or two above
    ground potential means that it is doing it's job of returning the current to
    the low voltage (neutral) side of the supply panel with little loss due to
    the resistance of the conductor.
    An open neutral is bad news and may results in electrical current trying to
    return to the supply panel through whatever is available.
    For example: Those so called GFIs (Ground Fault Interupters) actually work
    on the currents in the hot lead and the neutral being 'balanced'. If
    unbalanced due to defective neutral or a fault to ground the GFI disconnects
    the outlet/s to potentially save life.
    It is also bad to think of the ground wire as "Being the same as a neutral".
    It is not.
    The ground wire is there for safety in case something goes wrong. I would
    not like to have the grounded frame of my fridge being used as the return
    wire for the electric current operating the fridge compressor!
    Have fun, safely!
     
  6. Terry

    Terry Guest

    Beeper: I have duplex outlets in my kitchen wired this way. the upper socket
    is wired, say to the red, and the lower socket of the duplex to the black.
    This is not to get 230 volts between upper and lower but does allow two 115
    volt loads to be plugged in, effectively doubling the current capacity of
    the outlet. With 14 AWG that circuit should be fused/breakered at 15 amps. A
    double pole breaker should be used to disconnect both sides 'legs' of the
    supply to that circuit simultaneously.
    Terry
     
  7. John Larkin

    John Larkin Guest

    Well, I'm an electrical engineer, and we don't refer to "legs". By
    "different phases" I meant that the voltages are, well, not in phase.
    Zero degrees and 180 degrees are different phases to me. Zero and 120
    are also different phases, by my standards. Zero and zero are the same
    phase. A good test is to short them together and see what happens.
    Unless you're an engineer, in which case they are often correctly
    called 'Phases.'

    But definitions are personal property, so call them what you will.

    John
     
  8. Rich Grise

    Rich Grise Guest

    But with a 240V center-tapped transformer, they are exactly in phase -
    simply opposite polarity, with respect to the center-tap.

    Cheers!
    Rich
     
  9. Tom Biasi

    Tom Biasi Guest

    Yes, they are 180 degrees out of phase "in phase."
    Tom
     
  10. John Larkin

    John Larkin Guest

    As I said, to see if they're in phase, merely connect them together
    and see what happens. Report back and I'll interpret the experiment
    for you.

    John
     
  11. Rich Grise

    Rich Grise Guest

    But they're the same signal! How can a signal be out of phase with itself?
    Picking up a voltage from opposite ends of a transformer winding does
    not introduce any phase shift, nor does center-tapping that winding and
    grounding the center-tap.

    If you think they're out of phase, please show me the component that
    delays the signal by 1/120 second.

    Sorry.
    Rich
     
  12. Beeper

    Beeper Guest

    Terry, you just said what I was hoping to hear. It is OK if the two seperate
    circuits are mechanically connected(if you will) If one trips, the other one
    trips also. This was not true in my case and that's why I questioned it. How
    about everyone else who knows residential code. Mechanically connected? yes
    or no?
     
  13. Rodney Kelp

    Rodney Kelp Guest

    If you run two separate circuits with 14-3, I don't see the neutral sharing
    aspect. Each 14-3 leg has it's own neutral.
    They should have used 12-3 anyway. And if you add circuits, run new lines
    from the breaker panel with a new breaker.. Don't add to existing wiring.
     
  14. John Larkin

    John Larkin Guest

    Then why does the power company run them in on separate wires?
    It can't.
    Take the A and B signals from a residential 120/240 circuit. Use a
    time-interval counter to measure the delay from identical points and
    slopes on the A and B signals, both referenced to N. You will find the
    delay to be very close to 8.333 milliseconds.

    Repeat experiment with a phase meter. Repeat with an oscilloscope. All
    will indicate a 180 degree phase difference.

    The only sensible way to refer to such a power system is "two phase."

    Sorry.

    John
     
  15. Rich Grise

    Rich Grise Guest

    On Thu, 24 Feb 2005 09:00:16 -0500, Beeper top-posted (see below for
    context)
    They should be mechanically connected (ganged) if you will be using the
    two hots for a 220 (240) load down the line. If you don't have anything
    like that, and are absolutely certain that you never will, then
    independent breakers should be fine. The most the neutral will ever have
    to carry will be the current to the one side; otherwise they subtract,
    as Terry has pointed out.

    If each circuit has its own neutral conductor, then ganging breakers
    doesn't make any sense in the first place - they're completely independent
    circuits.

    Hope This Helps!
    Rich

    [Context]:
     
  16. Rich Grise

    Rich Grise Guest

    By this logic, the positive pole of a battery is 180 degrees out of
    phase with the negative pole.

    Thanks,
    Rich
     
  17. John Larkin

    John Larkin Guest

    No, phase is meaningless for DC.

    John
     
  18. John Fields

    John Fields Guest

    ---
    No, they're not.

    If you use one end of the transformer secondary as a reference, the
    center tap will be in phase with the other end of the winding, (that
    is, one positive-going peak will occur at the same time as the other
    positive-going peak) but if you use the center tap as the reference
    the ends of the winding will be 180° out of phase with each other.
    That is, the positive-going peak of one will occur when the
    negative-going peak of the other occurs.
     
  19. John Fields

    John Fields Guest

     
  20. If two circuits are run in a single 14/3 cable, the breakers feeding
    that cable _must_ be mechanically interlocked, so that they will
    switch on or off together, whether or not you are using them as a 220
    V supply. The reason for this is so that if you turn off one circuit
    to work on it, you won't get burned by the other, which will terminate
    in the same box.
     
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