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Home wiring: hot neutral & conjoined circuits

Discussion in 'Electrical Engineering' started by Joe, Sep 16, 2004.

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  1. Joe

    Joe Guest

    I was replacing a receptacle on what I'll call circuit 1, which is a 20-amp
    appliance circuit in the kitchen. The breaker was off, confirmed with outlet
    tester. The wire nut came off of the neutral bundle, I went to put it back
    on, touched two of the NEUTRAL (white) wires, and got a shock. I got my
    multimeter and found 120 volts across two neutral wires. I found this VERY
    strange.

    But the plot thickens: my wife then asked when the microwave would be back
    on. She was right...the microwave was off, but the microwave is on a
    different circuit (also a 20 amp appliance circuit), which I'l call circuit
    2. I tested an outlet on circuit 2, and the outlet tester indicated that hot
    & ground were switched.

    So, I turned off circuit two, and the hot neutral in circuit one went away.
    Turned both circuits back on, and everything works fine.

    This seems very strange to me...can anyone shed some light on this?

    Thanks,
    Joe
     
  2. Joe

    Joe Guest

    Thangs Greg, a handle tie is now on my Home Depot list.

    This answers a lot of questions for me, including why a 3-wire cable was
    coming into that box. However, I'm still having trouble figuring out why
    there was voltage across the two neutrals. Since the neutral is shared,
    there is only ONE neutral in this circuit--how can there be another neutral
    that is out of phase?
     
  3. Joe

    Joe Guest

    Yes, apparently this is a shared neutral circuit. I was not aware of any
    such thing before I read Greg's post. A three-wire cable runs into the box,
    with red being tied to a 2-wire cable going back out of the box, so I
    presume that black is hot for my circuit (circuit 1) and red is hot for the
    microwave circuit (circuit 2), with a shared neutral.

    They are on top of each other in breaker box, so out of phase as you
    suggested. Voltmeter confirms: 240 volts between hot 1 and hot 2.

    Planning on getting a handle tie for the breakers per Greg's recommendation.
     
  4. Jimmie

    Jimmie Guest

    When our kitchen was overhauled I added two shared neutral circuits, 4 sets
    of outlets on 4 breakers, When you take the cover off of the outlets there
    is a label that says shared neutral cirxuit. Also labeled breaker box the
    same way. .
     
  5. Guest


    I would not recommend ever sharing a neutral even on single phase
    services. Handle ties for (2)120V circuits are not cool. Even on
    single phase services the phase 'legs' or hots alternate vertically in
    the panel. So any breakers that may be handle tied have 240V between
    them. If the neutral is lost between the panel and the shared point of
    connection you can easily have 240V across 120V equipment connected to
    these circuits. Plus since this is single phase you can easily
    overload the neutral ampacity wise. Three phase neutral sharing across
    individual circuits is even more hazardous since the the potential
    fault current is much higher than with single phase.

    In short the proper fix for this is to run an additional neutral,
    which since you are dealing with cable means running a new cable for
    one of the circuits. I would not handle tie these breakers!

    Matt


    --
    "It's not what folks don't know that gets 'em in the most trouble,
    it's the things they do know that ain't so" Will Rodgers

    "Any sufficiently advanced technology appears as magic" Arthur C.
    Clarke
     
  6. Kilowatt

    Kilowatt Guest

    I use plain Jane Usenet to access the groups.
    This message was displayed in alt.engineering.electrical
    This is the only message that has been echoed to Usenet.
    Can someone tell me where the original thread originated from?
     
  7. Kilowatt

    Kilowatt Guest

    That explains it.
    Thanks


     
  8. Louis Bybee

    Louis Bybee Guest

    Shared neutrals have been quite common for some time, and as you point out
    usually aren't problematic when installed properly, and used with resistive
    loads.

    However, where shared neutrals are used with harmonic rich loads (switching
    power supplies, electronic ballasts, some HID Lighting, etc.) problems with
    the neutral conductor can, and likely will occur. Triplen harmonics can be
    additive in the neutral with true RMS current levels significantly higher
    than any single phase conductor even where the current measured on each
    phase conductor appears to be well balanced, and the conductors are
    appropriately phased.

    Louis
     
  9. Louis Bybee

    Louis Bybee Guest

    The subject was home wiring, but the post I responded to made reference to a
    208Y120 3 Phase system as one where the neutral was commonly shared. A
    shared neutral in a 3 phase system regardless of the load type used to be
    the norm. Now that more has been learned in regards to additive harmonics on
    shared neutrals of such systems more attention has focused on wiring
    practices and components used, especially where a large portion of the known
    load is harmonic rich. A few examples of that are cabling systems with an
    oversized neutral available, and K rated transformers.

    Louis--
    *********************************************
    Remove the two fish in address to respond
     
  10. Guest

    I did not really state that correctly, you are right about neutral
    currents roughly, the current relationship you describe is accurate
    for straight up resistive loads only. Most loads on 120v branch
    circuits are more complex and can have harmonic issues. As far as
    being qualified, I am a licensed master electrician with I over 25
    years experience, admittedly mostly industrial, not much residential.
    Many of the higher quality job specs I have worked with addressed this
    issue and required dedicated neutrals for all branch circuits where a
    neutral is required. In the last 15 years or so I have been more
    involved with service and less with installation and I have seen many
    examples of bad events from shared neutrals on 120v branch circuit, a
    few folks injured, and tens of thousands of dollars in equipment
    destroyed because of neutral failures on these circuits. I have not
    installed such a circuit in many years and never will again. If you
    insist on using these circuits at least install a single breaker that
    is designed to open two circuits rather than handle ties on two single
    circuit breakers, a weak or damaged spring in one and both may fail to
    open.

    Matt





    --
    "It's not what folks don't know that gets 'em in the most trouble,
    it's the things they do know that ain't so" Will Rodgers

    "Any sufficiently advanced technology appears as magic" Arthur C.
    Clarke
     
  11. Guest

    I won't use them. Seen far too many bad events from such. I'm not
    condemning those who use them properly, but I will never recommend
    them. I understand that residential/light commercial construction is a
    very tight margin industry, I have been fortunate that most my time
    has been with higher end industrial work. I have seen many spec sheets
    that addressed this issue and required dedicated neutrals for general
    use branch circuits. Also harmonics and noise concerns are are growing
    issue due to changing load types. Another thing, I don't care for
    handle tied breakers on two single circuits, handle tied breakers are
    less reliable than one designed to open two circuits. As for space in
    panelboards, again I guess I am used to larger boxes, so thats not an
    issue from my view. I think you will see this issue readdressed by the
    IEEE and NFPA soon due to issues still evolving on fire prevention,
    consumer/worker safety, harmonics, line noise and limiting fault
    currents.


    __240V__ <---- on 'homerun' neutral loss !
    |
    |

    120V________________________ LOAD____ __

    | |

    | |
    N _________ LOST/BAD _____________| 240V

    |

    | |
    120V________________________ LOAD____| __|


    I realize that I am showing 0 voltage drop across the loads, that is
    not uncommon on some equipment controls.

    Matt


    --
    "It's not what folks don't know that gets 'em in the most trouble,
    it's the things they do know that ain't so" Will Rodgers

    "Any sufficiently advanced technology appears as magic" Arthur C.
    Clarke
     
  12. Romy

    Romy Guest

     
  13. Dan Lanciani

    Dan Lanciani Guest

    | What happens if a home owner then later moves one of the
    | breakers so that both line feeds are served by the same
    | neutral? In that case you could have a fire.... a situation
    | that could sneak up on anyone but a fully qualified
    | electrician that looked the entire system over before making a
    | change... a common neutral wired somewhere in the attic could
    | be easily missed.

    Our house--wired by a licensed electrician--included a number of shared
    neutral circuits with their hots connected to the same leg. Even as a
    mere home owner I was disturbed by this, so I fixed it...

    [...]
    | I guess that would be safe enough...it would be an obvious
    | situation to someone seeing the 3 wire into the panel... they
    | would know its a comon neutral. Ive never seen it though.
    |
    | is it common?

    Shared neutral circuits are quite common. Incorrectly wired shared
    neutral circuits are, unfortunately, not entirely uncommon...

    Dan Lanciani
    [email protected]*com
     
  14. John Gilmer

    John Gilmer Guest

    When you have a shared neutral you run 3 conductor (plus ground) cable.
    The "extra" conductor is RED.

    Even to a near clueless homeowner, the RED color is a CLUE!

    Note that most 240 volt loads just use "normal" 2 conductor cable with the
    white being used for one of the "hots." This is so routine that often
    even electricians don't bother to mark the white wire inside the service
    panel: if a white wire is connected to a 2 pole breaker, it's HOT. Of
    course with GFCI breakers becoming more common ...

    Clearly, it's possible to crease all kinds of dangers by switching about the
    wires in the service panel. I suppose the possibility of someone screwing
    up a shared neutral circuit is one of the dangers but I would not lose sleep
    over it.
     
  15. Romy

    Romy Guest

    ---Snip---
    It's pretty common to use shared neutrals, especially when used with three
    phase wiring. As for the dangers, it's not really "dangerous" when done
    properly by a qualified person. The only problem is when people who don't
    really know what they're doing, start "doing" things. Same danger as
    somebody who doesn't know how to change their brakes on their car, decides
    to do so...

    Romy
     
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