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Thread: How does the neutral wire prevent MWBCs from operating at 240v?

  1. #101
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    Quote Originally Posted by jaggedben View Post
    Grounding only one node of a circuit does not create a short circuit on any kind of circuit, anywhere, whatsoever. Grounding has nothing to do with phase angles, either.
    We are in perfect agreement.
    Advise is a dangerous gift, even from the wise to the wise.

  2. #102
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    The Experiment

    Managed to run the experiment this afternoon. Here are my results.

    Some notes I take away from this thread:


    The MWBC is a series/parallel arrangement of sources & loads.

    In a perfect world with two perfectly balanced loads & sources no electric current would be present in the neutral or parallel section of the circuit.

    The idea that the currents cancelled each other out only applies in the mathematical model, in the physical world there is no electric current in the neutral or parallel portion of the circuit, no electric current enters the node.

    With unbalanced loads the series path tries to operate as a voltage divider, but a proportional electric current in the neutral keeps the voltage across the loads even.
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    Advise is a dangerous gift, even from the wise to the wise.

  3. #103
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    O, another MWBC debate? love these.

    one thing i don't see to much about is how MWBC power is distributed. having all the loads used most often on one side of CT xfrmer is not good.

    the issue with MWBC is if one load decides to run south, the load on the other side of MWBC can be damaged if both are on at the same time.

  4. #104
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    Quote Originally Posted by FionaZuppa View Post

    the issue with MWBC is if one load decides to run south, the load on the other side of MWBC can be damaged if both are on at the same time.
    Please enlighten us on this.

    What exactly do you mean by load running south? We have multiwire feeds to buildings nearly all the time, loads do go south, often not effecting anything else. Now if you meant if the neutral goes south, then I get that - though like I said we have multiwire feeds supplying buildings nearly all the time so same goes for the main supply as it does for a (multiwire) branch circuit.
    I live for today, I'm just a day behind.

  5. #105
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    Quote Originally Posted by kwired View Post
    Please enlighten us on this.

    What exactly do you mean by load running south? We have multiwire feeds to buildings nearly all the time, loads do go south, often not effecting anything else. Now if you meant if the neutral goes south, then I get that - though like I said we have multiwire feeds supplying buildings nearly all the time so same goes for the main supply as it does for a (multiwire) branch circuit.
    MWBC with loads that are not compromised, a lost N will not be an issue, as long as both loads are on at the same time, the loads turn on and work. w/o the N one load by itself will not turn on.

    take two 120ohm loads, one on one side the other on the other side. 240v CT 1ph xfrmer.
    both are 1amp on half the xfrmer when only one is on. both on at the same time is still 1amp across each load with is now across the whole xfrmer. so what happens if the one load goes "bad" and is now just 60ohms. when both on you'll get more amps on the good 120ohm load, which is bad, right?

    next, take 20 MWBC's, all red hots on one pole, all black hots on the other pole. now, if all the most used loads are on the red hots then you'll be loading the xfrmer heavily on only half the full winding. one half of the CT xfrmer gets real hot, the other side stays cold, etc.

  6. #106
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    Quote Originally Posted by FionaZuppa View Post
    MWBC with loads that are not compromised, a lost N will not be an issue, as long as both loads are on at the same time, the loads turn on and work.
    Only with equal loads; otherwise, the smaller (higher resistance) load will see higher voltage.

    w/o the N one load by itself will not turn on.
    If you mean if one load's switch is open (turned off) off, then none will be energized.

    take two 120ohm loads, one on one side the other on the other side. 240v CT 1ph xfrmer.
    both are 1amp on half the xfrmer when only one is on. both on at the same time is still 1amp across each load with is now across the whole xfrmer.
    Agreed.

    so what happens if the one load goes "bad" and is now just 60ohms. when both on you'll get more amps on the good 120ohm load, which is bad, right?
    With an intact neutral, both loads still see 120 volts, but the "bad" 60-ohm load will pass twice the current, or 2 amps.

    next, take 20 MWBC's, all red hots on one pole, all black hots on the other pole. now, if all the most used loads are on the red hots then you'll be loading the xfrmer heavily on only half the full winding. one half of the CT xfrmer gets real hot, the other side stays cold, etc.
    If the center-tapped secondary was physically constructed that way, probably true.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  7. #107
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    Quote Originally Posted by FionaZuppa View Post
    MWBC with loads that are not compromised, a lost N will not be an issue, as long as both loads are on at the same time, the loads turn on and work. w/o the N one load by itself will not turn on.
    There's enough additional information required for those statements to make any sense that, as stated, they end up being untrue. i.e. a lost neutral will almost always cause big problems. It will cause a higher resistance load to burn up and a lower resistance load to run weak when they share the same neutral. This has nothing to do with either load being 'compromised.' Even a neutral that develops significant resistance can do it. Look at the math (see post #80).

    You're probably right about badly unbalanced loads not being good for transformer longevity, but I would wager that bad cases of that are few and far between.

  8. #108
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    Quote Originally Posted by LarryFine View Post
    If the center-tapped secondary was physically constructed that way, probably true.
    how else does a 1ph CT xfrmer get constructed? the N comes off the center point of two secondary 120v coils (or literally tied to the center of the winding during the winding process). hence the 1ph 2pole CT xfrmer.

  9. #109
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    Quote Originally Posted by FionaZuppa View Post
    how else does a 1ph CT xfrmer get constructed? the N comes off the center point of two secondary 120v coils (or literally tied to the center of the winding during the winding process). hence the 1ph 2pole CT xfrmer.
    You're picturing a number of turns of wire wrapped around a core, a tap attached, and then the same number of turns wrapped again. That means the first half of the winding is physically closer to the core than the second half, allowing for greater electromagnetic coupling. If two completely separately-wound coils are placed side-by-side, one coil could run even hotter than the other by being more physically separated.

    Many transformers are constructed by winding with two parallel wires (called bi-filar winding), so both halves are identically coupled to the core. These two windings can then be placed in series for a center-tapped configuration, or in parallel for a single-voltage output with twice the current capacity, which describes a typical dual-voltage transformer. Of course, primaries as well as secondaries can be constructed this way.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  10. #110
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    Quote Originally Posted by FionaZuppa View Post
    MWBC with loads that are not compromised, a lost N will not be an issue, as long as both loads are on at the same time,
    And are equal.
    Si hoc legere scis nimium eruditionis habes.

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