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Thread: imbalance?

  1. #1
    DIY Member tregg's Avatar
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    Default imbalance?

    I've heard two licensed electricians refer to the white wire as carrying only the imbalance as if there's not much shock danger there. My foreman though said the imbalance acutally burns worse than the black wire. That's in a simple series light bulb circuit. So if you were to bare the white and touch it while you are grounded what would happen? Isn't there a 120v drop created by the lamp? But still wouldn't you get a percentage of the amps? (because now you've made a paralled circuit) Someone else said these guys don't know what they're talking about and went on to talk about imbalance actually refering to 3 phase systems with unequal loads on each phase.
    Last edited by tregg; 06-20-2013 at 05:42 PM. Reason: clarity
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    Retired Defense Industry Engineer jadnashua's Avatar
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    It's all about reference point...any current that flows from the 'hot' black wire, flows through the load into the white wire, so the white wire actually carries the same load as the hot. It's just that the white wire is referenced to ground, so if YOU are at ground, there's no difference, thus no impact. The current will travel the path of least resistance, and the white wire is generally it. Now, if you want to talk about parallel circuits, there are ways you could be in a situation where it's an issue which is why situations where this is more likely to happen, they specify the use of GFCI devices. These measure the outgoing current and compare what is returning on the neutral or white wire, and if they differ, that means some current is 'leaking' and could be going through you, so it shuts down the power flow altogether.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Electrical Contractor/Instructor jwelectric's Avatar
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    Default

    In order to give you a good understanding of the correct answer will take a while so I shall give you a short one.

    A series circuit is a voltage divider and the current is the same throughout the entire circuit as current only has one path to travel

    A parallel circuit is a current divider and the voltage stays the same throughout the entire circuit as current will have different paths to travel.

    When we have a multiwire circuit the neutral will carry only the unbalance of the two or three hot conductors. In our homes there will only be two hot conductors.
    Should I open my service panel and use an ammeter to check the amperage on one side of my service entrance conductors and have 60 amps and the other had only 45 amps then the neutral would only have 15 amps on it.

    To explain what is happening here there will be a total of 45 amps of series and 15 amps of parallel current flowing in that service.

    Now ask those who say touching the neutral is not dangerous to show you just how un-dangerous it is by giving you a demonstration.

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    Moderator & Master Plumber hj's Avatar
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    Default neutral

    YOU and the neutral wire, are at the same potential, as long as the neutral wire is properly connected. IF it becomes disconnected, then YOU will be become part of the circuit at whatever amperage it is carrying. That is how people get killed when they remove a water meter, or section of piping, when it is the ground and is carrying the neutral load due to a failed neutral conductor. In the Chicago area, we learned very quickly NOT to separate unions by grabbing the pipes on both sides of it. You knock them apart with a hammer or wrench and look for a spark between the two pipes. Two out of phase 120 volt lines do not need the neutral IF the load on each is equal, the neutral would carry the "imbalance/difference" if the loads are not equal. Conversely, if the two lines are "the same phase" then the neutral would carry the "total" amperage of both lines and could be overloaded without the safety of a circuit breaker to control it.
    Licensed residential and commercial plumber

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    DIY Member tregg's Avatar
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    Default

    Quote Originally Posted by hj View Post
    YOU and the neutral wire, are at the same potential, as long as the neutral wire is properly connected. IF it becomes disconnected, then YOU will be become part of the circuit at whatever amperage it is carrying. That is how people get killed when they remove a water meter, or section of piping, when it is the ground and is carrying the neutral load due to a failed neutral conductor. In the Chicago area, we learned very quickly NOT to separate unions by grabbing the pipes on both sides of it. You knock them apart with a hammer or wrench and look for a spark between the two pipes. Two out of phase 120 volt lines do not need the neutral IF the load on each is equal, the neutral would carry the "imbalance/difference" if the loads are not equal. Conversely, if the two lines are "the same phase" then the neutral would carry the "total" amperage of both lines and could be overloaded without the safety of a circuit breaker to control it.
    Thanks all. This especially is good info!
    "PULL LEMULE"

  6. #6
    Electrical Contractor/Instructor jwelectric's Avatar
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    We have become accustom to referring to a 240 volt single phase circuit as being two 120 volt phases that a good understanding of just how current flows throughout a circuit becomes hard to understand.

    What we need to keep in mind when trying to understand current flow, what the circuit itself is called. It is a 240 volt single phase. This being said and understood then the concept of two phases of 120 volts is wrong.

    In a simple two cell flashlight we have the same thing happening as with a 240 volt multiwire circuit. Change the 1.5 volt with the number 120 and rotate the batteries 60 times a second and we will have an example of a 240 volt AC circuit.

    The windings (batteries) will be positive on one end and negative on the other end. Should we decide to read just one battery (winding) then we would insert one probe in the center and depending on which end we went with the other probe would determine if the reading was positive reading or negative reading. This would mean that at the center we would have both a positive and a negative reading thus known as the neutral point on the power source.

    We can do a little experiment to see how this works. Wire a multiwire circuit to two keyless with a switch in all three conductors, black, red, and white. Now energize the circuit and close either the black or red switch now the neutral switch. Light comes on. Now open the switch on the neutral, light goes off. Now close all three switches and both lights come on. Now open the white switch and neither light goes off. They become a series 240 volt circuits with each bulb dropping the voltage the load calls for. If bulbs are equal then nothing will be noticed. If it was two phases out of sic with each other then instead of the lights burning one or both would blow.

    If the bulbs are different then the lower wattage bulb will get brighter and the higher wattage bulb will dim. The 240 volt winding did not do something magical and is still doing the same thing it was doing when its center was tapped.

    What we think of when we say to use both phases is reality is use both legs in order to keep the load down on the neutral whereas should we use only one leg we would have a parallel 120 volt circuit with the neutral being protect by parallel overcurrent devices that would be carrying its own load while the neutral or return path would be carrying the entire load.
    Touching a loaded neutral while you are grounded creates a parallel path for that current to flow on. It would depend on how much resistance the path through your body was to the amount of current that would flow. The neutral of the service of which one would be touching is connected to earth. From the point one touches the neutral and where this earth connection is will depend on the current that flows through that path.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    It can get complicated, but again, it's what you choose as the reference point. On something like an old CRT computer monitor or TV, the filaments may only be running at 5vdc, but they're in a circuit that may have 25,000vcd in it. If the references aren't right, all you'd get are sparks on the low-voltage circuit, but they reference one side of the 5vdc ps to the 25,000vdc supply, and therefore, you have 24995vdc and 25000vdc, so the filaments don't arc and burn up - the difference is 5vdc. Current flow requires a voltage difference (pressure) to move the electrons. Similar to when you scuff your feet across a carpet...you may have built up a charge of many thousands of volts, but there's very little current. If you don't know what you're doing, keep in mind that all it takes is something like 5ma, that's 0.005A to kill you if it is applied just right. Wire has resistance, so even from one end to the other, things can be different. It's what you don't know that is dangerous....lots of dangerous things in even a home electrical system. Don't mess with it unless you know what you're doing.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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