Shared neutral principle

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  #1  
Old 06-05-05, 01:41 AM
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Shared neutral principle

Three wire plus ground shared circuits are common in kitchens. I understand how to wire these, but don't understand how they can work safely. If I have two 20 amp circuits, with say an 18 amp load on one and a 19 amp load on the other, I've got two #12 hot wires supplying power the receptacles, but only one #12 neutral supporting a 37 amp load.

I am evaluating the wiring in a small older apartment. It's been upgraded since it was built and now has THHN in EMT to a junction box, there connected to BX to the receptacles and other fixtures. There are three incoming hot wires to this junction box, but only a single neutral that is connected to each of the BX neutral wires. Is this adquate?

Thanks
 
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Old 06-05-05, 02:12 AM
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Are you aware that the breakers must be on opposite phases of the lines.
making it 240 volts from one hot to the other hot wire?
 
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Old 06-05-05, 03:18 AM
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When you have two items plugged in, the power flows from one hot line thru item one, thru item two, to the second hot line.
But any differential current will flow thru the neutral.
That will be 18 amps minus 19 amps = 1 amp flowing thru the neutral.
18 amps flowing thru one phase, and 19 amps on the other phase.
If the two items pull the same amount of current the neutral current will be zero.
The two items end up sharing, 240 volt line.

If the two breakers get placed on the same phases/lines then you can over load the neutral wire, the total current from the two items will flow thru the neutral.
 

Last edited by GWIZ; 06-05-05 at 02:23 PM. Reason: Clarity
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Old 06-05-05, 04:03 AM
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Yes, I did understand that these shared neutral circuits were connected to each side of a 240 v circuit. I just didn't understand how this could work safely and since I didn't, I wasn't sure if this could work for three circuits. Thanks for your explanation.

Should I infer from your response that shared neutral can only work for two circuits, connected as you described, and there would be no way that three circuits would be safe, sharing a single neutral? Two would have to be on the same 120 v bus.
 
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Old 06-05-05, 04:23 AM
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GWIZ explained how this works with normal household wiring, where you have 240 volts on two wires.

However, you cannot apply this to three wires no matter what you do in a normal residence. Using a shared neutral for three wires only works where there is three phase power.

It sounds like you have a serious situation that should be immediately fixed.
 
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Old 06-05-05, 10:35 PM
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Originally Posted by GWIZ
When you have two items plugged in, the power flows from one hot line thru item one, thru item two, to the second hot line.
But any differential current will flow thru the neutral.
That will be 18 amps minus 19 amps = 1 amp flowing thru the neutral.
18 amps flowing thru one phase, and 19 amps on the other phase.


If the current flowed through a load on one leg then a load in the other leg wouldn’t the loads on the two legs in series with each other?
Isn’t this what happens when there is an open neutral on a 3 wire circuit?
I thought the reason the neutral carried only the unbalanced current had to do with the positive and negative voltage peaks on the two legs and the direction of flow.
The two legs of a single phase service are of a single secondary winding…there is only one “phase”.
 
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Old 06-05-05, 10:57 PM
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Thanks, racraft, for the verification. I began to suspect this. When you renovate a 70 year old apartment, you can face a legion of kludges done by shoddy workman. May their pensions be in proportion to the quality of their work. I will find an electrician.

Keep the theoretical comments coming. Maybe I will eventually overcome the lack of high school physics.
 
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Old 06-05-05, 11:03 PM
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If the two 120-volt loads have exactly the same impedence, then yes, they could be considered to be in series with each other across a 240-volt source. And yes, when you have an open neutral on a shared neutral circuit (something you should take extraordinary measures to make sure never happens), then all the loads on one side are in parallel with each other and collectively in series with all the loads on the other side. And yes again, the reason the neutral only carries the unbalanced currents is because the two current wave forms are 180-degrees out of phase with each other.

In my opinion, whether you call the two legs of residential service "phases" or not is not a very important question. True, perhaps "phases" is not the most precise term, but you won't go very wrong thinking of them as such. I say that if you want to call them phases, go ahead--it doesn't hurt anything, especially if you never intend to work with three-phase power. But of course you are correct, all the power in a house is "single-phase".
 
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Old 06-06-05, 03:56 AM
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Originally Posted by Juhl
""
" If the current flowed through a load on one leg then a load in the other leg wouldn’t the loads on the two legs in series with each other? "

I agree with JOHN. (Yes)
--------------------------------
" Isn’t this what happens when there is an open neutral on a 3 wire circuit? "

I agree with JOHN. (yes)

With an open neutral situation.
The danger is, if the resistance/impedance of the two loads are not the same.

If you have a 15 amp toaster in series with your TV that draws 1-1/4 amps.
The 15 amp toaster has the resistance of 8 ohms.
Say the TV has 96 ohms of resistance.

The resistance of the toaster is so low that the TV will have close to the full 240 volts across it, damaging most TVs. With that open neutral situation.

The neutral keeps the voltage across each load at 120 volts. (for lack of better wording)
-------------------------------------
" I thought the reason the neutral carried only the unbalanced current had to do with the positive and negative voltage peaks on the two legs and the direction of flow. "

Not sure if I understand your wording.

The resistance/impedance of the items is what makes an unbalanced or balanced load.
You would come up with the same thing if the power supply is DC. Two 120 volt battery's in series and the center connection as the common neutral point.

----------------------------------------
" The two legs of a single phase service are of a single secondary winding…there is only one “phase”. "

If-if your transformer has two hot lines that are center tapped for a neutral common.
Checking the phase angle from the neutral to line 1 then neutral to line 2.
one hot line will be 180 degrees out of phase with the other hot line,
That makes line 1 the opposite polarity of line 2 in respects to the neutral as common.
I see that as two phase's.

Don't take this in a bad way. how do you define "Phase" ?
*
 
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Old 06-06-05, 07:09 PM
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I take it that this is _supposed_ to be a theory discussion

1) Current and voltage for alternating current
The first thing to remember about voltage and current measurements of _alternating current_ circuits is that these measurements are _averages_. The '120V' power in your home is generally _not_ 120 volts, but instead is a continuously varying voltage which, if you take the 'root mean square' average, averages out to 120V. The particular form of average (RMS) is used because if you apply AC with 120V RMS to a resistive load, the heating result is the same as if you apply true 120V DC.

2) On shared conductors.
_Circuits_ in which current actually flows require complete closed paths for the electrons to actually go from the source, through the load, and back to the source. A given bit of wire can be part of two separate circuits, with separate current flowing in each circuit. In this case, the _total_ current through that wire is the sum of the current caused by circuit A and the current caused by circuit B. When you take this sum, you much take into account the direction of the current flow. If the current caused by circuit A is 10A with the electrons moving 'left' through the wire, and the current caused by circuit B is 15A with the electrons moving 'right' through the wire, then the _net_ electron flow in the wire itself would be 5A to the 'right'.
It doesn't matter how many circuits share a given bit of wire, the net current in the wire is the _sum_ of all the currents, with appropriate sign conventions maintained. You must call electron flow in one direction positive, and in the other direction negative, and add them all up.
3) On alternating current shared conductors.
For _alternating current_ circuits, the above theory still applies, but only in the instantaneous case, meaning only if you measure the _instantaneous_ value of the current and take your sum. But since the current is constantly changing, what you really want is some mathematical technique that provides the answer given the RMS current flow. To do this, we need more information. We need both the RMS average current, and we need to know the frequency, and the phase angle. In your home, the frequency is a constant 60 Hz, so we don't need to consider it. But the phase angle is critical.
If you have two circuits, and they are operating with the same phase, then the net current flow in the shared conductor simply adds.
If you have two circuits, and they are operating with exactly opposite phase, then the net current flow in the shared conductor _subtracts_, so that if one circuit has 10A and the other circuit has 12A, then the _net_ current flow in the shared conductor is 2A.
If the phase angle between the two circuits is something else, then you need to do vector addition to figure out the net current; feel free to look up the math, its quite cool.
4) On shared neutral circuits in your home.
The supply to your home is arranged so that you have _two_ hot legs from the transformer and one _neutral_ conductor.
Because of the nature of transformers, the voltage of these two hot legs is almost exactly opposite phase. This causes the current flow to also be essentially opposite phase. If you have two separate circuits, each tapped from a different transformer hot leg, then the current flowing through these circuits is essentially out of phase, and if these two circuits share a conductor, the net current in that conductor is _reduced_.
However, if you have two circuit fed from the _same_ transformer hot leg, then the current in the shared conductor will be increased.
5) On 'phases'
Counting 'phases' is always confusing, because there are several different definitions of 'phase' that are used at the same time. You pretty much have to depend upon context to understand the term, and if the context is confusing than 'phase' can be almost meaningless.
The critical difference between single phase service and 'poly-phase' service is that in polyphase service you have multiple alternating current circuits arranged so that there can always be full power delivered to a balanced load. With single phase service, the available power continuously cycles up and down, as the AC cycle progresses. But with three phase power, you always have the same aggregate power available, with each phase doing its individual cycle a fixed amount out of phase with the other.
By this important distinction, the two hot legs of residential single phase service are completely different from the three hot legs supplied for commercial three phase service.
However for many uses, it is entirely reasonable to say that each transformer hot leg produces a different phase, and that residential service provides two hot legs 180 degrees apart. It is confusing because there is an industrial two phase service (which hasn't been state of the art for years, but is probably still in service _somewhere_. In 2 phase industrial service, you have two transformer hot legs supplied, which are 90 degrees out of phase. One leg or the other will deliver power to the load, all the time, making this industrial two phase service quite different from residential service.

- Jon
 
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Old 06-07-05, 06:21 PM
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I installed a shared neutral circuit about 18 months ago, drawing on advice received here, and the circuit has performed perfectly. But I continue to read the new posts on the subject, which occasionally creates some doubts in my mind.

After reading the current thread (at least the parts I could understand!), I want to confirm that the two hot wires in my circuit are on opposite phases, rather than the same phase. I installed a double pole breaker in the panel - had two spare adjacent slots that accomodated the breaker. So I have the correct breaker (double pole and the right brand for my panel), and it fit nicely into available slots, and the circuits are working fine. But is it correctly operating on opposite phases? Do I need to test? If so, how? I realize that in taking this "intuitive" approach, I really don't understand the "phase" concept, but my main concern is just to verify that my installation is OK.
Thanks
 
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Old 06-07-05, 07:09 PM
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The voltage between the two hot wires of a multi wire circuit should measure 240 volts. If it measures 0 volts, you don't have a multi wire circuit, but rather a fire hazard.

If you don;t have a voltmeter, you can sometimes use a neon light tester, as many of them will work on both 120 and 240 volts.
 
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Old 06-08-05, 01:50 AM
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Thanks Jon, I enjoy reading your posts they are always very informative.

I agree, the term phase has a broad definition and the NEC does not offer a specific definition. In my opinion, since the only use of the term "phase" by the NEC relates to single or poly phase services and the conductors derived from these services, it is confusing when the phase conductors of a single phase service are referred to as different phases. While the phase angle of the pos. and neg. voltage peaks is 180 degrees apart on AC phase conductors, it is not the only phase angle that can compared, the phase angle between voltage and current can vary depending on the type of load served. By applying the broad definition of “phase” one could say two circuits on one leg, one serving a resistive load and one serving an inductive load, were on different phases, therefore a shared neutral would only carry the differential current. Those who understand the context know this is ridiculous. I think if the term were just used to describe single or poly phase services and conductors, as the NEC seems to, there would be less confusion.
 
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Old 06-08-05, 06:14 PM
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If the term "PHASE" is confusing to some, they can also be refered to as "LEGS" or LINE A & LINE B. Different areas of the country have different terms for describing the same thing.
 
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Old 06-09-05, 03:43 PM
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I perceive a serious mis-conception as applied to a single-phase, 220/120 volt, 3-wire, A.C. power system. The mis-conception consists of the theory that the single-phase "source" is comprised on 2 voltages 180 degrees out-of-phase with each other.The mis-conception is based on using the Neutral as a "reference-point" for voltage-readings and then applying a "polarity" value to the readings in addition to the voltage-value.The "polarity" concept is the reason for the mis-conception.

The Ground-floor level of a building is designated "Elevation Zero". The basement-floor is 10ft. below the Ground-floor and is "Elevation -10" with respect to the Ground-floor elevation-value.The floor 10 ft. above the Ground floor is "Elevation +10" with repect to the Ground-Floor elevation-value.The total height with the basement floor as the reference-level is +10 +10 += +20; the top floor 20 ft above the basement floor. With the top-floor as the reference-level, the distance is -10 -10 = -20; the bottom floor is 20 ft below the top floor.When Using the Ground-floor as the elevation-value, the "+" and "-" elevation-values are dis-regarded.

We now progress to a 3-wire, 100/200 volt D.C. "source, two 100-volt batteries in series. We have 3 terminals--- "Neg", "N" ("Neutral"), and "Pos". With the "Neg" terminal as the voltage-reference, "N" is 100 volts more "+" than the "Neg" terminal, and the "Pos" terminal is 100 volts more "+" than the "N" terminal, so the "Pos" terminal is +200 volts with the "Neg" terminal as the voltage-reference.

If "N" is the voltage-reference point we have two opposite-but-equal values- "-100" and "+100" . To conclude the 2 values are "out-of-phase" is an obvious mis-conception.The mis-conception in an A.C. system consists of using the Neutral as the voltage-reference point and than applying "instantaneous" polarity-values to the two equal-but-opposite voltages. The two voltages are no more out-of-phase than in a D.C. system.

"Phase" is applied to a time-difference, or a value-difference at a specific time.A single-phase supply can be "split" into 2 circuits 90 degrees "out-of-phase" by inserting a capacitor in one of the circuits, a capacitor having a "time-constant".

The Neutral-current is a 3-wire circuit can be calculated exactly with a ingenious but simple "network theorm"
 
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