neutral sharing

how come equiptment or appliances CAN share a neutral in a 3 phase system, but you CANT do that in residential 240 volt single phase/
split phase systems?. why cant a 14/3 feed a fridge and dishwasher?. the main service shares one neutral..so whats up?
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Well, electrically, you can.
But when you have three balanced loads, one on each phase of a three-phase system, the currents cancel out at the neutral, so there is very little current flowing in the neutral conductor back to the supply (I'll ignore issues with switching-power supplies and high harmonic loads).
With the residential circuit, if the two loads are balanced and fed from the opposite 'hot legs', the same thing can happen, the neutral current cancels out.
If somebody screws up and wires to circuits from the *same* hot leg to a common neutral, then obviously the neutral conductor is carrying the sum of the two load currents and may overload.
There are also problems if you go to work on one of the circuits, open the neutral leg and there is a load on the other circuit (that now also has an open neutral).
If the two circuits are run in metal conduits and they are not in the *same* conduit, the mis-match between 'hot leg' and 'neutral' current will induce currents in the metal conduit, causing heating and losses.
For these reasons, the NEC does not allow this sort of connection in residential circuits. There is a type of multi-branch circuit that allows this in commercial installations, but I'd have to look it up.
daestrom
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On Thu, 14 Sep 2006 01:05:53 GMT, "daestrom"

Really?? Where does the NEC forbid residential multiwire branch circuits? Article 100 defines them and Article 210.4 gives the rules for multiwire branch circuits and there is nothing there that forbids them in residential occupancies. The explanatory material in the 2005 NEC handbook specifically references 240/120 Volt single phase three wire circuits and gives several examples of allowed usage.
As long as the original poster's fridge and dishwasher are "line to neutal loads" article 210.4 seems to allow a multiwire branch circuit, subject of course to the provision that both breakers are in the same panelboard (and not on the same phase!) and there are situations where a double pole breaker would be required (likely not a good idea in this scenario...).
If there is a section of the NEC that disallows multiwire branch circuits in a residential occupancy, kindly provide the reference.
E. Tappert
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No, you got me. I was thinking there was an explicit ban for residential, but cannot find it.
It does mention if multi-wire circuits supply the same 'yoke', then provisions must be made to disconnect both ungrounded conductors at the same time. This is usually done, as you said, with a double-pole breaker.
Sorry about that, I should have looked closer. But multi-wire does require some additional precautions, sometimes pigtails in the neutral line are needed; and if run in conduit must all be in the same run.
daestrom

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daestrom wrote:

300.13 Mechanical and Electrical Continuity — Conductors. ... (B) Device Removal. In multiwire branch circuits, the continuity of a grounded conductor shall not depend on device connections such as lampholders, receptacles, and so forth, where the removal of such devices would interrupt the continuity.
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Tom Horne

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daestrom wrote:

In general, including cable, all conductors of the same circuit have to be in the same run (same cable).
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daestrom wrote:

example....if you have 6 outlets on the same single pole circuit and each outlet has an appliance plugged into it, all with different loads. whats happening on that common neutral wire?
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Not exactly what I was talking about.
If you have six outlets on a single pole circuit, and six more on a second single pole circuit and they share *one* neutral return, then the current in the neutral (groundED) conductor could be almost double the circuit breaker trip rating if both single pole circuits are from the same 'hot'.
When both circuits are from the same side of the line (same 'hot'), then the neutral currents from all the loads will *add* and overload the neutral conductor. If the two single pole circuits are from opposite 'hot' legs, then their load currents will cancel and there will be almost no current in the neutral returning to the source.
Get it right, you save the cost of a conductor, get it wrong you could overload the neutral conductor.
daestrom
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daestrom wrote:

No One who cannot execute a multiwire branch circuit in compliance with the applicable electric code for their jurisdiction has any business wiring anything.
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wrote: | daestrom wrote: |>
|> |>> |>> daestrom wrote: |>> |>>> |>>> If somebody screws up and wires to circuits from the *same* hot leg to a |>>> common neutral, then obviously the neutral conductor is carrying the |>>> sum of |>>> the two load currents and may overload. |>> |>> example....if you have 6 outlets on the same single pole circuit and |>> each outlet has an appliance plugged into it, all with different loads. |>> whats happening on that common neutral wire? |>> |> |> Not exactly what I was talking about. |> |> If you have six outlets on a single pole circuit, and six more on a |> second single pole circuit and they share *one* neutral return, then the |> current in the neutral (groundED) conductor could be almost double the |> circuit breaker trip rating if both single pole circuits are from the |> same 'hot'. |> |> When both circuits are from the same side of the line (same 'hot'), then |> the neutral currents from all the loads will *add* and overload the |> neutral conductor. If the two single pole circuits are from opposite |> 'hot' legs, then their load currents will cancel and there will be |> almost no current in the neutral returning to the source. |> |> Get it right, you save the cost of a conductor, get it wrong you could |> overload the neutral conductor. |> |> daestrom |> | | No One who cannot execute a multiwire branch circuit in compliance with | the applicable electric code for their jurisdiction has any business | wiring anything.
And you somehow believe that daestrom's assumption that there was a prohibition against this in the code means he does not know how to correctly wire it? He sounds like a commercial electrician, to me. While I might question the breadth of what he would allow to be done in a given situation, I do get the general impression that his understanding of electrical systems provides him the ability to understand unsafe situations that are not necessarily addressed by the code.
Suppose you have a single phase share neutral circuit with different loads on each pole. Now suppose these two loads have (at times) a very low power factor, where it is possible for one of them to have leading current and the other to have lagging current at the same time?
Not that I would ever expect the above situation in my new house, but I will be specifically prohibiting any shared neutral circuits except where there is a specific cause to have one for a reason other than to save on a conductor.
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snipped-for-privacy@ipal.net wrote:

It's your house and therefore your privilege to specify anything that you are willing to pay for. My statement was not directed at Daestrom and he's been here long enough to know I don't casually snipe at people. I stand by my statement because I believe that inability to safely wire a multi wire branch circuit indicates a lack of understanding of basic electrical principals needed for the competent installation of electrical work.
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On Thu, 21 Sep 2006 20:16:55 GMT Thomas D. Horne, FF EMT
| It's your house and therefore your privilege to specify anything that | you are willing to pay for. My statement was not directed at Daestrom | and he's been here long enough to know I don't casually snipe at people. | I stand by my statement because I believe that inability to safely | wire a multi wire branch circuit indicates a lack of understanding of | basic electrical principals needed for the competent installation of | electrical work.
I personally do know how to wire a shared neutral circuit correctly.
No electrical installation is perfectly safe. It's all relative. And I believe a shared neutral branch circuit serving more than one device has a relatively greater level of hazard when wired correctly and to code using state of the art methods, when compared to a basic two wire circuit also wired correctly and to code using state of the art methods. And I believe that level of hazard to be sufficiently high so as to warrant not using them everywhere it can possibly be avoided, which is virtually all locations.
Part of this is due to having seen numerous wirenut failures. The big motivation is to avoid as many wirenut connections as possible. This would allow me to run a shared neutral circuit to a single duplex device that has the hot side tab broken off, as no pig tails would be needed for that.
But that is not the only motivation. Any wiring connection could fail. And it could be the neutral. This is one reason there is a rule against multiple wires in one hole on the neutral bus. But there are still some other possible causes for a failure right there.
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I agree. Unfortunately, homeowners don't know what they don't understand!
I recently did some work at my house adding and rearranging circuits, and found a couple of shared neutrals in unexpected places. The end result is correct due to a lot of tracing and verifying, but I wonder what would have happened if someone without that knowledge had done it. If the circuits are lightly loaded there might be no problem for a long time, until a new load is plugged in.
Ben Miller
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Benjamin D. Miller, PE
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Ben Miller wrote:

My position is that it is not feasible to idiot proof the world. Anyone who engages in work involving something that can kill and does so without a full understanding of the necessary physical principles and safety precautions is a Darwin award candidate.
The logical extension of the no multiwire branch circuit position is that there should never be two separate circuits in any box, all device connections should be by pigtail, neutral and grounding connections should be made up with irreversible crimps, the main breaker for every home should be located in the meter enclosure and shunt tripped open by anyone opening the homes lighting and appliance panel cover, and so on. It is not the job of government to protect people from themselves. Any attempt to write idiot proof building codes inevitably imposes burdens on the rest of us that would be a totally unwarranted intrusion into our lives.
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Tom Horne

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wrote: | Ben Miller wrote:
|> |>> I stand by my statement because I believe that inability to safely wire a |>> multi wire branch circuit indicates a lack of understanding of basic |>> electrical principals needed for the competent installation of electrical |>> work. |> |> I agree. Unfortunately, homeowners don't know what they don't understand! |> |> I recently did some work at my house adding and rearranging circuits, and |> found a couple of shared neutrals in unexpected places. The end result is |> correct due to a lot of tracing and verifying, but I wonder what would have |> happened if someone without that knowledge had done it. If the circuits are |> lightly loaded there might be no problem for a long time, until a new load |> is plugged in. |> |> Ben Miller |> |> |> | | My position is that it is not feasible to idiot proof the world. Anyone | who engages in work involving something that can kill and does so | without a full understanding of the necessary physical principles and | safety precautions is a Darwin award candidate. | | The logical extension of the no multiwire branch circuit position is | that there should never be two separate circuits in any box, all device | connections should be by pigtail, neutral and grounding connections | should be made up with irreversible crimps, the main breaker for every | home should be located in the meter enclosure and shunt tripped open by | anyone opening the homes lighting and appliance panel cover, and so on. | It is not the job of government to protect people from themselves. | Any attempt to write idiot proof building codes inevitably imposes | burdens on the rest of us that would be a totally unwarranted intrusion | into our lives.
IMHO, your position on this is absurd.
I'm opposed to having multiwire branch circuits in my house, and in most other areas I influence decisions for, based on the principle that if there is a single wire open failure, it should cause the circuit to stop working for reasons of safety. This is not about "not understanding" it. This is in fact about understanding the reality of a work in which things can fail despite our best efforts. But practice is to assume that some unexpected failure may take place, and to design to minimize the impact of failures, with special emphasis on those that can result in damage beyond what will fail, especially if this could involve injury or death to animals or people.
If there is no need for a 3-wire circuit, why have one? To save a few dollars on wire cost?
I'm not proposing to eliminate multiwire circuits from the code for the sake of preventing others from the risks I see in it. But I would support such a code change as a means to force appliance manufacturers that now use a 3-wire circuit to make them work on a 2-wire circuit. If they were to make versions that worked on 2-wire circuits with no code change to get that to happen, then I would see no need to support any such code change. My objective is only to have the choice to make all my circuits be 2-wire only to the maximum extent possible. And yes, I am indeed looking for a circuit that can sense a radical imbalance in voltage between phases on a single phase service and shunt trip the main off in that case. Actually, simply independently checking for out of range voltage on both phases may be all I need.
Europeans surely know there is no fundamental need for things like clothes dryers and electric stoves to have both 120 and 240 volts. They can run them from 240 volts alone. Certain sub-circuits in these appliances that in American models would use 120 volts could be made to use 240 volts as is already done in Europe. They can be made to work safely on line-to-line and thus work safely in both locations, even if the supply is L-N as it would be in Europe. The tumbler motor in the dryer can be rewrired to run 2 windings in series instead of parallel. Electronics running on lower DC voltages can use switch mode power supplies that can select the voltage or be autoranging. Lights can be powered from the DC power (and will even be more reliable as a side effect of the thicker low voltage filament).
I do agree that idiot proofing the building codes is a misdirection. But to the extent that more restrictive codes in the direction of being safer results in the necessary materials, procedures, equipment, and appliances that are compatible with the safer methods being more readily available at a lower cost, then I do see a general benefit from such restrictions. If safe practices are done by only 1% then the market is to small for things that are needed for those safe practices. But if we can make such safe practices be carried out by a much larger percentage of the market, such as 95%, then these things will be there for those of us that act safe not to comply with code, but to actually be safe.
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What i understand you can but the 2 hot leads must be tied together on the same breaker for example a dw and a gdusing 14/3 tied to a 2 pole 15 amp breaker...
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matt wrote:

Multiwire Branch Circuits are allowed for residential circuits. It should be noted that a GFCI Circuit Breaker cannot be used to protect a multiwire branch circuit. However, GFCI receptacles work just fine on multiwire circuits. Also, by 300.13(B) pigtails are usually required for the grounded conductor at outlets.
2005 NEC REF: I. General Provisions 210.1 Scope. This article covers branch circuits except for branch circuits that supply only motor loads, which are covered in Article 430. Provisions of this article and Article 430 apply to branch circuits with combination loads.
210.4 Multiwire Branch Circuits. (A) General. Branch circuits recognized by this article shall be permitted as multiwire circuits. A multiwire circuit shall be permitted to be considered as multiple circuits. All conductors shall originate from the same panelboard or similar distribution equipment. FPN: A 3-phase, 4-wire, wye-connected power system used to supply power to nonlinear loads may necessitate that the power system design allow for the possibility of high harmonic neutral currents. (B) Devices or Equipment. Where a multiwire branch circuit supplies more than one device or equipment on the same yoke, a means shall be provided to disconnect simultaneously all ungrounded conductors supplying those devices or equipment at the point where the branch circuit originates. (C) Line-to-Neutral Loads. Multiwire branch circuits shall supply only line-to-neutral loads. Exception No. 1: A multiwire branch circuit that supplies only one utilization equipment. Exception No. 2: Where all ungrounded conductors of the multiwire branch circuit are opened simultaneously by the branch-circuit overcurrent device. FPN: See 300.13(B) for continuity of grounded conductor on multiwire circuits.
300.13 Mechanical and Electrical Continuity - Conductors. (B) Device Removal. In multiwire branch circuits, the continuity of a grounded conductor shall not depend on device connections such as lampholders, receptacles, and so forth, where the removal of such devices would interrupt the continuity.
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On 13 Sep 2006 20:48:45 -0700 snipped-for-privacy@electrician2.com wrote:
| matt wrote: |> how come equiptment or appliances CAN share a neutral in a 3 phase |> system, but you CANT do that in residential 240 volt single phase/ |> split phase systems?. why cant a 14/3 feed a fridge and dishwasher?. |> the main service shares one neutral..so whats up? | | Multiwire Branch Circuits are allowed for residential circuits. | It should be noted that a GFCI Circuit Breaker cannot be used to | protect a multiwire branch circuit. However, GFCI receptacles work | just fine on multiwire circuits. Also, by 300.13(B) pigtails are | usually required for the grounded conductor at outlets.
What makes you think that a GFCI Circuit Breaker (or even an AFCI Circuit Breaker) cannot protect a multiwire branch circuit? They can, and do.
Perhaps you are referring to the use of two separate single pole GFCI Circuit Breakers. Of course those will fail. And it should be obvious that something is funny since the correct wiring of a GFCI breaker involves attaching the breaker's white pigtail wire to the neutral bus, and attaching the circuit's neutral wire to the neutral lug on the breaker (so that both wires pass together through the brealer's current sensing transformer).
Square-D has 2 pole GFCI breakers up to 50 amps with neutral (the 60 amp breaker does not have the neutral pass through, so it is limited to 2-wire 240 volt circuits). Their AFCI breakers have only single pole versions. Cutler-Hammer does have 2 pole AFCI breakers.
| 2005 NEC REF: | I. General Provisions | 210.1 Scope. This article covers branch circuits except for | branch circuits that supply only motor loads, which are | covered in Article 430. Provisions of this article and Article | 430 apply to branch circuits with combination loads. | | 210.4 Multiwire Branch Circuits. | (A) General. Branch circuits recognized by this article | shall be permitted as multiwire circuits. A multiwire circuit | shall be permitted to be considered as multiple circuits. All | conductors shall originate from the same panelboard or | similar distribution equipment. | FPN: A 3-phase, 4-wire, wye-connected power system | used to supply power to nonlinear loads may necessitate | that the power system design allow for the possibility of | high harmonic neutral currents. | (B) Devices or Equipment. Where a multiwire branch circuit | supplies more than one device or equipment on the same | yoke, a means shall be provided to disconnect simultaneously | all ungrounded conductors supplying those devices or equipment | at the point where the branch circuit originates. | (C) Line-to-Neutral Loads. Multiwire branch circuits | shall supply only line-to-neutral loads. | Exception No. 1: A multiwire branch circuit that supplies | only one utilization equipment. | Exception No. 2: Where all ungrounded conductors of the | multiwire branch circuit are opened simultaneously by the | branch-circuit overcurrent device. | FPN: See 300.13(B) for continuity of grounded conductor | on multiwire circuits. | | 300.13 Mechanical and Electrical Continuity - | Conductors. | (B) Device Removal. In multiwire branch circuits, the | continuity of a grounded conductor shall not depend on | device connections such as lampholders, receptacles, and | so forth, where the removal of such devices would interrupt | the continuity.
I see no prohibition of GFCI protection by circuit breaker in the cited code. I've never seen it in my copy.
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Yes you can. And it is typical to find the Dishwasher and the Disposal on a split receptacle sharing the neutral in a 12/3 cable under the kitchen sink. And you do put it on a 2-pole breaker or use a "Handle-Tie" to join them at the panel.
MLR
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if you have 6 outlets on the same single pole circuit and each outlet has an appliance plugged into it, all with different loads. whats happening on that common neutral wire?
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