Shared neutral with 2 breakers - common application?

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Old 10-20-09, 07:35 PM
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Shared neutral with 2 breakers - common application?

I lost a real lengthy post earlier, explaining every detail.

Renter lady had both a tv and computer tower both go out on the same weekend, where neither work now, nor can be 'reset'. They are on same cirtcuit that I discovered used a shared neutral with 2 15A breakers, that have potential 240 volts between them, due to the buses they are on.

Testing in all kinds of ways revealed no voltage problems nor problems outside with the wires or in the panel box. No loose connections.

But I did shift the one 120 wire to another breaker, on the same 120 leg, in case something strange caused some current to go up that other 120 wire.

Anyone know the typical application for this method, and where the joinery is done with each hot wire, to the single neutral wire?
 
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Old 10-20-09, 07:49 PM
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I'm sure the pros will jump in here...but if I understand what you did you may have created a big problem. Multiwire branch circuits (shared neutral) must have the hot wires attached to breakers on opposite phases...otherwise you have serious danger of overheating/fire as the neutral wire will be overloaded. With hots on the opposite phase, the neutral carries the difference of the loads on each phase. With hots on the same phase, the neutral carries the sum of the loads...which often is more than the wire is rated to carry.

Example... on opposite phases, circuit "a" load = 10 amps, circuit "b" = 6 amps, shared neutral carries 4 amps, OK for 14g cable. On same phase, neutral would cary 10+6 amps = 16 amps...overload.

Sorry for the convoluted explaination and I may have used some wrong terminology.
 
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Old 10-20-09, 08:10 PM
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And it is best to use a 240v breaker. Have the two devices been tried on other circuits? Was there a storm when the failure occurred?
 
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Old 10-20-09, 08:11 PM
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What you have is a multiwire circuit. If installed properly they are a legal circuit.

The breakers must be positioned in the panel such that there is 240V between them. The use of a double-pole breaker is strongly recommended. When the hots are on different phases, the current in the neutral wire is the difference between the hot legs. When the hots are on the same phase, the current in the neutral is the sum of the hot legs which can create a dangerous overload on the neutral.

The problem on this circuit is probably related to a loose or broken neutral somewhere in the neutral. When this happens, it allows the voltage to divide unevenly between the hots. For example you could have 180V on one leg and 60V on the other. This can cause damage to electronics and appliances. I would explore this possibility to diagnose the trouble. Again, the use of a double-pole breaker is strongly recommended. Note that you can get a shock from a shared neutral circuit if only one of the breakers is turned off.
 
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Old 10-20-09, 08:12 PM
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Tom hit the nail on the head. Using a 3-wire MWBC (Multi-Wire Branch Circuit), you have two 15A circuits using 3 wires. Though you have 240v between each breaker, you will have your standard 120v between each hot and neutral.

Sometimes, if the neutral wire becomes loose or disconnected (either due to old age, poorly made connections, or inadvertently disconnecting something), you can end up with higher voltage on one set of receptacles and a lower voltage on the other set.

Have you tested the voltage at these receptacles?

The MWBC should be 'broken out' at the first box the circuits hit. Usually, it's wired like usual from that point on.
 
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Old 10-20-09, 09:32 PM
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The MWBC is pretty common and if done right it is not a issue at all and I do that in both resdetnail and commercal side.

As few other gave you few good pointers and if you are running cable make sure you have the same set if you going to use MWBC like example red and black on cable A that will tie to two pole breaker A.

I know it was used to be ligit to run single poles breakers before but not a really safe methold but it was done pretty often as long you know which " phase " you land it on.

The 2008 NEC code change that a bit so anytime you replace the MWBC breaker make sure you get two pole breaker the handle tie is ok for some area.

Yeah at the first junciton box before you break out to single circuit always pigtail the netural conductor { my SOP is use the crimp so that way someone know this is a MWBC they can not able lift the netural at all like that }

Merci.
Marc
 
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Old 10-21-09, 08:17 AM
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Originally Posted by ibpooks View Post
The breakers must be positioned in the panel such that there is 240V between them. The use of a double-pole breaker is strongly recommended. When the hots are on different phases, the current in the neutral wire is the difference between the hot legs. When the hots are on the same phase, the current in the neutral is the sum of the hot legs which can create a dangerous overload on the neutral.

The problem on this circuit is probably related to a loose or broken neutral somewhere in the neutral. When this happens, it allows the voltage to divide unevenly between the hots. For example you could have 180V on one leg and 60V on the other. This can cause damage to electronics and appliances. I would explore this possibility to diagnose the trouble. Again, the use of a double-pole breaker is strongly recommended. Note that you can get a shock from a shared neutral circuit if only one of the breakers is turned off.

I'll be! I just learned something. Now I have to quickly undo what I did. But now I trade possibly one problem for another?

If they did not do this 3-wire business, we would not be having this issue! This is nuts, to do something like this, where there can be a potential problem, when there are certain ifs, whatever those ifs are! Crazy.

Now what?! How in the world is a person going to find some improper or otehrwise bad wire joinery with this system? Where is the likely place in a LR, Kichen, short hall, BATH, 1 BED, basement, apt, that has 3 way circuits only between inside and outside hall lights only. 1/2 that apt. is on that one 15 amop circuit!!, which now is really scary what you said about me putting the wire on the same leg.

I have to hurry abnd run over there and change it.

No storms. Like I said, the outages could be a coincindence. But what if not?! The voltagfes were good yesterday. How can one find it now? IF there IS anything to find?

Disconnecting the red and black wire off each circuit breaker, I gort about 180 ohms between them. But after I mapped out the 3-wire loop, it appears maybe this could normally be expected afterall, perhaps due to resistance ina long loop of 14ga.wire.

I need an explanation I can understand, how the current likely could go up to 180 on one leg and down to 60 on the other leg, when al the wires are tight, including the neutrals, and no sign of overheating in the box.

I think it was explained here in thsi thread, but now I can't think straight since I have to leave for the moment and rush over there and put that circuit back the way it was!

See you soon.
 
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Old 10-21-09, 09:11 AM
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Originally Posted by ecman51` View Post
If they did not do this 3-wire business, we would not be having this issue!
Every circuit has a potential for failure. MWBC fail in ways that other circuits do not, but they also have advantages in reducing voltage drop and saving on wire cost.

Now what?! How in the world is a person going to find some improper or otehrwise bad wire joinery with this system?
As with any broken neutral, it's tedious. It could be at any box on the circuit.

I need an explanation I can understand, how the current likely could go up to 180 on one leg and down to 60 on the other leg, when al the wires are tight, including the neutrals, and no sign of overheating in the box.
When all the connections are good, the voltages stay at or very close to 120V and 120V. If the neutral is broken, then the voltage divides based on the impedance of the loads on each hot leg.

Voltage divider - Wikipedia, the free encyclopedia

Or go to this calculator which simulates the effect of a MWBC with a broken neutral.

BeyondTTL Resistor Voltage Divider Calculator

Put 240 in the VH box, 0 in the VL box and then play around with the resistance of the loads R1 and R2 to see how the voltage changes based on the ratio of the loads on each hot leg.
 
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Old 10-21-09, 09:20 AM
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Because of potential liability since this is an apartment I would suggest you get an electrician.
I need an explanation I can understand, how the current likely could go up to 180 on one leg and down to 60 on the other leg, when al the wires are tight, including the neutrals,
Are any of the receptacles back stabbed? If so the wires need to be moved to screws. Backstabs often cause a less secure connection. Since this is a MWBC it's best pigtail the neutrals at each receptacle and connect the pigtail to the receptacle. This helps prevent a lost neutral if a connection at a screw is lost. You also need to check all existing wirenuts, including at lights and switches.

As to how it works. Without the neutral fixtures on the opposite phase become series wired. In a series wired circuit unless the resistance/impedance is equal the voltage is split unequally between the two loads.
 
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Old 10-21-09, 10:31 AM
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Originally Posted by ecman51` View Post
I lost a real lengthy post earlier, explaining every detail.

Renter lady had both a tv and computer tower both go out on the same weekend, where neither work now, nor can be 'reset'. They are on same cirtcuit that I discovered used a shared neutral with 2 15A breakers, that have potential 240 volts between them, due to the buses they are on.

Testing in all kinds of ways revealed no voltage problems nor problems outside with the wires or in the panel box. No loose connections.

But I did shift the one 120 wire to another breaker, on the same 120 leg, in case something strange caused some current to go up that other 120 wire.

Anyone know the typical application for this method, and where the joinery is done with each hot wire, to the single neutral wire?
A drawing always helps visualize what you have in the walls. In the drawing I've attached you can see how the receptacles are on opposite hot legs. From any of those receptacles you can have 2 wire with ground onward power for lights or other.



You need to get organized. I sounds like the tv and computer are now inoperable. As mentioned this is likely as result of an opened shared neutral causing the 120 volt circuit to become a higher voltage that damaged the equipment.

It is also not clear how your multiwire is done but the fact your tenants tv and computer will no longer operate lends to the above configuration. A multiwire that is split at a single junction box as this drawing below shows.... can only deliver damaging voltage if the shared multiwire neutral is opened at the junction box or the neutral bus at the panel



Your multiwire may or may not have pigtails to the receptacles.

Turn the breakers off to the multiwire and identify every light, receptacle etc.. that loses power.

Unplug everything from this multiwire branch circuit. Turn any light switches off.

You say that you have tested and all voltages are normal?... but are they normal when tested to neutral with the above instructions completed and power on?? By this I mean do you show zero or near zero volts. Do not expect a high voltage reading like 180 or so.

If so then you likely have a loose neutral and the most common place to find an intermittent open on the neutral is at a receptacle. Back stabs in particular but could be inside a wirenut you will have to look....

One way to begin to get an idea of what your wiring configuration might be is to see if the black and red are present in a receptacle box with one connected to the receptacle and the other passing thru. If one or the other is missing then you are either at the end of a branch or a branch that is not sharing the common neutral like the above 1st drawing shows.

It is also going to be very difficult to actually determine that things are ok due to your correct voltages without a resistance/load on the multiwire branch circuit.

You might use a lamp with a bulb to determine if you have any high voltage problems. You should get a very bright light if your neutral is opened but be careful it is possible that it could pop if the voltage is too the high side of 240 volts.

Again make sure you do not operate any valuable equipment on this multiwire till you can assure proper connections have been made.

If you think you have the 2nd drawings configuration then you need to find that common junction box where the neutral was split and check that connection for tightness.
 

Last edited by Bruto; 10-21-09 at 12:07 PM.
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Old 10-21-09, 10:32 AM
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I just got back. I did what you guys said, and put the wire back so that 240 is between the two, once again. But before I did, I removed both red and black wire and ohms tested again(like yesterday), both with my analog, then digital. I had readings jumping all over. That concerned me.

But then I wondered if the jumping meters was due to my test leads not beign clamped to the wires. Yes, that was the problem. So then I alligator clipped them on, and now between red and black, with the neutral completing the loop, the reading was rock steady for along time, at 32 ohms (not 180 ohms as I thought). Since it is a loop, there has to be continuity(as long as at least one thing is plugged in or turned on, otherwise the loop would be broken).

It was suggested here to use a double pole breaker. Why? What is technically wrong with 2 single breakers the way they had it? How could this protect any better? And this has been this way for years. The building is from 1955 and no upgrade has been done to the place since I have worked for that landlord for the last 23 years. (Probably originally it had fuses?)

This incident is the first time we have ever heard of anything about anybody blowing anything. Many many tenants have come and gone, with their various electrical items.

And since all the lights in the entire apt. are also on that circuit - since she has never seen flickering lights, or lights going bright or orange, I am leaning to think that the tv and computer tower coincidently blew (on back to back days -not at the same instant), with no cause fom this circuit.

But I still would like to grasp if there really is a possibliity that 240 volts could even be possible, across outlet terminals, with this 3-wire set up. Maybe it can't. No light bulb has ever exploded from some overcurrent, that I have ever heard of, either. If it were at all possible that such a thing could happen, even say if a neutral were loose or something, how could they (say the NEC) allow such a 3-wire system, considering the expensive eqiuip. people plug in today. Losing a light bulb is one thing - losing possibly a huge theater size tv and computer is another.

I am pretty decent at mapping circuits and I am going to try to draw out how the 3-wire circuit can work, with outlets, and see if I can figure out how such a problem could happen. Unless any of you here are good at doing line drawings on the screen here, and submitting a drawing showing how that could actually happen. (People who do do that...do they draw it on paper first?, then photograph the drawing, then submit on Photobucket?)
 
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Old 10-21-09, 10:48 AM
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Originally Posted by Bruto View Post
A drawing always helps visualize what you have in the walls.
Gee -thanks! That is just what I requested!


You need to get organized. I sounds like the tv and computer are now inoperable.
The wiring is operable. It is only the tv and computer that went poof over the weekend, on Saturday, then Sunday. No proof there was a problem with the wiring, ever. But obviously very concerning when the tenant sort of demanded that there better not be anything wrong with the wiring causing her to blow items and to maybe possibly blow the replacement huge heavy console beautiful tv she just had her son and 4 other people help get up to the upstairs apt., on a dolly. If that tv were to blow, I'd go hide!

We could have had an electrician come out. But he, like I would be doing.....what? All kinds of accessing and looking, running up the time ticker, while nothing maybe even bad gets discovered, based on her 2 failures wihout any other evidence?

And there are no other reasonable outlets off that 3-wire circuit to plug into! The LR and BED outlets are all on that one circuit!

As mentioned this is likely as result of an opened shared neutral causing the 120 volt circuit to become a higher voltage that damaged the equipment.
That is what I have to study in your drawings, to see if that is even possible, and where an open neutral might be. A loose white wire on a recepticle? Could that really cause this?

As stated in my last post, surely the NEC could not allow such a thing. Not with all the cases you read(and that I have personally gone out and fixed) of loose backstabs that cause circuits to go out. Going out is one thing. Being boosted to 240 volts is quite another. I would have no respect for the NEC if such 3-wire system wiring is allowed , where 240 could even remotely enter that loop. That would be stupidity.

I am going to have to go back to work soon, and surely will study your drawings later. Absolutely. You went through a lot of work and I appreciate it.

....................................................................

I just looked at your first drawing. It looks to me that if the neutral came loose or disconnected that the outlet would just be dead.
 
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Old 10-21-09, 11:18 AM
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Originally Posted by ibpooks View Post


When all the connections are good, the voltages stay at or very close to 120V and 120V. If the neutral is broken, then the voltage divides based on the impedance of the loads on each hot leg.

But I do not see how that is possible, in brutos first easy-to-follow schematic. If you see how, please explain.

It looks like the only way (to me, anyway, at this juncture)is if somehow 120s volt came up that neutral wire. Perhaps if a wire in a box, lacked some insulation, and touched a neutral wire, that also had to have lacked insulation at that same spot. But if that happened, then the circuit breaker should trip. And it never did..

The only other way I could see it, and not trip a breaker, is if my bare wires touching explanation occured, AND the neutral was loose in the box, or bad somewwhere upstream toward the pole. And I have seen no evidence of that.

There are 3 other apts. here, and nobody else has said anything.
 
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Old 10-21-09, 11:35 AM
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As stated in my last post, surely the NEC could not allow such a thing. Not with all the cases you read(and that I have personally gone out and fixed) of loose backstabs that cause circuits to go out. Going out is one thing. Being boosted to 240 volts is quite another. I would have no respect for the NEC if such 3-wire system wiring is allowed , where 240 could even remotely enter that loop. That would be stupidity.
Be careful to criticize what you may not understand.... The NEC and the CEC allow this and it is more common in the CEC. In fact your apartment service entrance is a shared neutral from the transformer. Lose that neutral and everything in all your apartments 120 volt circuits floats to high voltage....not just a single branch circuit.
You also misunderstand the principles. A multiwire done correctly is not a problem but over time can be (as any circuit) but the code authorities are not concerned what you may operate on the premise wiring. They write the codes to provide minimum safety to humans and protection for the dwelling and proper operation. Not what you plug in..... They cannot be concerned with what may go wrong 20 years from now with premise wiring.....wiring will eventually experience problems due to age or abuse ..... nothing is forever. A multiwire just has a few more serious problems than a typical 2 wire with ground branch circuit.

We could have had an electrician come out. But he, like I would be doing.....what? All kinds of accessing and looking, running up the time ticker, while nothing maybe even bad gets discovered, based on her 2 failures wihout any other evidence?
This is possible but it is prudent to check the wiring as your doing. Once you understand the multiwire this is simply a time thing to verify connections at that point you should be fine as will her new TV. Or provide a single branch circuit to the receptacle she will be plugging the tv into....there are many options.

It was suggested here to use a double pole breaker. Why?
Only to assure that opposite legs are obtained and that both legs are denergized. If two single poles then you may not realize you have a multiwire and only turn one breaker off. Using single pole breakers there are a few possible issues one is the obvious...not getting opposite legs and the other is if you remove the shared neutral wire from the neutral bar or any other point during troubleshooting or maintenance and power is on the multiwire you just turned the branch circuit to a 240 volt branch circuit. See the below drawing.... The open neutral puts the hot legs in series via the neutral connection to the devices. Current can now flow back and forth between legs because the return path over the shared neutral has been lost as is the 120 volt potential. Any 120 volt equipment or appliances plugged into the receptacles depending on impedance will receive high or low voltage as previously explained.

 

Last edited by Bruto; 10-21-09 at 12:09 PM.
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Old 10-21-09, 11:51 AM
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See this link for a detailed example.....

Understanding the Dangers of Multiwire Branch Circuits

And a graphic from the same author....

 
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Old 10-21-09, 12:31 PM
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Originally Posted by ecman51` View Post
As stated in my last post, surely the NEC could not allow such a thing.
The NEC covers only minimum safety standard of premises wiring. The power quality is left entirely to the discretion of the engineer, electrician or other person designing the system. Generally people (both builders and homeowners) choose cheaper installations over quality and especially over long term quality. Even still your MWBC has operated with only this minor malfunction for over 50 years -- I'd count that as a successful installation.

This type of over-voltage is really no different than if a power company transformer malfunctions, car hits a pole or lightning strikes the wires outside. Anything connected to the power line or external data lines risks damage if not protected by adequate surge protectors, line conditioners, voltage regulators, UPS or any other type of power quality device.

One other note I'd like to add is that unless this was a particularly old computer I doubt a run of the mill overvoltage killed it. Nearly all computers, digital TVs and other consumer electronics are designed for international markets and can operate seamlessly on voltages from 100V to 250V without malfunction.
 
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Old 10-21-09, 02:34 PM
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Originally Posted by Bruto View Post





Only to assure that opposite legs are obtained and that both legs are denergized. If two single poles then you may not realize you have a multiwire and only turn one breaker off. Using single pole breakers there are a few possible issues one is the obvious...not getting opposite legs and the other is if you remove the shared neutral wire from the neutral bar or any other point during troubleshooting or maintenance and power is on the multiwire you just turned the branch circuit to a 240 volt branch circuit. See the below drawing.... The open neutral puts the hot legs in series via the neutral connection to the devices. Current can now flow back and forth between legs because the return path over the shared neutral has been lost as is the 120 volt potential. Any 120 volt equipment or appliances plugged into the receptacles depending on impedance will receive high or low voltage as previously explained.
I see. I follow. For some reason I did not see that on the first drawing I earlier responded to. Now I am going to go back and look at that drawing and compare to the one here, where you actually traced how that circuit can super energize. Very scary to have something designed like this. If the power company had a failure outside your home, I guess one just blows that off as bad luck. But to have a house wired 3-wire now seems awfully risky.
 
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Old 10-21-09, 03:01 PM
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Originally Posted by ibpooks View Post
The NEC covers only minimum safety standard of premises wiring.
Huh. Well....if that's the case. It just seems like some contradiction when from what seems like electrical inspectors and the codes are like the IRS, if you know what I mean. And then to say that minimal standards are covered. Huh.

Even still your MWBC has operated with only this minor malfunction for over 50 years -- I'd count that as a successful installation.
But when one considers that IF someone say, as was mentioned, started disconnecting wires live (as often electricians do) or something, and did not think about a 3-wire and/or otherwise that an important needed neutral was lost......this seems nuts. An accident waiting to happen....whether next week or 50 years in the future. I am a person who leaves nothing to chance. Such a wiring system leaves stuff to chance.

But answer me this: If say a condition arose, that spiked the current on one leg up to about 180.....if that really were to happen...shouldn't a circuit breaker trip? And not just some appliance fry, without tripping the breaker?

The lights (all apt. lights are on that circuit) never dimmed or got bright, she said. Nor did the breaker trip. And to send out an electrician to go all over the house, like a mole, opening stuff up, willy nilly, running up a huge tab, if there was not even a problem? Can you imagine?

Please comment on the light and no breaker tripping clue.

This type of over-voltage is really no different than if a power company transformer malfunctions, car hits a pole or lightning strikes the wires outside. Anything connected to the power line or external data lines risks damage if not protected by adequate surge protectors, line conditioners, voltage regulators, UPS or any other type of power quality device.
Maybe there is nothing within reason they can do about this. But they could have, regarding the house wiring.


One other note I'd like to add is that unless this was a particularly old computer I doubt a run of the mill overvoltage killed it. Nearly all computers, digital TVs and other consumer electronics are designed for international markets and can operate seamlessly on voltages from 100V to 250V without malfunction.
Ahhh. There indeed are 2 switch settings on the back. One is for like 115 and the other 230.
 
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Old 10-21-09, 03:12 PM
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Originally Posted by french277V View Post
Yeah at the first junciton box before you break out to single circuit always pigtail the netural conductor { my SOP is use the crimp so that way someone know this is a MWBC they can not able lift the netural at all like that }

Merci.
Marc
That sounds like a good idea. I wonder if they did that. Maybe they just twisted the wires and soldered and friction taped it, as how you often find stuff with old wiring. When they do just that sort of thing, one is not as quick to start unraveling it.
 
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Old 10-21-09, 03:38 PM
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But answer me this: If say a condition arose, that spiked the current on one leg up to about 180.....if that really were to happen...shouldn't a circuit breaker trip? And not just some appliance fry, without tripping the breaker?
You have some misconceptions or a typo error. Do you mean voltage spikes to 180 ? If so voltage is not what trips a breaker. Current flow trips breakers. Nothing changes the current... it is fixed by the loads plugged into the branch circuit. So if I have 4 light bulbs at 60 watts each (2 amps) and a counter top mixer at 9 amps then if all are 'on' I have 11 amps. Voltage will not change the current.
 
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Old 10-21-09, 04:14 PM
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Originally Posted by ecman51` View Post
started disconnecting wires live (as often electricians do) or something, and did not think about a 3-wire and/or otherwise that an important needed neutral was lost
Electricians are very aware of shared neutral circuits as they are used in virtually every commercial building you set foot in. It is less common in residential, but dealing with one is basically a daily occurrence. Furthermore, working live unnecessarily will earn you a nice unpaid vacation at many companies.

I am a person who leaves nothing to chance. Such a wiring system leaves stuff to chance.
As I've said, every circuit has the chance for failure -- some in much more dangerous ways than a MWBC. Perhaps you are someone who is willing to pay more upfront for a more reliable system, but not everyone is.

But answer me this: If say a condition arose, that spiked the current on one leg up to about 180.....if that really were to happen...shouldn't a circuit breaker trip?
The MWBC only affects voltage. Total current remains the same. Breakers trip only on excessive current.

The lights (all apt. lights are on that circuit) never dimmed or got bright, she said.
If the lights are also on the MWBC, then it suggests that there is not a problem with the neutral (at least up to that point where the lights split off) in the circuit.

Ahhh. There indeed are 2 switch settings on the back. One is for like 115 and the other 230.
That is the old style of power supply where voltage must be manually selected so it could have been damaged by high voltage. In the newer power supplies the voltage setting is automatically adjusted.
 
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Old 10-21-09, 05:17 PM
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Originally Posted by ibpooks View Post
Furthermore, working live unnecessarily will earn you a nice unpaid vacation at many companies.


The MWBC only affects voltage. Total current remains the same. Breakers trip only on excessive current.
Then what could make the appliance fry?

That is the old style of power supply where voltage must be manually selected so it could have been damaged by high voltage. In the newer power supplies the voltage setting is automatically adjusted.
Manually selected or automatically adjusted voltages? ??? Explain more.
 
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Old 10-21-09, 06:26 PM
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Ecman51.,

As you see what Ibpooks mention unpaided vaction due unnecessery live work and I have couple guys whom work with me they took a perment pink slip they are done due they did not follow the safety rules about live work.

Next question is how appainces can fry ?

Ok as far the appainaces are set for 120 volts like motor , controls etc but once you get up over 140 volts and higher you will run into issue due the electronic board can not take overvoltage unless automatic switching device { see bottom for more details } the reason why the higher voltage destory some appainces due you have very high ohm devices and it don't take much to blow it apart { that one thing you can not change the Ohm law it always the same reguarding of voltage it run thru }

The other question about automatic voltage slector and many new electronic devices are set up to automatic adjust to proper voltage level and lock it in but few can switch like surge or dip it can able automatic follow the voltage supply { only steady voltage }

The same thing with electronic flourscent ballast most new one have multi voltage senseing unit which it will read what voltage it is and lock it in and run in correct way.

Like example one ballast will work with 120 or 277 volt supply that is two most common but yeah if like 120-277 volts that mean it will run any voltage in that range.

on older computer and few other appiances they have manual voltage slection to slect the ballast or transfomer operation to correct one if some reason get overvoltage it will protrated with correct step to blow some electronic stuff no quesiton asked.

Merci.
Marc
 
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Old 10-22-09, 08:22 AM
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french,

So it is NOT just the total current in amps, but the voltage?, is what you are saying?

.........................................................

I have had the chance now to give this matter a little more thought. I understand it completely as to the crossing that can occur, and how the one leg can enter the other leg.

This is similar to my 24 years of experience with trailers and the old 50 amp boxes with cartridges fuses, how when one leg's cartridge fuse blew, you would get orange lights when you turned on the 240 volt stove, from a backfeed entering the leg whose cartridge had blown. I dealt with this alot, until wiring was updated to 100-200 amps with circuit breakers. The only difference between that, and this subject, is how, rather than having a dead leg where backflow can enter that dead leg but then get to neutral, but with no overcurrent or anything,....no harmeful effects really -

-with this 3-wire scenario though, you can have a backfeed that goes up an already energized circuit , and up the voltage on one circuit and lower it on the other.

I was laying in bed thinking about all this, and the relevency of how to complete this backfed loop, you must actually have an appliance turned on or a light turned on, for this to occur. Because of this, and the fact I have (pretty much...unsure about fridge tho')identified what is on those 2 circuits (red wire one and black wire one), it never dawned on me until this morning, after considering bruto's drawings, that I really need to turn on the microwave and toaster, and whatever else in the kitchen (these are on the black wire), one at a time, so I do not blow the circuit breaker - and then see how many volts I have in the LR (and other places where I have identified power on the red circuit.)

The lady told me also that all she has to do is run either her toaster and coffee maker in the kitchen, or, the coffeemaker and microwave, and it trips the black wire circuit breaker. Whether or not that has anything to do with the possible overcurrent problem associated with the red wire/LR circuit, or if it is simply something perhaps due simply with amp draw and the 15 amp circuit, is something I am going to look into. I can't remember if the fridge is on that black wire circuit.

I want to go back to that apt., as soon as possible, to test these things I mentioned, while at same time, taking a volt reading in LR while doing so. But the lady is gone to work. I'll call her up, set up a time, do these things, then get back to you guys with my findings.

But here is one more question I have, without studying those drawings again(but I will): IF there is this backfeed.... shouldn't then say if I were to turn off the black circuit breaker, that I would STILL get some current on that black circuit somewhere(as long as appliances were all turned on), if the ground got lost on that circuit somewhere? And if this is true, that I could use this finding, as well, to likely confirm if I had a bad neutral somewhere? (I do know it is not in the box, that I do know.)
 
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Old 10-25-09, 01:27 PM
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Maybe good idea to change out ground wire clamp on water pipe?

I checked out the lady's apt. again yesterday. I ran the kitchen's black wire circuit (fridge, microwave, toaster, coffee maker) to see then if I got a voltage to spike up the red living room circuit or not. Nope. All is well. No matter how I test, I show no variation in voltages.

She also said that she has never seen her lights flicker. And the lights are on that red wire circuit throughout the house!

She said heads are going to roll if she lose another tv, or equip.. This is the 2nd tv in the last year. She does not trust my abilities. She knows I am just a maintenance man,.... even though I proved to her I never made a mess in her apt., while sheetrock taping and mudding, while she sat on the couch and watched me, without sanding, .... as she threatened then that she had better not see dust get all over all her stuff.

She does not want to have to pay an electrician to have him not find anything, either. I did give her that choice after she learned the landlord will not pay for an electrician to search for a potentially non-existant problem. But would pay only if one was found.

The lady may just be unlucky. The landlord says that she buys cheap stuff.
..........................................
But my real thought here today is that I noticed the main ground wire that is for all 4 rentals, is very green in the area of the damp block wall above the main throw switch box. (Elsewhere it is coppery color looking).

The 6 ga.? copper wire originates inside that fused main throwswitch box where it is clamped to all 4 of the neutrals. That all looks good in there. The copper wire is clamped to the copper water pipe between the basement floor and meter.

But the clamp is real corroded. I do have continuity though beween the copper wire and the soil side of the clamp. But this might not prove the copper wire can pass full current if needed to do so, if the pole lost neutral, let's say.

Is it always wise to throw that main disconnect before changing out the clamp, in case the neutral were lost.....or to just do it any way?" Would a licensed electrician be mindful to do that?
 
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Old 10-25-09, 01:50 PM
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How hard would it be to run a second 2-conductor cable to the point where the current 3-conductor branches off? If you did that you could abandon one leg of the three conductor and use two single pole breakers. This would not eliminate all voltage spikes but at least eliminate spikes from a bad neutral on the current MWBC.
 
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Old 10-25-09, 02:43 PM
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Do you have the answer to my question, regarding the clamp?

Answering yours - the landlord is too cheap. He will fix a known proven problem, but this is not proven.

I accessed another apt.'s panel box this morning, and found it is wired the same way, with the 2 circuits having the shared neutral.

The wiring has been this way for years and years, and nobody has ever said anything about a problem of any sort. The biggest problem actually is unrelated, and should be fixed; the whole kitchen, including fridge, is all on one circuit!. He won't even fix that. He just tells people that they can't be plugging in all this stuff at once, if the subject comes up.

The apts. used to be on that old fashion combo bus fuses/glass fuses 60 amp service. And he told me that most of each apt. is wired on those 2 15 amp circuits.

He is not about to rewire out of the goodness of his heart. Electricians give out rewire bills often into the thousands of dollars.
 
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Old 10-25-09, 03:24 PM
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Certainly cleaning up the clamp is a good idea but the ground is only for safety should something fail. The circuit should work even with the ground disconnected so that is not causing your problem though it could be a safety issue.
 
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