why i HATE shared NEUTRALS!!

[George Stolz]

What tool are you referring to?

Just a Vol Con Low Impedance tester. In retrospect, I realize that the tick tracer's almost as effective when used properly (for the situation), but I've run into a couple situations where the Wiggy would come in handy, so that was my excuse. I had another poorly stabbed watt-hour-meter a couple weeks ago, causing a phase to disappear under load. It would have been a lot clearer with a Wiggy than the tick tracer.

I figure it's inexpensive enough, and accurate enough, light enough, and more importantly rugged enough, to sit on my bags full time. I sure wouldn't want to put my ampclamp on my bags, it's too expensive.

 

[ramsy]

Thx George. My tic-tracer fails to dectect most of these balanced neutrals also. Not sure why?

My jolting was also on top a ladder, touching ceiling grid, while wiring 277vac flourescent fixtures. Not sure if that neutral served seperate full boats, but it was tied to the same j-box and balanced, since my tic tracer showed nothing on it while churping at the phases just before they were de-energized.

The mild jolting was more frightening than injurous, and my grief was directed as the bundle of white wire sharing the same wire nut. Glad to hear a solinoid tester might work better than a tic tracer. Will try my SQ-D Wiggy next time.

 

[George Stolz]

In either case, the testers would only show the voltage if the neutrals were severed; but the tick tracer could have a false positive once it's no longer grounded (inductance), so the Wiggy would be more honest. Instead of not believing a glowing tick tracer, I'm going for believing the Wiggy, should the need arise.

 

[brother]

In either case, the testers would only show the voltage if the neutrals were severed; but the tick tracer could have a false positive once it's no longer grounded (inductance), so the Wiggy would be more honest. Instead of not believing a glowing tick tracer, I'm going for believing the Wiggy, should the need arise.

I suppose an amprobe wouldnt get it either if neutral was balanced.
Take the following EXTREME scenerio in COMMECIAL buildings. You have a 3 hot 1 common neutral circuit, Black, red, blue (full boat). Your working on the 'blue' circuit' . You shut it off/lock out tag out, the 'blue circuit'.

You test the circuit and it is off and also put an amprobe on the neutral and you have a 'true' neutral' no amps cause the black and red circuit is perfectly balanced.
Of course the black and red circuit do NOT pass through the same j box nor are in the same conduit leaving the panel so there is NO hint of that neutral being shared. Also that black and red circuit is on and carrying a LOAD (someone in another part of the commercial office is using their computer and copier).

You still treat the wires as being HOT even the neutral even after you losen the wirenut and check the neutral to ground, you find voltage for a moment then POOF!! black and red circuit fried the computer/copier in about 2 or 4 offices down from you cause soon as you disconnected that neutral there was 208 volts going to them.
Someone later comes screaming about their computer being fried and running from about 2 or 4 offices down from where you working, you dont hear about till later that day LOL .

Tell me what could have been done to AVOID frying that computer, you searched for the shared neutral, looked for and tested for, used an amprobe and u still had no hint of the neutral being shared. What else could you have done short of turning OFF the whole PANEL ?? Im still learning so let me know.

 

[roger]

You test the circuit and it is off and also put an amprobe on the neutral and you have a 'true' neutral' no amps cause the black and red circuit is perfectly balanced.

In a Wye system the neutral would have the same current flowing on it as the Black and Red circuits. It would only be a true neutral (zero amps) if all three phases were on and balanced.

You can see this for yourself using the following formula

SQRT[(A*A+B*B)-(A*B)]

Roger

 

[celtic]

Of course the black and red circuit do NOT pass through the same j box nor are in the same conduit leaving the panel so there is NO hint of that neutral being shared.

How is that a shared neutral?

 

[infinity]

You still treat the wires as being HOT even the neutral even after you losen the wirenut and check the neutral to ground, you find voltage for a moment then POOF!! black and red circuit fried the computer/copier in about 2 or 4 offices down from you cause soon as you disconnected that neutral there was 208 volts going to them.
Someone later comes screaming about their computer being fried and running from about 2 or 4 offices down from where you working, you dont hear about till later that day LOL .

If the loads on the black and red circuit were perfectly balanced an open neutral would not cause the voltage at each computer to be 208 volts. They would effectively be in series and would divide the 208 volts between them.

For example if you had a 100 watt lamp on the black phase and a 100 watt lamp on the red phase, the loss of the common neutral would put the two loads in series with a voltage drop at each lamp of 104 volts. The lamps would not "fry" but would become noticeably dimmer.

 

[LarryFine]

I suppose an amprobe wouldnt get it either if neutral was balanced.
Take the following EXTREME scenerio in COMMECIAL buildings. You have a 3 hot 1 common neutral circuit, Black, red, blue (full boat). Your working on the 'blue' circuit' . You shut it off/lock out tag out, the 'blue circuit'.

You test the circuit and it is off and also put an amprobe on the neutral and you have a 'true' neutral' no amps cause the black and red circuit is perfectly balanced.

On two legs of a "full boat", the neutral will not have zero amps, but will have exactly the same current as the missing third leg had when it was on and the three legs balanced.

Of course the black and red circuit do NOT pass through the same j box nor are in the same conduit leaving the panel so there is NO hint of that neutral being shared.

If the black and red legs are not in this box, then neither would there be their shared neutral. The hots and neutrals of any circuit must always be grouped. In other words, any place the neutral is accessible and carrying more than one circuit's load, the corresponding hots will be in the same box.

There is no excuse for not being aware that you are looking at a shared neutral. I've never opened a neutral junction that I didn't know was carrying current. A quick look inside the panel will tell you if the neutral is shared, and by which other circuits.

I always place shared-neutral-circuit breakers side-by-side, even if it requires re-labeling the panel, mainly because it minimizes the possibility of same-phasing ond overloading a neutral. I hate seeing three hots from a full-boat conduit land on breakers spread out in a panel.

Your argument is more towards the use of handle ties for shared-neutral circuits than it is against shared neutrals. The bottom line is that you should always be aware when you're looking at a neutral that supplies, or possibly supplies, more than one circuit.

Also that black and red circuit is on and carrying a LOAD (someone in another part of the commercial office is using their computer and copier).

Again, you should anticipate this, identify the other circuits, and lock-out and de-energize them, routinely.

You still treat the wires as being HOT even the neutral . . .

Yes, always.

Tell me what could have been done to AVOID frying that computer, you searched for the shared neutral, looked for and tested for, used an amprobe and u still had no hint of the neutral being shared. What else could you have done short of turning OFF the whole PANEL ??

I believe I have.

Im still learning so let me know.

That's the best thing you wrote. Keep up the great attitude!

If the loads on the black and red circuit were perfectly balanced an open neutral would not cause the voltage at each computer to be 208 volts. They would effectively be in series and would divide the 208 volts between them.

For example if you had a 100 watt lamp on the black phase and a 100 watt lamp on the red phase, the loss of the common neutral would put the two loads in series with a voltage drop at each lamp of 104 volts. The lamps would not "fry" but would become noticeably dimmer.

True. But now, try the same experiment, but with one 100-watt load and one 1000-watt load.

 

[infinity]

True. But now, try the same experiment, but with one 100-watt load and one 1000-watt load.

This is true but Brothers example said that the loads were the same, hence my example.

You test the circuit and it is off and also put an amprobe on the neutral and you have a 'true' neutral' no amps cause the black and red circuit is perfectly balanced.

 

[jwelectric]

Brother;

Help me understand what you are saying as I am a little confused.

Of course the black and red circuit do NOT pass through the same j box nor are in the same conduit leaving the panel so there is NO hint of that neutral being shared. Also that black and red circuit is on and carrying a LOAD (someone in another part of the commercial office is using their computer and copier).

If all the circuits are not together why would the grounded (neutral) be carrying the current of the other circuits?

Of course the black and red circuit do NOT pass through the same j box

Did the Blue and a grounded (neutral) break off as a two wire circuit from somewhere?
Would this not be just a two wire circuit at this point?
Or did the original installation consist of one grounded (neutral) looping through all the conduits and boxes?

nor are in the same conduit leaving the panel so there is NO hint of that neutral being shared.

How was the grounded (neutral) installed in this installation?
Was there a grounded (neutral) installed with the conductors in each conduit?

I am having a hard time visualizing how these installations could share the same neutral.
Should the Blue and a white leave a junction box that contained a multiwire circuit at that point the neutral would not be sharing the load of the other circuits.
Should the Blue wire leave the panel with a white wire and the Black and Red wire leave the panel with another white wire then the neutral with the Blue wire would not be sharing the load of the other two.
It would take some thinking and a violation of the code to have only the Blue and white conductor present on a junction box or conduit and it still share the load of the other two circuits.
I am having a hard time visualizing a junction box or piece of equipment that only contains one blue and one white that is sharing a neutral.

If you would be so kind would you take a few minutes and explain to me how this installation is installed where the three are sharing the neutral but all three are not present.

 

[George Stolz]

Here's a picture:

Should the neutral be broken at the Black phase, #3 fixture, how does the computer get damaged?

 

[Shockedby277v]

Nice pic. What program did you use to make that??

 

[jwelectric]

Here's a picture:

Should the neutral be broken at the Black phase, #3 fixture, how does the computer get damaged?

George you know that I love you to death and I would not ask this question except for the fact that I love to pick on you as much as I can.

In the picture that you posted you show arrows pointing to four wire cables. One leaving the panel, one between the red and blue and one between the blue and black.

Why would you continue the four wire cable between the red and blue when a three wire cable would work and between the blue and black when a two conductor cable would work?

Now I know that you have money to burn but with the cooper prices being what they are I would have to take a mortgage on the house to buy this much four wire cable when I wouldn’t be using all the conductors.

Now before you get mad and come over to my house and beat me about the head I think this picture says it all. Good job!

Edited for spelling

 

[ramsy]

If you would be so kind (to) explain how three (phases) are sharing the neutral but all three are not present.

George posted a link that showed the best examples of this.

I suppose an amprobe wouldnt get it either if neutral was balanced.

..Tell me what could have been done to AVOID frying that computer, you searched for the shared neutral, looked for and tested for, used an amprobe and u still had no hint of the neutral being shared.

My tic tracer wont pick up a balanced neutral either.

Bottom line, class 0 rubber gloves. Leather gloves are not dielectric rated, even if more comfortable, but may be better than nothing.

Another solution is Common Mode Voltage (CMV). That neutral can be checked, without lifting it. According to my ShureTest manual:

Common Mode voltage (CMV). DVM must resolve fractions of 1 volt.
1) G-N > 0.0 vac RMS. Any CMV potential shows an active neutral.                                
2) G-N > 2.0 vac RMS. Harmonic current is likely.
3) G-N=> 2-4 Branch may simply be loaded near capacity, but still usable.

This CMV methods is usually done on recepticles. The problem with j-boxes is getting a voltage-test lead into the wirenut, much less a Scotchguard brand wirenut, as desribed earlier in this thread.

 

[brother]

Brother;

Help me understand what you are saying as I am a little confused.



If all the circuits are not together why would the grounded (neutral) be carrying the current of the other circuits?


Did the Blue and a grounded (neutral) break off as a two wire circuit from somewhere?
Would this not be just a two wire circuit at this point?
Or did the original installation consist of one grounded (neutral) looping through all the conduits and boxes?


How was the grounded (neutral) installed in this installation?
Was there a grounded (neutral) installed with the conductors in each conduit?

I am having a hard time visualizing how these installations could share the same neutral.
Should the Blue and a white leave a junction box that contained a multiwire circuit at that point the neutral would not be sharing the load of the other circuits.
Should the Blue wire leave the panel with a white wire and the Black and Red wire leave the panel with another white wire then the neutral with the Blue wire would not be sharing the load of the other two.
It would take some thinking and a violation of the code to have only the Blue and white conductor present on a junction box or conduit and it still share the load of the other two circuits.
I am having a hard time visualizing a junction box or piece of equipment that only contains one blue and one white that is sharing a neutral.

If you would be so kind would you take a few minutes and explain to me how this installation is installed where the three are sharing the neutral but all three are not present.

Ok , im sorry i dont have a program to put up pictures, but ill try to explain my HYPOTHETICAL scenerio. heres a link to the 'y' distrubution' system.

http://www.powervar.com/english/pdf/wp202.pdf#search='shared%20neutral%20circuits%20balanced'

heres the other, ive put this one up before http://www.eh.doe.gov/paa/oesummary/oesummary2005/oes2005-15-01.pdf

it has some of the info im talking about. I will have to agree, there would HAVE to be an ILLEGAL/AGAINST CODE installation for this to happen, and i have seen illegal installations.

In a common single phase 120/240 volt residential service...(called a 3 wire service - having two hot legs and one neutral at the main)...this translates to 2 circuits being allowed to share the neutral return to the panel...as long as they are on the 2 different legs of the 120 volts (so that one breaker lands on one section of the buss...and the other breaker lands on the other section of buss) The two 120 volt incoming legs are 180 degrees away from each other...technically, they could be referred to as leg 1 being +120 volts, leg 2 being -120 volts...and because - at any given moment during the 60 cycles (written as 60 Hz) one is Up while the other is Down...the neutral loads effectively cancel one another. For example, let's say breaker 1 has a 9 amp load....and breaker 2 has a 12 amp load. IF they were on the same buss and sharing that neutral....the total neutral load would be ADDITIVE...making for a 21 amp load on the neutral - overloading it and you do NOT want to see this. But - if they are opposite legs of the buss....the positive neutral cancels the negative neutral...so the Plus 9 amps on breaker 1 cancels the Negative 9 amps from breaker 2....leaving the neutral to carry only the unbalanced left over portion...or 3 amps ( clearly a much better scenario then the 21 amps that would cause a shared neutral melt-down over time).

This is a quote i found froma another site. This is my understanding as well.
If you had a circuit with just 2 hots (on different phases of courseJ), and they both had the same amps, would they not cancel each other out and you have a balanced system?? a true neutral, for example if ckt 1 was 10 amps and ckt 2 was 10 amps would they not cancel each other??. I have to do my review on my theory. but i thought this was how it went. I had a link showing this but now i cant find GRRRRRR

 

[roger]

Brother, in your post at 12:58 this morning you were talking about a three phase Wye installation, in your latest post you have changed to using a single phase installation, they don't work the same when sharing a neutral in a MWBC.

Let's go back to your 12:58 AM post and give the Black and Red a value of 10 amps each, now using the formula I gave you,

SQRT[(A*A+B*B)-(A*B)]

The first part of the equation would be 10 x 10 + 10 x 10 = 200

The second part would be 10 x 10 =100

Then we would subtract the second part from the first 200 - 100 =100

The Square Root of 100 = 10

So, the neutral is carrying the same current as A and B phase circuits.

You do not have a "true neutral" when dealing with two phases of a "full boat", so in your earlier scenario an ammeter would show current.

Roger

 

[roger]

Here is what you are trying to describe in the Single Phase Scenarios of your last post.

A True Neutral

An Unbalanced Neutral

A Non Edison Circuit

Remember that these graphics aren't true in a Wye MWBC


Roger

 

[George Stolz]

Ok , im sorry i dont have a program to put up pictures, but ill try to explain my HYPOTHETICAL scenerio.

You can just verbally amend mine, and if you'd like, I can amend it on my Mac and repost it with your changes. That's part of the reason I posted it with labels (Lights 1 - 7).

I will have to agree, there would HAVE to be an ILLEGAL/AGAINST CODE installation for this to happen, and i have seen illegal installations.

So, essentially, we're coming to an agreement that the MWBC is perfectly fine, but it tends to act as a magnifying glass to problems of misinstallations.

If you had a circuit with just 2 hots (on different phases of courseJ), and they both had the same amps, would they not cancel each other out and you have a balanced system?? a true neutral, for example if ckt 1 was 10 amps and ckt 2 was 10 amps would they not cancel each other

On a single phase, 120/240 system, yes. On a 120/208, no.

My tic tracer wont pick up a balanced neutral either.

To be clear, are you talking about an intact neutral? I thought no grounded conductor would be read by a tick tracer, reagrdless of how many amps are passing through the conductor. There is no potential to ground (normally) so there would be nothing for the tick-tracer to detect.

ShockedBy277, I have an iMac with Appleworks 6 on it. (Although, I just bought a new emachine on credit. I'm having a heck of a time getting my iMac to play nicely with this Windows machine. The networking works all right, I can access the internet through the emachine's modem connection, but getting files from the Mac to the emachine is so far, impossible.)

Mike, I wouldn't be happy if I didn't give you at least a toehold to pick on me.

 

[brother]

So, essentially, we're coming to an agreement that the MWBC is perfectly fine, but it tends to act as a magnifying glass to problems of misinstallations.


On a single phase, 120/240 system, yes. On a 120/208, no.

Well i dont agree with the 'perfectly fine' but i will agree that it is code. And should be installed by well trained individuals.

Here is what you are trying to describe in the Single Phase Scenarios of your last post.A True Neutral

Thanks roger, thats what i was looking for.
Well ok, maybe the scenerio would not be the same in a 120/208 circuit. but what about the picture roger shows. lets say there is a 3rd circuit in that 120/240. so you have 2 legs on the same phase lets say its 5 amps and the other is 10 amps on the same leg and the next phase has the 10 amps Seeing that current is additive on the same leg. So you have 15 amps - 10 amps = 5 amps. So the neutral is carrying back 5 amps.

So your working on that circuit and you turn off that 3rd circuit (5amp), now you have a 'true neutral.' You use your amprobe, and no amps, u proceed to disconnect the neutral and POOF!! somewhere in that expensive and Big 5000 square foot house the $3000. tv gets fried. Homeowner wouldnt turn off the electronic equipments cause you were working on a different circuit in the house. What else could you have done to try to make sure that neutral wasnt shared by the other circuits. Remember they were not in the same cable/conduit.

 

[roger]

So your working on that circuit and you turn off that 3rd circuit (5amp), now you have a 'true neutral.' You use your amprobe, and no amps, u proceed to disconnect the neutral and POOF!!

Brother you would not have a "POOF" in this scenario, this "true neutral" can be removed without affecting anything.

When we have a single phase "true neutral" condition (improbable) the circuit becomes a simple series circuit dropping equal voltage across the loads and conductors to zero at the center of the circuit and the neutral is doing nothing.

Roger