ECN Forum
Posted By: sparkync sharing neutral - 01/18/06 12:43 AM
Is there any conditions for 120 volt circuits where you can't share the neutral?
I am wiring circuits for computers in a customer's new business location. I have a 3 phase service and am planning on pulling 3 phase conductors and 1 neutral. Wondering if I am missing something on these computer circuits. Also have some copiers that I am planning on doing the same with. Any comments yea or nay will be appreciated. I believe I'm right according to code, but just double checking. Thanks, Steve
Posted By: Dnkldorf Re: sharing neutral - 01/18/06 12:51 AM
I'll tell you don't do it.

I Don't share nuetrals in a commercial enviroment for many reasons, from harmonics, to burning the place down due to a loose nuetral on one of the multiwire circuits you ran..


Dnk...
Posted By: Roger Re: sharing neutral - 01/18/06 01:02 AM
Steve, for electronic circuits it is a good idea to use an oversized neutral (super neutral) for three phase multi-wire branch circuits or use two wire circuits.

Other than that, multi-wire branch circuits are some of the most sensible wiring methods used.

Roger
Posted By: renosteinke Re: sharing neutral - 01/18/06 01:24 AM
A lot of attention has been given to "isolated grounds" for computers and the like. This also a real market sor surge suppression and UPS (back-up power) systems.

While not required by code, nor ever mentioned in the specs for any such system, it is my practice to NOT share the neutral between separate circuits.
By running a dedicated nuetral, I avoid the whole harmonics issue. More important, to me, is that I also eliminate one source of a voltage surge- a loose neutral connection. I'll even go so far as to identify the neutrals in the panel and junction boxes.

You are correct in your understanding of the code- this is just one area where I chose to go "beyond" code minimums.
Posted By: Dnkldorf Re: sharing neutral - 01/18/06 01:41 AM
Typically, for commercial applications, I will also run oversized wiring too.

By doing that, you avoid the multiwire disasters, and you also reduce line losses, where some believe, this is an advantage of a multiwire cicuit.


Dnk...
Posted By: iwire Re: sharing neutral - 01/18/06 01:45 AM
Gee I have no disasters and I run a lot of multiwire branch circuits.

Quote
By running a dedicated nuetral, I avoid the whole harmonics issue.

What do you do at the panel?

[This message has been edited by iwire (edited 01-17-2006).]
Posted By: Dnkldorf Re: sharing neutral - 01/18/06 01:54 AM
Bob, went on one last week, someone ran a 3 circuit, one nuetral to multiple outlets, simlilar to what the poster is asking.
Aparently, a nuetral connection came loose in a recptacle box. All the surge protectors plugged in smoked and melted, the printer/copier burnt up, and needed boards, fire dept called, the whole mess.

For the cost of wire, and the potential for disaster, I don't do it.

Just for fairness, multiwires are good for lighting, if both circuits are matched closely.


Dnk...
Posted By: Roger Re: sharing neutral - 01/18/06 02:05 AM
Dnk, it's not just the cost of wire, it's also natural resources including fuel for generation.

We literally install thousands of multi-wire circuits every year and I only know of one open neutral problem we have had in the last ten years, which had been tampered with by the particular facilities maintenance staff.

John, a dedicated grounded conductor to one ungrounded conductor is not a neutral. [Linked Image]

I have seen more open service neutrals than branch circuit or feeder neutrals by far.

Roger

[This message has been edited by Roger (edited 01-17-2006).]
Posted By: iwire Re: sharing neutral - 01/18/06 02:06 AM
I will install what the specifications allow or require.

My preference on longer circuits is multiwire. [Linked Image]
Posted By: gfretwell Re: sharing neutral - 01/18/06 03:20 AM
Reno is right about IG.
I was an installating planning rep at IBM for 5 of my years there. Isolated Ground is a waste of money. IBM stopped mentioning it in the Carter administration but the legend lives on in the mind of "experts". If you like the customer, save them some money and talk them out of it. If they really insist, money is not an issue for them so sell them "monster" super neutrals at the appropriate price. Then you can take the family to Hawaii this summer.
Posted By: e57 Re: sharing neutral - 01/18/06 07:58 AM
3-wire, and 4-wire circuits are your friends. You be nice to them, and they be nice to you... You give them a nice balanced load with some tight connections, and they give you a little extra money in your pocket for not running extra cables, conduit, and 1-2 extra conductors. In fact, it needs to be specified for me to do anything else.

Granted harmonic issues are on the rise due to the type of loads that generate them are on the increase. But I don't see that as a reason to stop using them. Lost neutrals can be problematic, but only if you dont follow a few simple rules in usage of them.
  • Tight connections that are not totally dependant on the cheapest wire nut known.
    (WW-3 - Twisting does help here!)
  • Balanced, or even relitively balanced loading.
  • Circuit design, branching away from 3-wire, and 4-wire circuits once you have reached an area, to keep the number of connections on the shared neutral portion down. Instead on daisey chaining through every point. Get rid of one circuit right off by taking a HR to a dedicated load, find a load to match for the next, and so on.



Oh, I heard a wierd copper statisic today. We have now surpassed what is left in the ground, (expected yield) compared to what is already in use. But that could just be a ruse to keep the price up...



[This message has been edited by e57 (edited 01-18-2006).]
Posted By: Dnkldorf Re: sharing neutral - 01/18/06 12:06 PM
To the guys that like the use, do you mark the shared nuetral somehow in j/device boxes?


Dnk..
Posted By: renosteinke Re: sharing neutral - 01/18/06 04:13 PM
What to do about harmonics at the panel?

Pretty much nothing. In these set-ups, my panel neutral (and feed) is already a lot larger than double the size of the #12 branch circuit conductors. Then, there is a short run to the transfer switch, with the monster battery-bank UPS and isolated transformer close at hand.
Get harmonics through those, and you probably shoud re-consider your appliances!

:-)

And, yes, I absolutely agree that twisting the wires helps!
Posted By: tdhorne Re: sharing neutral - 01/18/06 05:59 PM
If you are really paranoid about open neutrals then you could consider using crimp caps on the multiwire neutral splices. The critical item in using them effectively is to use the manufacturers crimp tool and set it for the sleeve you are applying. As they are not readily opened there would be less chance of an open neutral.
--
Tom Horne
Posted By: resqcapt19 Re: sharing neutral - 01/18/06 06:31 PM
John,
Quote
By running a dedicated nuetral, I avoid the whole harmonics issue.
No, all you have done is to mask one symptom of the harmonics issue. You have reduced the excessive heating and voltage drop in the neutral that can because by excessive harmonics. All of the other power quality problems are still there.
Don
Posted By: Dnkldorf Re: sharing neutral - 01/18/06 09:46 PM
Let's think about this.....

He wants to run 3 hots with a nuetral.

ASSUMPTION: he has a 208Y120 service.

So if we have 2 of the circuits at 20a each and the third circuit has nothing plugged into it.

We should have 20a on the nuetral? no biggie so far. However, with laser printers, power supplies, copiers ect. introducing harmonic currents on the nuetral also, you see we could be taking chances with the nuetral becoming overloaded. Now lets for fun do this for a whole commercial building for todays electronic equipment. We start to have problems.

They other problem I have with them, is the 1 loose connection problem.

I'll give you that some multi-wire circuits can be used for lighting, maybe a hair salon and such. But for todays typical commercial applications, I'll choose to stay away from them.

I don't think there is no right or wrong here, just preference.....


Dnk...
Posted By: resqcapt19 Re: sharing neutral - 01/19/06 12:28 AM
Has anyone ever seen an article written about harmonic problems that was not written by someone with an economic interest in solving the problem? I know a number of electrical engineers who do not specify oversized neutrals for large office building where we have all been told that there would be a "harmonic problem". They have never experienced problems. I have read of very very few documented cases of problems caused by harmonics. Most failures are a result of poor installation and not harmonics.
Don
Posted By: iwire Re: sharing neutral - 01/19/06 12:39 AM
dnk you are right it's just a preference thing and by all means do what makes you happy.

Quote
We should have 20a on the nuetral? no biggie so far. However, with laser printers, power supplies, copiers ect. introducing harmonic currents on the nuetral also, you see we could be taking chances with the nuetral becoming overloaded.

1) What is 12 AWG rated at 75 C?

2) Many times the cables we use are super neutral.

Quote
Now lets for fun do this for a whole commercial building for todays electronic equipment. We start to have problems.

Perhaps, but the use of multiwire or two wire circuits only deals with the branch circuits.

All the panels and feeders will be subjected to the same amount of harmonics using multiwire or two wire circuits.

It is common in the buildings we wire that all the 208 neutrals, including the panelboard neutral bars will be 200% of the phase conductors.

All that said, I agree with Don, harmonic currents are not new, they existed before PCs came along. The only new thing is need to fix the problem.
Posted By: Dnkldorf Re: sharing neutral - 01/19/06 12:52 AM
I am not sure you can fix the problem as you say, the problem in inherent with the devices we use.

Using filters, mitigating transfomers and such help indeed.

Everyone has their own way, some for different reasons or for different results.

Don, I had a problem/project that I am just finishing. After months of measuring and consulting with suppport enginneering from Cutler Hammer, we eliminated excess heat from the system, directlty caused by harmonics.
The savings are qiute pleasing.

Dnk...
Posted By: e57 Re: sharing neutral - 01/19/06 08:58 AM
"consider using crimp caps on the multiwire neutral splices" Like the idea, but experiance drives me the other way. Did many years of service work where crimps in current carrying conductors would fail. They heat slightly in use, and due to evironmental temp changes, and eventually work loose under load, and start to arc. I don't even like them for grounding. But if twited AND crimped, I might be OK with that...

"So if we have 2 of the circuits at 20a each and the third circuit has nothing plugged into it."

(Assuming they are 20 circuits...) If you have a circuit pushing a full 20A there are going to be problems anyway. [Linked Image] Try to keep the loads even and well below 80%... Even if you had that scenario with 2 @ 20A on a wye system the neutral load won't be more than 20A. And if one was slighly higher or lower, it would go down, instead of up. Say A were @ 15A, B @ 10A, C @ 0A, N @ ~13A... #12 THHN is actualy rated for 30A. (Before derating, or 240.4d) With a 2-wire circuit on a wye system @ xA, the neutral is xA, with no chance for the benefit of an unbalanced load to offset it. IMO

"To the guys that like the use, do you mark the shared nuetral somehow in j/device boxes?" We normaly group the conductors of one roundhouse together with some tape, or a different colored lettering on each. So say I had two, 4-wire circuits... One circuit would have all the conductors in that circuit with a wrap of tape normally. We do the same for 2-wire circuits so we don't mix them up... Some of the larger supply houses, if you order in advance, can get you different colors on the lettering, but that is only usefull if you are doing a WHOLE job, not very usefull on remodels. And you can get stripped wire in different colors if you have more than one transformer in the building.
Posted By: Dnkldorf Re: sharing neutral - 01/19/06 12:04 PM
E57, I thought the same thing a while back.

---------------------------------------------

Charlie, I am still missing something here.
Her are my readings:

Aphase 22a
Bphase 24a
Cphase 23a

The nuetral has 30a on it. I keep coming up with 1.3 or 1.7 amps. Is the math wrong or do I have harmonics problems?

---------------------------------------------

This was a discusssion we had on the subject last yr, when I stumbled upon this problem.

It is very possible to have more current on the nuetral, than on the phase conductors.

This is the whole link, https://www.electrical-contractor.net/ubb/Forum7/HTML/000372.html


Dnk....
Posted By: mxslick Re: sharing neutral - 01/19/06 07:12 PM
Quote
I'll give you that some multi-wire circuits can be used for lighting, maybe a hair salon and such.

Nope, I have to disagree. In my brother-in-law's first salon location, the lighting (all 2 x 4 troffers) was wired with a shared neutral and two of the three phases used. We (meaning me) were constantly replacing ballasts.

Now in the new shop (see "The case of the missing phase.." thread) I wired all lighting with dedicated neutrals per phase and we have lost ONE ballast in 15+ years. Hmmmm......I seriously doubt that the quality of the troffers has gotten better over the years. [Linked Image]

In the cinema business, sound racks are frequently served with more than one circuit. By 20 years of experience I can positively state that the racks wired with shared neutrals have more problems with induced noise and premature amplifier failure than those where dedicated neutrals are used. In fact, I will VOID WARRANTY on any sound equipment wired/installed with shared neutrals.

Some people can argue that from a strict engineering/theoretical standpoint that it shouldn't matter, as long as all connections are tight and the circuit is not overloaded.

But electricity, despite many years of application, is a strange beast and will behave in unexpected and unpredictible ways. To quote from a book I have, "Industrial and Control Power Systems Handbook"; {Pub. McGraw-Hill, auth. F.S. Prabhakara, Robt. L. Smith Jr., Ray P. Stratford} :

Quote
One prominent systems engineer has stated that is mathematically impossible to tie the {power} grid together, but the work and study of electrical industry engineers has made it possible.
{added text by me} Bold emphasis mine also.

In summary, shared neutrals for branch circuits of any kind are not a good idea. As I've said many times on the forum, just because the Code allows it doesn't mean it's always safest or right.

The arguement of increased cost, especially on large jobs, doesn't wash with me. If you're spending that kind of money on large job, what is the REAL cost of the extra wire? In actual percentages, please? On a 20-screen cinema, I would spend maybe an extra $50.00 on wire. On a total materials bill of over $4,000.00

Just my 25 cents. (Used to be 2 cents, inflation you know. [Linked Image] )

edited to fix first quote

[This message has been edited by mxslick (edited 01-19-2006).]
Posted By: ghost307 Re: sharing neutral - 01/19/06 07:43 PM
Remember that GFCI and AFCI breakers won't work without a neutral that is dedicated to the phase wire.
Sharing the neutral will just end up guaranteeing a callback to rewire the circuit.
Posted By: iwire Re: sharing neutral - 01/19/06 08:09 PM
mxslick

Quote
Some people can argue that from a strict engineering/theoretical standpoint that it shouldn't matter, as long as all connections are tight and the circuit is not overloaded.

But electricity, despite many years of application, is a strange beast and will behave in unexpected and unpredictable ways.

I sure would like someone to explain the magic that must happen at a panel as all the two wire circuits turn into a large multiwire circuit at the panel. [Linked Image]

This is a serious question.

What makes all these problems 'go way' if you run two wire circuits into a panel.'

It makes no sense at all.
Posted By: Dnkldorf Re: sharing neutral - 01/20/06 12:58 AM
Bob, could you be confusing a 240V 180 degree shift system vs a 208V 120 degree system, in regards to the balanced nuetral currents?

Dnk...
Posted By: wa2ise Re: sharing neutral - 01/20/06 02:57 AM
A possible problem with a multiwire circuit might be: If one side of the multiwire with shared neutral has a large transient load (like an air conditioner or fridge turning on, or heavy loading of computers and other such electronics on it(big spikes of current at every peak of the powerline voltage), the neutral and the hot that is serving the big loads will have voltage drop, half of which will appear on the neutral. But that voltage drop on the neutral will look like a voltage *increase* on the othr side of the multiwire circuit, and if severe enough it could blow up loads on it. Separate neutrals won't have this happen. But the neutral gets shared at the panel, but that neutral us usually much heavier and have miminal voltage drops.
Posted By: e57 Re: sharing neutral - 01/20/06 05:03 AM
wa2ise, very well said... a 4-wire circuit is still a "circuit", and one would consider it common sense to apply like loads, not say sensitive AV equip, and HVAC. Or a MIG welding rig and an HDTV... Thats just crazy talk! [Linked Image]

Anyway, I'm with Bob on this... It is still there at the panel and service. You could even inheret problems from the building next door... Short of a seperate transformer or the like, you really can not get rid of, aleaviate the voltage portion of harmonics, surges or spikes off the system. The benifit of a 2-wire is that you will reduce the chances of voltage drop related issues due to load, and current portion of harmonics to a greater degree, but even they are still present at the panel although to a lesser degree.
Posted By: mxslick Re: sharing neutral - 01/20/06 07:22 AM
Quote
I sure would like someone to explain the magic that must happen at a panel as all the two wire circuits turn into a large multiwire circuit at the panel.

This is a serious question.

I'd like someone to explain it to me, too. But all my experience so far bears out the use of shared neuts has been nothing but trouble. Even when the workmanship and all connections are good.

Quote
What makes all these problems 'go way' if you run two wire circuits into a panel.'

As a guess, I would say that the size of the neutral buss and feeder in relation to the instantanious current flow presents a much lower impedance than the smaller conductor of the shared neutral and/or return flow to opposite phases, and thus the current (or harmonics) see a lower impedance at the panel, creating less heating and other nasties. (Maybe why some swear by the "superneutral" concept in multiwire.)

Quote
It makes no sense at all.

My quote from the book was meant to point out the magic of the grid system as well. Taken to the mathematic literal sense, I understand that to tie multiple sources of generation reliably without voltage or frequency control issues, the load MUST be precisely matched to the generation available. Obviously a huge generator governor cannot respond to someone turning on (or off) a toaster, for example. It is only the resistance, reactance and impedance of the miles of lines, transformers, etc. that such load diversity doesn't cause problems. I try not to think about it too much, it gives me a headache. [Linked Image]

The poco guys I know tell me stories of things that aren't supposed to happen, should never happen and are theoretically impossible to happen..but do. There are so many variables involved I feel there's no way in heck we'll ever get a definitive, one size fits all answer.


But that's another whole topic, so back on track...
Posted By: Roger Re: sharing neutral - 01/20/06 11:15 AM
Mxslick, in the last seven years I've writen P.O.'s for around 5000 flourescent fixtures (electronic ballast) of which 95% have been connected to MWBC's, there has been less than 30 ballasts changed in the 1 year warranty period on these projects combined.

The problem you were having was most likely another PQ situation.

To those who do not want to use them, I say don't, I will continue to.

Back to the wasted resources,

Hypothetical situation; A home has (4) 20 amp circuits (loaded to 15 amps each) wired as two wire circuits. We use #12 conductors, at 100’.


#12 = 1.98 ohms per K’ x .100 = .198

I^2 R heat loss

15 x 15 x .198 x 8 = 356.4 watts

Now if we took the (4) circuits and installed them as multi wire branch circuits, we would reduce the current carrying conductors to 4. (remember hypothetically 15 amps so each grounded conductor is truly neutral.)

15 x 15 x .198 x 4 = 178.2 watts.

Now if we give this a time frame per day, times 365 days, times umpteen billion nation or world wide circuits, we have impact on the aforementioned resources. [Linked Image]


Roger



[This message has been edited by Roger (edited 01-20-2006).]
Posted By: iwire Re: sharing neutral - 01/20/06 11:26 AM
Quote
I'd like someone to explain it to me, too. But all my experience so far bears out the use of shared neuts has been nothing but trouble. Even when the workmanship and all connections are good.

And all my experience tells me otherwise. [Linked Image]

I work for a very large EC, the past 6 years in the division that handles warranty issues and I see no difference in the frequency of repairs between two wire and MWBCs.

But lets think about that, both of us have biases and neither of us did any real testing or logging.

In other words our experience means little. [Linked Image]

I am not picking a fight, I agree with Roger if you do not like MWBCs do not use them. I will continue to use them.

Many of the building we do are quite large, MWBCs help greatly to overcome voltage drop.

Bob
Posted By: Dnkldorf Re: sharing neutral - 01/20/06 12:54 PM
Just so we compare apples, Let's look at it like this.

In Roger's above example, the IR losses would be cut in half. Let's call it 50%...

Now then, Roger's example is based on (4) 15a loads. Total 7200W..

The losses using 2 wire is 5%..
The losses using 3 wire is 2.5%.

So even know 50% in losses, are only really a 2.5% savings, but it is savings.

Now, let's say we use 2 wire, 10ga.

Using the same loading, 2 wire installation equates to 223W. The 12 ga was 376W.

Just being fair here, this experiment was on a 240V residential power. Not 208V 3ph.

I think we all can agree, that 3 ph power and nuetral currents, do not fit the above examples, and I think this is what the original poster was intersted in.

Good discussion....


Dnk....
Posted By: Roger Re: sharing neutral - 01/20/06 01:23 PM
Dnk, apply any equal numbers (unequal if you want) to two sets of A,B,and C phases of wye connected MWBC's verses (6) two wire circuits and do the math, you will see a savings as well.

When you do this assume linear loads for simplicity.

We all agree that certain loads need special consideration in the way of oversized neutrals or even two wire circuitry, but to think this is a problem with all MWBC's is simply not the case.

As an aside, how many people here can swear that they have seen a melted neutral conductor or connection due to additive hamonics?

Roger
Posted By: walrus Re: sharing neutral - 01/20/06 07:38 PM
Quote
Let's think about this.....
He wants to run 3 hots with a nuetral.

ASSUMPTION: he has a 208Y120 service.

So if we have 2 of the circuits at 20a each and the third circuit has nothing plugged into it.

We should have 20a on the nuetral?
Don't believe one would have 20 amps on the neutral in this case. The phases are out by 120 degrees so it wouldn't it be something less than 20??
Posted By: Roger Re: sharing neutral - 01/20/06 08:55 PM
Walrus, it would be 20, see the formula below.

Using 20 amps on A and B;

sqrt of (IA^2 + IB^2) - (IA x IB)

(IA^2 + IB^2) - (A x B)
20x20 + 20x20 - 20 x 20
400 + 400 - 400
800 - 400 = 400

sqrt of 400 = 20


Roger
Posted By: mxslick Re: sharing neutral - 01/20/06 09:13 PM
Bob:

Quote
I work for a very large EC, the past 6 years in the division that handles warranty issues and I see no difference in the frequency of repairs between two wire and MWBCs.

But are you referring to connected equipment or the wiring system itself? In my case, only one of many experiences was due to wiring system failure (loose wirenut); all the other problems were despite the fact the wiring itself was o.k.

Quote
But lets think about that, both of us have biases and neither of us did any real testing or logging.

Sort of. In my case, I never used a 'scope or power quality analyzer on the circuits, I just followed my instinct and pulled in the extra neutral. Wham, bam. Problem gone. Remember, my situation is usually noise, hum or erratic automation issues, along with occasional amplifier repeated failures. My file of old service reports (from only the last five years, though) was really the only logging. And admittedly, the percentage of neutral problem repairs in relation to my total workload is quite low, maybe 2% total. {Most cinema service calls are for mechanical breakdowns or non-power related sound issues.} I have a hunch your experience with this issue reflects about the same overall percentage.

Quote
In other words our experience means little.

To each other, maybe. I like hearing of other's experiences, that's how I learn new things. To ourselves, I disagree. [Linked Image] We are, after all, in two different working environments. But we do have the same objective: a quality, cost-effective and trouble-free installation. [Linked Image]

Quote
I am not picking a fight, I agree with Roger if you do not like MWBCs do not use them. I will continue to use them.

You are the last person I'd ever suspect of trying to pick a fight. [Linked Image]
But since I don't like MWBC's, I won't use them. They do have a place and proper application, my work isn't one of them.

As dnk said, good discussion!

edited to fix html

[This message has been edited by mxslick (edited 01-20-2006).]
Posted By: e57 Re: sharing neutral - 01/21/06 01:38 AM
Quote
As an aside, how many people here can swear that they have seen a melted neutral conductor or connection due to additive hamonics?
A total of ONE (for sure) in over 15 years... It was a maxed out panel feed with a reduced neutral. Panel was scoped before and after the new feeder and panel to have high harmonic content, but still can only say it was a contributing factor. The only other PQ problem sever enough for mention was a building that kept killing anything with a transformer... UPS's, E lighting inverters, and ballasts, etc. Loads of 4-wire circuits, but for more than a year on that project, no sign of a single burned neutral on the any new or existing wiring. New POCO transformer, and a seperate one, and service for the elevator (the main culprit) corrected the problem.

Harmonic issues do exist in varying degrees, but are rarely enough to cause issues great enough to think about change of wiring method. In just about all of the burned neutral wire-nut problems I have seen over the years, I can point more toward poor connection practices than anything else! Un-twisted wires in a cheap spring nut...
Posted By: walrus Re: sharing neutral - 01/21/06 01:19 PM
Roger
Thanks for posting that equation, I'm guessing its some quadratic solution. Can you post a link? where I can find an explanation for how they derive it?? I'm asking as this has puzzled me for some time. I had a location where I was feeding a pump with 2 motors. I shared a neutral for these 2 120 loads. After I had got this pump running I switched on both motors and checked the current in neutral with a clamp on, expecting to see 0. Surprise,I didn't. This was fed from a sub panel installed by others. It was 2 phases of a 208 y service. Anyway the motors name plate showed the motors at 9 amps and they checked correctly on hot conductor but the neutral showed 4 amps when both were running. I finally realized why I didn't see 0 but couldn't explain why I was getting 4 amps. Using your formula I should see nameplate on neutral.?
Posted By: Roger Re: sharing neutral - 01/21/06 10:27 PM
Hello Walrus, the formula I showed is a simple standard formula for wye neutral current, it was shortened using just two phases from the following.

neutral current = sqrt of
(IA^2+IB^2+IC^2)-(IAxIB)+(IBxIC)+(ICxIA)

To go into more depth of wye neutral current you can go HERE .

Roger
Posted By: electure Re: sharing neutral - 01/22/06 01:12 AM
The multiwire paranoia seems pretty silly to me. I've done commercial service work for about 30 yrs of my "electricianing". I have yet to see a problem that could be directly attributed to a shared neutral. Loose connectons, sure. 2 within the last week.
There are an awful lot of modular wiring (like Reloc) systems in use with fluorescent and HID lighting. I've yet to see one with individual neutrals, and still haven't seen one burnt up because of it.

Oversize neutrals are fine if you want, but considering: You can't buy a 3 phase 6 wire device, disconnect, panelboard, transformer, or service, I fail to see the point of running individual neutrals on branch circuits.

It reminds me of the $158 audiophile duplex receptacle that was discussed here once. It had gold plated contacts. It would then be connected to a home's copper wiring, and at the service, to the PoCo's aluminum service drop and distribution system.

Iwire and I were the original guys that "agreed to disagree" on many subjects.
We actually agree on so many things that it's kinda scary. [Linked Image] I'm with with Roger and him 100% on this one.

I had some typos



[This message has been edited by electure (edited 01-21-2006).]
Posted By: walrus Re: sharing neutral - 01/22/06 03:02 PM
Roger
Thanks for the link, not sure my feeble brain 20 something years removed from my last Calculus class can handle it [Linked Image]. Its going to take a few reads thats for sure.
Posted By: renosteinke Re: sharing neutral - 01/22/06 09:28 PM
Even paranoids have enemies! :-)

I've been a firm believer in avoiding shared neutrals ever since Mr. Murphy came in one night, and cost me $350 in fried power supplies.
I will concede here that Mr. Murphy had some help, in that a) there were an awful lot of wires tied together under that wire nut (my fault for a poor re-making of that mess!) and b) someone had seen no reason to run his neutrals all the way to the panel (maybe an entire 8 feet away), and had combined them in the junction box.

So, yes, my concern is with poor connections, far more than harmonics. I've found cooked neutrals, but harmonics had nothing to do with it!

Another mess cost me $600 when I opened a neutral to add a receptacle to the circuit. Though I had taken pains to shut off my circuit, and power down ever appliance in the area....well, the wires were not twisted together under the nut, and a money changer in the next room was somehow tied in- along with another circuit. The money changer said "hello" to 220 volts, and died. Oops.

The heck with voltage testing....maybe we ought to start putting an amp clamp on neutrals as well.

So- even though I like the theory- I try to avoid sharing the neutral. Just making things a little more "Murphy resistant."

I conside neutral connections to be even more critical than hot-wire connections.
Posted By: Larry Fine Re: sharing neutral - 01/23/06 03:16 AM
Reno, you just pointed out a good reason for handle-tying multi-wire circuits' breaker handles. Plus that assures no overloading the neutral by same-phasing the hots.
Posted By: mxslick Re: sharing neutral - 01/23/06 07:41 AM
Folks, please bear with me as I am now going to rant a bit... [Linked Image]


Quote
Though I had taken pains to shut off my circuit, and power down ever appliance in the area....well, the wires were not twisted together under the nut, and a money changer in the next room was somehow tied in- along with another circuit. The money changer said "hello" to 220 volts, and died. Oops.
{bold emphasis mine}

Arrrggghhh! It is precisely this kind of thing happening which makes me so adamantly opposed to shared neutrals!! Here's a case of a qualified, experienced electrician taking all proper and reasonable steps and WHAM!!

Be realistic, in any environment, there WILL be others adding onto/changing wiring, moving breakers around, replacing receptacles and fixtures!!

It doesn't make a darn bit of difference how experienced one is, if you don't have x-ray vision or the ability to sniff out electrons there will always be the unexpected changes waiting to bite you in the tail!

Be honest, folks: how many of you turned off what you thought was the right circuit and got surprised because of backfeed, mislabeled disconnects/breakers, or accidentally opening a neutral you thought was dead?

and:

Quote
You can't buy a 3 phase 6 wire device, disconnect, panelboard, transformer, or service, I fail to see the point of running individual neutrals on branch circuits.

Apples and oranges....no one ever said that feeders, panelboards, transformers or services need to be set up with discrete neutrals. [Linked Image] We're talking branch circuits, which are subject to a lot more "unqualified" repairs or modifications.

I think the point will become quite clear when something bad happens to a circuit on your "watch". Again, not a reflection on anyone's workmanship, please do not take it as such. Just consider that it would only take a change in load factors (those pesky harmonics again [Linked Image] ), or even a simple lack of knowledge by someone attempting to add on to or repair something on that circuit.

I'll say it yet again, just because Code allows it doesn't make it safest or right. Code is and always will be a minimum standard.

finally:

Quote
Reno, you just pointed out a good reason for handle-tying multi-wire circuits' breaker handles. Plus that assures no overloading the neutral by same-phasing the hots.

Sounds like in this case even if the handles were tied, it wouldn't have helped. Reno's description implies that some questionable changes or taps were involved.

As others have noted in this thread, it all boils down to one's own experiences and/or biases on this issue.

Thanks for bearing with me, we now return to our regularly scheduled program.... [Linked Image]

edited for spelling


[This message has been edited by mxslick (edited 01-23-2006).]
Posted By: e57 Re: sharing neutral - 01/23/06 08:06 AM
Oh.... Forgot to share a funny story....

I worked for a kinda family shop, (3 brothers) youngest of which was to take me out and show me the ropes...

During his long diatribe about "Always follow the neutral back to the buss bar so you know you're pulling the right... Doh.... Go inside and see if I blew any thing up!" I go inside and the horrorfied customer is sitting in front of her smoking monitor. That wasn't funny, but that guy explaining it to his brother was...

Anyway, things happen... And it is common sense to shut adjacent circuits in any box off when messing with neutrals. Go as far to have customers up-plug computer or other equipment while you are doing so. Best thing to do is put an amp clamp on any neutral if it has a remote possibility of being shared. That is still not a guarantee, but does help you to say no if it does have any current.
Posted By: Dnkldorf Re: sharing neutral - 01/23/06 12:28 PM
I have never found a "smoked" nuetral in my travels yet, But, I have come across nuetral wire that brittle.
Touch them and the insulation starts to peel off. I've seen nuetrals loose at the bar alot.

This looseness, I have attributed to 2 possibilities. One, a bad installation. Two, the wires are getting hot and cooling, and the screws are backing out somehow.

But I don't know if the second is fact.

Anyone seen this happen?


Dnk....
Posted By: Tiger Re: sharing neutral - 01/23/06 01:42 PM
If I turn a circuit off and it runs into a conduit (in the distribution panel)with five ungrounded conductors and three grounded conductors, I cut the power to all the ungrounded conductors.

Dave
Posted By: mxslick Re: sharing neutral - 05/01/06 09:53 PM
Due to the AFCI thread, I hereby go:

Bumpa-bumpa!! [Linked Image]
Posted By: dlhoule Re: sharing neutral - 05/02/06 07:10 PM
Dave,
"If I turn a circuit off and it runs into a conduit (in the distribution panel)with five ungrounded conductors and three grounded conductors, I cut the power to all the ungrounded conductors."

Good practice, and it still doesn't guarantee you won't have a problem. I've seen many cases where more than one conduit leaving panel went into same jbox.

One of the reasons I am strong advocate for identifying every gounded conductor in every jbox, panel, or anyplace it can be seen.
Posted By: Scott35 Re: sharing neutral - 05/03/06 02:16 AM
Thanks to mxslick for bumping this thread back to life [Linked Image]

Wanted to make a short reply to it back in January, but somehow forgot to (like being busy is any excuse [Linked Image]...)

Anyhow, I'll make this short and sweet - if possible [Linked Image]

First off, let me point out that there are a lot of valid posts made in this thread already! My contributing 2¢ is kind of meaningless, but I felt the urge to chime in and act cool!

Secondly - and most importantly why I am writing this whole thing in the first place!;
To the original poster ("OP") sparkync,
did any of this thread assist with your installation questions???

Pretty much, the installation part is mainly up to you - as to the choice of using Multi Wire Branch Circuits, or going with an array of Two Wire Branch Circuits... with the following exceptions:

1) Are there any Specifications within Contract Documents, that exclude using Multiwire Circuitry for given equipment? - and I must exclude the typical "Boilerplate" stuff that firms toss into Contract Documents, including "Basics" in the beginning of Division 16 of the Project Manual, or Text Blocks found in the "General Notes" page of the Planset.

What I am referring to would be exclusive pages within Division 16 of the Job Manual, which cover specifics for given equipment.

2) Does the Manufacturer(s) of given equipment demand their equipment to be connected only to "Dedicated 2 Wire Circuitry", in order to hold any warrantee?
This would be an issue for you in the event of ANY failure - the first thing they will do is pin the failure on the Circuitry, and boast this to your Client.

3) Was the project bid (proposed) to use 2 Wire Circuits for the specific loads (equipment)? If yes, than you should do as bid.

Other than this, there really is no valid reasoning either way. It's a matter of choice.

The NEC has no say in the types of circuits you _CAN_ or _SHOULD_ use for this equipment, it only mentions how to safely install oversized Common Grounded Conductors (Neutrals). This is, once again, a Design Issue, not a Safety Issue.

If you decide to use Multiwire Circuitry, feel free to keep the Common Grounded Conductor the same size as the Ungrounded Conductors, or if you feel it is needed, increase its size.
If you go the 2 Wire Circuitry route, be sure to take into consideration the fact that each 2 wire circuit will have 2 current carrying conductors; so when running multiple circuits through a single common raceway / conduit, derating the conductors will apply "faster" than with multiwire circuitry.

If you have concerns regarding excessive load current on the Common Neutral, try to limit the _Possible Loads_ to no more than 50% of the circuit's value.

Since these loads are what I would classify as "Intermittent" loads, not "Continuous" loads, if any equipment does have a real dirty level of distortion ( > 33% THD "across the board" ), it likely will only be at its peak for maybe 15 minutes of each hour - maybe 30 minutes of each hour - so there will be at least 50% of the nominal time period where the load draw is very low, thus allowing the conductors to cool down.

Lastly, in regards to this whole Harmonics / Common Conductor shared subject;

I was a victim of closed minded thoughts when it came to loads which produced harmonic distortion, and thought at first that the problem was bigger than it really is.

But think about it...
*** Viewing it as the culprit will be _Switch Mode Power Supplies_:

The Workstation PC does not draw the rated capacity _Continuously_, only in transient peaks. Most of the time, the SMPS is maybe running 20% above Idle.
So, having a Workstation with a THD of 40% being found in the 3rd, 9th, 27th and 81st Harmonic doing typical Workstation duty, how long will these high levels of current be flowing between the load equipment and the Secondary of the Transformer?

Same goes for Printers (LAN Printers, Local Printers and Print Server Stations), Copiers and Fax Machines - total peak load draw is not continuous.

File Servers may have a more continuous load draw, yet do they draw for 3 hours or more?

Intranet Servers may fall into this class - and should be figured as such when designing circuitry.

***Viewing it as the culprit will be _Lighting_:

Think back to the "Harmonics From Hell Scare" of the late 1980's; what was going on then?

This was the invent and introduction of Hybrid Component Ballastry.
Most likely, the "Primary Scare" was something _Coupled_ to the usage of these types of Ballasts.

To me it sounds like just another misquoted report or statement, which first began a frenzy of panic, then was taken advantage of by manufacturers of equipment and materials.

What I believe the scenario(or scenarios) were about was Lighting Circuits which were loaded to maximum (like 20 amp circuits loaded to 18 - 19 amps), and running for entire days - even weeks, using #12 conductors for multiwire circuitry, fed through raceways with 3 or even 4 additional multiwire circuits running through them, and were feeding Lighting Equipment with the new technology Ballastry that produced "higher than ever seen before" Harmonic Distortion back into the circuitry and the system.

I say this because of the numerous installs we had encountered during Tenant Improvement work, where these Lighting scenarios had been installed as described above.

It is not "News" that Magnetic Reactor type Ballastry produces Harmonic Distortion - sometimes in the order of as high as 20% THD, as do Induction Motors and other Reactive loads, so the issue of Harmonic Distortion has always been there in the Commercial / Industrial world.

To wrap this fricken miniseries of a post up [Linked Image]

I still will use - and even design projects, with the concepts of utilizing 4 Wire Multiwire Circuitry wherever possible (on projects with 4 wire wye secondaries!).

If a Client feels the need to have 2 wire circuits used, I will do the normal thing and explain (as much as possible) the reasoning, ideas, myths, urban legends, pros and cons for each type of circuit (if there is a chance to do so!!!).

It is also my part to be sure the crews doing the installations do not screw up Multiwire circuitry, or any circuitry what so ever!
Sometimes it is not so easy for me to do so, as I may design a project and merely have the "fun" of playing Project Manager on it "Vicariously" (only see the job 1 or 2 days a month), but most of the jobs designed by me will also be run by me.

As to how many barbecued items I have witnessed, which were caused by Multiwire Circuitry faults or excessive Harmonic Distortion, only a handful in over 26 years.

Saw a barbecued common neutral in a Panelboard at the Bus Termination. That one turned out to be a combination of a loose screw on the Bus and some joker had 6 Lighting Circuits across the common.
Took for ever to T Shoot that one!!! (it was on a project where the EC had high turn-over employees coming and going, so things were really messed up!).

Another one which comes to mind involved fried equipment in a newly developed Commercial "Office/Warehouse" shell project, back in 1989, - where there are multiple buildings and all suites are simple shells.

This one Tenant had a Copier, Fax Machine and 2 PCs go all Smoky on him - and of course, the Property Manager calls ranting about "Fire Trucks, Flames Leaping From Buildings" and similar chaos.

I was working for an EC that just loved to smoke Pot - endlessly! At least a 1/4 Ounce every dang day!
On top of this, during the time of the french fried equipment, he decided that smoking pot wasn't quite enough - that it would also be great to smoke speed too!

Now, he wasn't the easiest to talk with when sober, so just add some pot and speed, shake well [Linked Image], and you have a raging jackass!!!

Insisting that the barbecue was my fault - due to "Not Giving A Damn About Good Makeup On Neutrals", he had the entire crew open every single outlet, J. Box and etc. to verify connections.

All terminations were 100%.

I was at the Panelboard and did a "Short Circuit Test" with my screwdriver - between the Grounded Conductors Bus and the Panelboard Enclosure.
With a few minor sparks popping off from the semi conductive path, all the sudden the lighting becomes stable and bright!

Tell "Mr. Jack-Pot" that I suspect either a fault from the Utility Transformer (lost Neutral), or more likely, someone forgot to install the Jumper Bus Bars between Gear sections.

After Jack-Pot argues this and messes with other stuff for 2 hours, I go to the switchgear, open it up, and find loose carriage bolts on the last 4 meter section.

A good heads up, plus one additional rant for me!

Scott35
Posted By: macmikeman Re: sharing neutral - 05/03/06 05:09 PM
Think about this for a moment. Running dedicated neutrals will increase the probability of making a faulty connection by two thirds. You may never ever make a loose connection, but by the simple numbers of it, you are increasing your odds by a factor of 2/3.Therefore it can also argued that running dedicated circuits rather than mwbc is a less preferable method.
Posted By: Tiger Re: sharing neutral - 05/03/06 08:25 PM
I'm with you on this one Reno. I avoid shared neutrals except when I'm running 2 circuits for a distance with BX, and when I'm pushing 9 current carrying conductors in conduit. I also agree handle ties is the best safety solution.

As a courtesy measure, I twist the ungrounded conductors in the distribution panel all the way to the breakers. I also pigtail to outlets whether it's shared neutral or not.

Dave
Posted By: iwire Re: sharing neutral - 05/04/06 09:49 PM
I am still buying miles of 12/4 - 10/4. [Linked Image]

Me and multiwire circuits are good friends and friends don't let each other down.

The distances and number of circuits I have to supply in the types of occupancies I work in make multiwire branch circuits the smart choice.

Besides, I can not supply equipment designed around 3 PH 4W supplies with 3 two wire circuits. [Linked Image]
Posted By: pdh Re: sharing neutral - 05/04/06 10:25 PM
Scott35: You might want to take a look at the following waveform. Scroll down to figure 8 on the following web page:

http://www.zlan.com/waveforms.htm

This is actually a very severe level of harmonics, and rather typical of what I have seen a couple times I've put a scope on a switching mode power supply.. Notice that each spike is conducting for about 1/3 of the time, and 2/3 is spent not-conducting. Next, imagine superimposing the same waveform over itself for 120 degrees phase shift and 240 degrees phase shift. The spikes essentially won't overlap. And don't forget that these spikes are actually about 3 times higher in current than the peaks of a normal linear load with the same average current.

Of course if you have a mix of loads, and computers are just a small portion of those loads, then you're not really going to see much of an issue with triplens on your neutral, as long as the various overcapacity requirements in the code are followed.

However, in the case of a room full of computers (and I've worked in rooms of as many as 2500 computers), you end up with essentially no loads other than these very spiky switch mode loads. If enough computers are pushing the average current load to 12 amps on a 15 amp rated circuit, then you're going to see 36 amps average on the neutral. Those "double neutral" cable assemblies, panels, and transformers are really going to get more than just double the load.

I read a study from Square-D where an analysis suggested that in the typical office building, triplen currents would not exceed double, and rarely even reach it. But it appears that was a study of the overall office environment or building, and it appeared to have been done many years ago. In the age of the internet and a web based business presence, there will be more and more computer rooms running multiple servers and requiring dedicated circuits. A 4-wire 208Y/120 branch circuit to that room is simply going to be unsafe (36 amps on a #14 neutral). If you up the cable to have a #10 neutral, you can probably get away with it, though I think #8 should be required on the neutral for this example case.

Among the many things I do is specify data center designs for internet providers or business going online in a big way. I always include in my design a specification that all 120 volt circuits must have individual neutral conductors regardless of the system phasing. No shared neutrals should ever be allowed for this kind of load (even for single phase systems, but that involves some other more complex issues).

I've also considered connecting computers L-L at 208 volts. Most can handle it just fine. But this shifts the problem around a bit. What happens is you get 2 sets of spike currents on each line conductor instead of 1 in the L-N case. So that case of 3 sets of 12 amp average switch mode loads is going to be putting as much as 200% load on each line, instead of 173% as would be expected in the case of 3 sets of linear loads (so 15.47% more). Those 3 sets of 12 amp loads are now going to be 3 sets of 6.93 amp loads at 208 volts, so that means 13.86 amps on each line conductor (not 12). That's too much for the 80% margin of a 15 amp circuit. Up it to a 20 amp circuit (or spread the loads among more circuits) and this might work.

A good computer room design will involve UPSes. I've read that some UPSes (of the double conversion always online tyoe) will have less harmonic current than a regular switch mode power supply. That might be a way to avoid or mitigate most of these issues. But there is always the risk that someone could bypass the UPS in the event of failures (and it seems some UPSes do fail).

If the computer loads are a major portion of the whole building load (e.g. a building just for a large data center), then this issue has to be dealt with upstream, as well, likely even involving the POCO.
Posted By: SolarPowered Re: sharing neutral - 05/05/06 12:51 AM
One way to mitigate the effect of computers is to use computers with power-factor-corrected power supplies. These pull very close to a pure sine wave from the building wiring.

Generally speaking, the power supplies that can take something like 120-240 volts in without changing a switch setting are power-factor corrected. (The power-factor-correction circuit has a side effect of allowing the "wide-mouth" input.)

I recognize that one typically has little or no influence on what computers are chosen, so this little tidbit of information is probably useful in only a few situations.

[This message has been edited by SolarPowered (edited 05-04-2006).]
Posted By: sparkync Re: sharing neutral - 05/05/06 01:41 PM
Wow!! I've been busy and haven't checked this forum too much lately. I've done my job I referred to, and to some's dismay and others acknowledgement, I did use multi circuit neutrals. I did put the printers and office equipment on separate neutrals from the computers. I appreciate all the input, as it is very helpful now and down the road later. To those who would say that anyone could do our trade, they don't know what they are talking about [Linked Image] Maybe they could "make it work", but will it still be working right down the road? Sometimes I think "painting" might be a better trade, not so much "do's and don'ts [Linked Image] But it's in my blood now, so what do? Thanks again for all the input. Job went well, and I still have more wiring in the future for more computers when they get ready. Steve...
Posted By: dlhoule Re: sharing neutral - 05/09/06 12:38 PM
Roger

(IA^2+IB^2+IC^2)-(IAxIB)+(IBxIC)+(ICxIA)

Am I missing something or what? It seems to me the right hand portion of that statement should be all minus signs.

This isn't what I would call my area of expertise, but it would make sense to me if those were negative signs.
Posted By: Roger Re: sharing neutral - 05/09/06 02:04 PM
Larry, you're not missing anything, it is my mistake, [Linked Image] I just put to many sets of brackets in the last part for the addition method I use.

It should look like this;

(IA^2+IB^2+IC^2)-(IAxIB+IBxIC+ICxIA)

This lets me add all the parts of the second side of the equation together and subtract this sum from the first part.

It can certainly be done in individual steps as you pointed out and shown as follows. (correctly [Linked Image])

(IA^2+IB^2+IC^2)-(IAxIB)-(IBxIC)-(ICxIA)

Roger
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