ECN Forum Forums Code Related Discussion NEC & other Code issues sharing neutral

E57, I thought the same thing a while back.

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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?

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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....

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.

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} :

{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. )

edited to fix first quote

[This message has been edited by mxslick (edited 01-19-2006).]

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.

mxslick


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.

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

It makes no sense at all.

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...

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.

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!

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.

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.


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.)


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.

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...

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.


Roger



[This message has been edited by Roger (edited 01-20-2006).]

And all my experience tells me otherwise.

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.

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