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Shared Neutral

M

Maintech

Jan 1, 1970
0
Where in the NEC does it say that a single neutral can be "shared" between
two or more circuits? My question is this, that I've seen where someone has
ran up to seven 120v single phase circuits for HID lighting, yet there was
only two neutral wires, the same gage wire as the hot lines, i.e. 12 AWG,
20amp. I've also seen where a neutral is shared between two circuits for
120v receptacles. It has always been my practice that when running a circuit
off of a single phase breaker to whatever load that I run a separate neutral
wire for that circuit and keep it separated.
 
N

Nukie Poo

Jan 1, 1970
0
Maintech said:
Where in the NEC does it say that a single neutral can be "shared" between
two or more circuits? My question is this, that I've seen where someone has
ran up to seven 120v single phase circuits for HID lighting, yet there was
only two neutral wires, the same gage wire as the hot lines, i.e. 12 AWG,
20amp. I've also seen where a neutral is shared between two circuits for
120v receptacles. It has always been my practice that when running a circuit
off of a single phase breaker to whatever load that I run a separate neutral
wire for that circuit and keep it separated.
210.4 Multiwire Branch Circuits.
(A) General. Branch circuits recognized by this article shall be permitted
as multiwire circuits. A multiwire branch circuit shall be permitted to be
considered as multiple circuits. All conductors shall originate from the
same panelboard.
FPN: A 3-phase, 4-wire, wye-connected power system used to supply power to
nonlinear loads may necessitate that the power system design allow for the
possibility of high harmonic neutral currents.
(B) Dwelling Units. In dwelling units, a multiwire branch circuit supplying
more than one device or equipment on the same yoke shall be provided with a
means to disconnect simultaneously all ungrounded conductors at the
panelboard where the branch circuit originated.
(C) Line-to-Neutral Loads. Multiwire branch circuits shall supply only
line-to-neutral loads.
Exception No. 1: A multiwire branch circuit that supplies only one
utilization equipment.
Exception No. 2: Where all ungrounded conductors of the multiwire branch
circuit are opened simultaneously by the branch-circuit overcurrent device.
FPN: See 300.13(B) for continuity of grounded conductor on multiwire
circuits.
(D) Identification of Ungrounded Conductors. Where more than one nominal
voltage system exists in a building, each ungrounded conductor of a
multiwire branch circuit, where accessible, shall be identified by phase and
system. This means of identification shall be permitted to be by separate
color coding, marking tape, tagging, or other approved means and shall be
permanently posted at each branch-circuit panelboard.
 
S

SQLit

Jan 1, 1970
0
Maintech said:
Where in the NEC does it say that a single neutral can be "shared" between
two or more circuits? My question is this, that I've seen where someone has
ran up to seven 120v single phase circuits for HID lighting, yet there was
only two neutral wires, the same gage wire as the hot lines, i.e. 12 AWG,
20amp. I've also seen where a neutral is shared between two circuits for
120v receptacles. It has always been my practice that when running a circuit
off of a single phase breaker to whatever load that I run a separate neutral
wire for that circuit and keep it separated.

Yours is a good practice especially now days with switching power supplies.
A very simple answer for your question is that in single phase services the
phases are 180 degrees from each other. A 3 phase system is 120 degrees from
each other. Reducing this to a very simple analogy, you get a pulse of
electric from one phase, it dies then the next one comes along and dies.
Never using the wire at the same time.

Remember the NEC is the minimum standard nothing more. There are lots of
installations that require more than the minimum

I saw several years ago a new building that the engineer allowed 3 120
volt circuits shared by a 10 awg neutral. It was a computer facility and we
ended up repulling most of the building with separate circuits and neutrals.

I hope this helps,
 
J

jim

Jan 1, 1970
0
SQLit said:
Yours is a good practice especially now days with switching power
supplies. A very simple answer for your question is that in single phase
services the phases are 180 degrees from each other. A 3 phase system is
120 degrees from each other. Reducing this to a very simple analogy, you
get a pulse of electric from one phase, it dies then the next one comes
along and dies. Never using the wire at the same time.
How would switch mode power supplies effect a single phase (120/240) service
by overloading the neutral? In a 3 phase service, switch mode power
supplies can overload neutrals due to the third harmonic (and odd multiples
of the third harmonic) being additive since the phases are 120 degrees
apart. This doesn't occur in single phase systems.
Remember the NEC is the minimum standard nothing more. There are lots of
installations that require more than the minimum

I saw several years ago a new building that the engineer allowed 3 120
volt circuits shared by a 10 awg neutral. It was a computer facility and
we ended up repulling most of the building with separate circuits and
neutrals.
Why? If your neutral currents were this high (greater than 30A) due to
harmonics, you would have certainly damaged the transformer, unless it was
oversized and designed for high harmonic service.

The biggest concern for shared neutrals is a safety issue. The electrical
system must only be worked on by individuals who understand the
implications of the shared neutrals and will protect themselves
accordingly.
 
J

John

Jan 1, 1970
0
Reducing this to a very simple analogy, you get a pulse of
electric from one phase, it dies then the next one comes along and dies.
Never using the wire at the same time.
This is not a simple analogy, it is simply wrong. Nothing comes back on a
pure 240v (or 3 phase) neutral; they don't take turns!
 
J

John

Jan 1, 1970
0
Maintech said:
Where in the NEC does it say that a single neutral can be "shared" between
two or more circuits? My question is this, that I've seen where someone has
ran up to seven 120v single phase circuits for HID lighting, yet there was
only two neutral wires, the same gage wire as the hot lines, i.e. 12 AWG,
20amp. I've also seen where a neutral is shared between two circuits for
120v receptacles. It has always been my practice that when running a circuit
off of a single phase breaker to whatever load that I run a separate neutral
wire for that circuit and keep it separated.
You have to be careful that the current on the neutral does not exceed the
wire capacity, since there is no protection on the neutral. You also have
to be sure that the current through any metal opening is zero; either by
having hots and neutrals carrying the same current, or by having two hots
from opposite legs carrying the same current, or by some combination of
these two. Plastic isn't bothered by this.
 
D

daestrom

Jan 1, 1970
0
John said:
This is not a simple analogy, it is simply wrong. Nothing comes back on a
pure 240v (or 3 phase) neutral; they don't take turns!

On a pure 240V single phase setup, you're absolutely right.

On three phase, with non-linear loads (such as electronic power supplies),
the three phase currents do *not* add up to zero thoughout the entire cycle
and the non-zero result flows in the neutral.

For an example, take three power supplies that uses a full-wave bridge to
feed some capacitors, each connected to one phase and neutral of a
three-phase supply. The diode in the power supply connected to 'A' and
neutral only conducts for a brief portion of the cycle when the
instantaneous voltage is higher than the capacitor charge. While phase 'A'
is that high, either phase 'B' or 'C' is *not* necessarily high enough to
forward bias the diodes in those power supplies. So the 'return' current
for the 'A' power supply flows in the neutral. Similarly, when 'B' voltage
is high enough to cause its power supply diodes to conduct, phase 'A'
voltage may not be high enough to continue conduction through its power
supply's diodes. Result is 'pulses' of current on the neutral with a
fundamental frequency of 180 Hz.

daestrom
 
M

Maintech

Jan 1, 1970
0
Thank You SQLit

SQLit said:
Yours is a good practice especially now days with switching power supplies.
A very simple answer for your question is that in single phase services the
phases are 180 degrees from each other. A 3 phase system is 120 degrees from
each other. Reducing this to a very simple analogy, you get a pulse of
electric from one phase, it dies then the next one comes along and dies.
Never using the wire at the same time.

Remember the NEC is the minimum standard nothing more. There are lots of
installations that require more than the minimum

I saw several years ago a new building that the engineer allowed 3 120
volt circuits shared by a 10 awg neutral. It was a computer facility and we
ended up repulling most of the building with separate circuits and neutrals.

I hope this helps,
 
Z

Zzzap

Jan 1, 1970
0
Where in the NEC does it say that a single neutral can be "shared" between
two or more circuits? My question is this, that I've seen where someone has
ran up to seven 120v single phase circuits for HID lighting, yet there was
only two neutral wires, the same gage wire as the hot lines, i.e. 12 AWG,
20amp. I've also seen where a neutral is shared between two circuits for
120v receptacles. It has always been my practice that when running a circuit
off of a single phase breaker to whatever load that I run a separate neutral
wire for that circuit and keep it separated.


You can share neutrals on two or more different phases. You
can't share it on the same phase.

I have never used this technique on more than two phases, for no
other reason than having a 12-4 wire handy is unusual. The logic
is that sharing the neutral on the same phase the unbalanced
load will add to one another, and the same can be said on the
converse.

However, there are new problems with modern electrical systems,
such as places with electronic equipment. What building anymore
doesn't have computers or electronic balasts? Don't ever share a
neutral in such a case even in your own house if a neutral the
circuit might be used for an electronic device. Whole buildings
have had to be upgraded in the past 20 years because of
Harmonics. And harmonics indeed is almost an entirely new
problem of the modern age of computers.

http://www.powerquality.net/problem.htm

Other than this go ahead and share a neutral. However, you will
find out that the wire is a ton cheaper when buying 12-2 than
buying 12-3 or 12-4. That alone makes it prohobitive in such a
situation where every dollar counts like residential wiring.

To go one step further, even though you can share neutrals still
and stay in code, why do it? Electronic devices are coming in
our lives daily by the minute. Do you really want to upgrade in
another 10 years when the NEC no longer allows it and shared
neutrals are causing you problems?

Where your friend shared so many neutrals I would never do such
a thing. I am sorry to say I can't find it in the code, and
therefore cannot say for sure if what he did wasn't up to par.
From what I know he did wrong, as well as the references I found
also indicate he did wrong.

More references:
http://www.squared.com/us/products/circuitb.nsf/07a0210021262d45
862564b5006e4f84/78265ef31153098c85256c4e005921a1/$FILE/0760DB02
03R902.pdf

http://www.kennedyelectric.com/SureTest/Suretest-
SharedNeutrals.htm

http://www.psihq.com/iread/strpgrnd.htm
 
M

Maintech

Jan 1, 1970
0
Thanks CBEEBE1!!
Your post prompted me to go back and reread National Electric Code 210.4 C.
The problem I have with the "multiwire branch circuit" is that in 210.4 C is
the exception #2 that requires that "all ungrounded conductors, HOT, of the
multiwire branch circuit are opened simultaneously by the branch-circuit
overcurrent device".
Even in the facility I'm working in now, which is less then one year old,
the contractors shared one single neutral with three circuits off of a
208/120 panel, feeding 120 single phase receptacles. BUT the circuit
breakers are all single pole, i.e. I go to replace a damaged receptacle
which comes from breaker #1, which is single pole, I test to insure my
safety and lock it out, and yet that neutral is shared with #3 and #5 which
may have loads on them, and I'm unaware that the neutral is shared because
they where to "cheap" to install a three pole breaker that would alert me
that this is a multiwire circuit. What happens to me when I go and remove
that receptacle from it's box? ZAP!
This was the reason I posted this, I learned from you guys and I thank you!
I've only delt with single pole which prompted me to run a neutral for each
circuit, or two pole 240 volt.
This is the third facility I've worked in that I've seen crap like this and
I hate it. It almost prompts me to open up any panel and check the number of
hots and neutrals and tracing them before attempting any repair, or I'll
have to trace the conduit and turn off all the circuits contained within.
My second big gripe with this is that these installations where done by a
"UNION" contractor who claim to be the "professionals" in their field, but
yet with a little bit of study, a simple maintenance mechanic like me can
see where they SCREWED UP, but yet who pays the price, ME!!!
Thanks!!!!!!
 
U

User 1.nospam

Jan 1, 1970
0
Thanks CBEEBE1!!
If you really know the code you know that the above citation has nothing to
do with 3 branch circuits that supply receptacle or lighting circuits. 210.4
C

Also, the requirement to disconnect all ungrounded conductors applies to
(a)dwelling units (not commercial/industrial) and (b) devices mounted on the
same 'yoke'. 210.4 B
No Zap should occur, the neutral should only be a volt or two above
earth potential.

This is key to not being zapped if you work on an outlet on the circuit.
In multiwire circuits the outlet is NOT allowed to
be a neutral splice point.

The neutral on this type of circuit should be pigtailed to allow removal of
the device without opening the neutral.

It can be hard to deal with, but it may very well Not be a code violation.
(see above)
It almost prompts me to open up any panel and check the number of

Remember, that job was done by the "Lowest Bidder" (probably) and those guys
had a boss who got a bonus for finishing the job early and below budget. In
the final analysis you have to give part of the blame to the AHJ. In
essence, the AHJ makes the electrical code 'on site', meaning that he can
enforce or make exception to a code rule at his whim. While large
exceptions are very rare, small things are done all the time.
 
M

Maintech

Jan 1, 1970
0
Bill Coffel said:
No Zap should occur, the neutral should only be a volt or two above
earth potential. In multiwire circuits the outlet is NOT allowed to
be a neutral splice point.

Yea and you know what? If everyone followed the rules it would be a better
place, but they don't and I'm the one to suffer when they do use a recept as
a splice point. My big gripe is that in a dwelling unit that it's required
that the "hot" side be two pole breakers when there is a shared neutral, but
yet in an industrial arena where the electrical is worked on much more, this
is not a requirement. You know I hate it when I see any kind of spark from
any wires after I've followed the "Lock Out Tag Out" rules. Why can't this
requirement be extended to the non-dwelling side as well to tie the
breakers together.
Maybe I'm just being a bitch.
 
M

Maintech

Jan 1, 1970
0
Tom Horne said:
Let me say that this attitude that appointment to the position of
electrical inspector makes the appointee a petty tin pot god is very
rare among electrical inspectors in the areas were I have worked. I
have not worked everywhere but I have worked in quite a few places. The
vast majority of electrical inspectors with whom I've had any dealings
seem to actually enjoy helping people understand the code and apply it
in a safe, cost effective manner.

That having been said any inspector who "makes the electrical code 'on
site', meaning that he can enforce or make exception to a code rule at
his whim" is abusing the authority of her/his office. I have run into
three such inspectors on jobs were I was the principal electrician and
have fought two of them to a stand still. The issue in the third case
was too small to be worth fighting. There is always an appeal process
and it can be reviewed by the courts. During one appeal I was a witness
rather than a principle for the appeals board chairman said that they
always back up their inspector. The civil court overturned the
inspector and the appeals board on the grounds that "the appellant
clearly new more about the appropriate application of local government
police authority than the board did." Any inspector who makes up the
rules as they go along is an incompetent hack who does not know the code
well enough to apply it properly! A competent inspector is a real asset
to the electricians and homeowners with which they deal. Their
strongest asset is a complete understanding of the limitations imposed
by law on the exercise of the police power of the state.

This is Illinois Farm country, What inspector???
 
B

Ben Miller

Jan 1, 1970
0
Maintech said:
Yea and you know what? If everyone followed the rules it would be a better
place, but they don't and I'm the one to suffer when they do use a recept as
a splice point. My big gripe is that in a dwelling unit that it's required
that the "hot" side be two pole breakers when there is a shared neutral, but
yet in an industrial arena where the electrical is worked on much more, this
is not a requirement.

OSHA and the NEC assume that in an industrial environment the service will
be performed by qualified people. Hence the differing requirements from
residential.
You know I hate it when I see any kind of spark from
any wires after I've followed the "Lock Out Tag Out" rules. Why can't this
requirement be extended to the non-dwelling side as well to tie the
breakers together.

If you see a spark after you lock out a circuit, then you did not perform
lockout/tagout properly. The correct procedure requires you to verify that
power is off on the device before you start work. You would then discover
that it was still energized on one side. I agree that this is time wasted,
but at least you are safe. It would be easier for maintenance if there was a
marking stating that it was a multi-wire circuit. I have been in facilities
where every receptacle is marked with the breaker(s), panel location, etc.
that feed it, but these are few and far between. One that comes to mind was
a power plant.

Ben Miller
 
M

Maintech

Jan 1, 1970
0
Ben Miller said:
recept

OSHA and the NEC assume that in an industrial environment the service will
be performed by qualified people. Hence the differing requirements from
residential.


If you see a spark after you lock out a circuit, then you did not perform
lockout/tagout properly. The correct procedure requires you to verify that
power is off on the device before you start work. You would then discover
that it was still energized on one side. I agree that this is time wasted,
but at least you are safe. It would be easier for maintenance if there was a
marking stating that it was a multi-wire circuit. I have been in facilities
where every receptacle is marked with the breaker(s), panel location, etc.
that feed it, but these are few and far between. One that comes to mind was
a power plant.

Ben Miller
Thanks Ben for your quick responce! I do check the hot side of single phase
circuits to insure that I've shut down the correct circuit, but how do you
check the nuetral side, if shared, and unaware that it is, with a
multimeter? Your statement about "marking circuits as multi-wire" is my big
bitch here. They don't do it in the industrial side, where as in a dwelling
you should install two pole breakers. They don't do it in the industrial
side. I have come across where circuits are shared with ballast lighting,
i.e. sparks.
 
J

John Gilmer

Jan 1, 1970
0
Thanks Ben for your quick responce! I do check the hot side of single phase
circuits to insure that I've shut down the correct circuit, but how do you
check the nuetral side, if shared, and unaware that it is, with a
multimeter?

Well, even if there is a shared neutral AND the neutral is wired through a
outlet the other HOT wire will be nearby. The "extra" hot wire should be a
"clue!" AND your run of the mill sensitive DIGITAL multimeter will
register a few volts when the probe is held next to a hot wire. You can
also invest $20 (or MUCH less) in a pocket voltage tester that beeps and
lights when near a hot wire.
Your statement about "marking circuits as multi-wire" is my big
bitch here. They don't do it in the industrial side, where as in a dwelling
you should install two pole breakers.

If you only have one pole loads it is "excessive" to require two pole
breakers. In the case of lighting loads it is common to reduce lighting
levels by switching out 1/2 or 1/3 of the fixtures on a string. The
electrician with any real experience and common sense should be aware of
"funny" things that can happen when you break the neutral.
 
M

Maintech

Jan 1, 1970
0
Sorry guys! I guess I opened up a can of worms with my itty bitty gripe.
I'm just a maintenance guy that at least looks at the NEC when doing
electrical work unlike a many of my counterparts who use a cheap reference
guide like "Uglies" and when the shit hits the fan they'll use the excuse of
"I didn't know!". One thing I have learned from all this is to just pay
attention to the work at hand. I've worked in a place before where one side
of a 240v single phase and the ground where used to supply a nearby 120v
receptacle, hence the ground became the neutral. I still work in an
environment where some think that ground is just a neutral.
All my bitching and griping in here isn't going to get me anywhere in life,
but it does let me vent off some steam I get from my so-called counterparts
who don't know the difference between three phase delta or wye.
If you want to hound me more about my ignorance just remove 98wowsa from
the e-mail.
And hey........Happy Holidays!
 
U

User 1.nospam

Jan 1, 1970
0
Any inspector who makes up the
rules as they go along is an incompetent hack who does not know the code
well enough to apply it properly!

Here the inspectors are intimidated by well experienced contractors and AE
firms.
A new hospital addition is a case in point. The AE desiged the incoming
underground service like this. 4-3" conduits enter an occupied basement and
run approx. 80' (unprotected) across the ceiling and turn up into the main
switchgear on the first floor.

Same hospital, different building, the secondary conduits turn up out of the
first floor (buried) in the center of the building and run vertically into
the bottom of the main switchgear on the second floor. The only protection,
other than the metal conduit, is metal studs and sheetrock forming a chase
around the conduits.

This was made known up the line....but the influence of the hospital and AE
firm resulted in no correction.

Having said that, I agree that most AHJ's are honest and do their best to
hold to the NEC. It can be hard tho, when you see major violations like
these, to swallow a correction notice over some trivial problem.
 
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