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.
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
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!!!
The exception #2 quoted is an EXCEPTION to 210.4(c) which says,
"Multiwire branch circuits shall supply only line to neutral loads."
[NEC 1996] The way it reads, this means that LINE-TO-LINE loads can
be supplied only when all ungrounded conductors are opened
210.4(b) looks more significant to this question, but even here there
seems to be no problem unless the multiwire loads are "on the same
> Your post prompted me to go back and reread National
Phil Munro Dept of Electrical & Computer Engin
mailto: firstname.lastname@example.org Youngstown State University
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
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
This is key to not being zapped if you work on an outlet on the circuit.
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.
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.
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.
Here the inspectors are intimidated by well experienced contractors and AE
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.
OSHA and the NEC assume that in an industrial environment the service will
be performed by qualified people. Hence the differing requirements from
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.
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,
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.
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.
The expectation is that the work done in other than dwelling occupancies
will be done by qualified persons. The US NEC does require that the
continuity of the grounded conductor (dare I say neutral) not rely on
it's connection to a device. If that requirement is adhered to then the
absence of a common shut off will not create a hazard were lock out tag
out procedures are faithfully applied. If you skip the final testing
step then you did not adhere to LOTO.
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
And hey........Happy Holidays!
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
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
Exception No. 1: A multiwire branch circuit that supplies only one
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
(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.
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,
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