Shared Neutral

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.
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210.4(C)

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Also 225-7(b)
Allows shared Neutrals for outdoor lighting (such as a parking lot)

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on single phase services such as a house you can share 2 circuits to a neutral. on 3 phase services you can share 3 circuits to a neutral 7 circuits should have had 3 neutrals.

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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!!!!!!

someone
there
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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 simultaneously." 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 yoke."
Maintech wrote:

> Your post prompted me to go back and reread National

--
Phil Munro Dept of Electrical & Computer Engin
mailto: snipped-for-privacy@cc.ysu.edu Youngstown State University
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On Fri, 21 Nov 2003 19:51:35 GMT, "Maintech"

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.

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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

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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.

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It can be hard to deal with, but it may very well Not be a code violation. (see above)

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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.
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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. -- Tom H
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This is Illinois Farm country, What inspector???
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(remove NS to use the address) 614.937.0463 voice 208.975.1011 fax
http://worthingtonengineering.com
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Any inspector who makes up the

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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12
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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
--
Benjamin D. Miller, PE
B. MILLER ENGINEERING
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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.
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phase
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.

dwelling
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.
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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. -- Tom
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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!

12
circuits
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neutral
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.
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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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