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.
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.
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.
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.
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.
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.
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.
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.
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
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
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. (see above)
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.
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.
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.