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Are there any scenarios where a neutral wire can be overloaded on a Multi Wire Branch Circuit (MWBC) when properly wired?

Also, on the same correctly wired MWBC, are there any deficits to having electronic devices as loads?

Both questions apply to outlet and split lighting circuits.

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If correctly wired (to include proper size breaker and breaker properly located for 240V to the MWBC,) no way to overload the neutral. The breaker will trip.

The neutral carries maximum current when only one side of the MWBC is running a 120V load. The current reduces when the other side carries a 120V load to the difference between the loads, and if the loads are equal, the neutral current is zero (0)

The classic way to overload the neutral is to improperly rewire it so both hots are on the same phase (0 V between hots) - then the neutral will see double the rated current before a breaker trips. That is more probable on certain types of panel and likewise with old, non-handle-tied feeds, when somone ignorantly moves a breaker to the wrong phase feed. Using a common-trip two-pole breaker makes this impossible in most (all?) panels, though that is not required by code unless the MWBC feeds both 240V and 120V loads. Handle ties are (presently) required but were not in the old days (I don't know the code revision that changed on, but it's more than 40 years ago, I think.)

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  • This doesn't account for fault conditions, though, correct? A fault condition can happen in any circuit...
    – FreeMan
    Feb 6 at 18:44
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    The properly sized breaker accounts for fault conditions. That's its job. Unless it's Zinsco, Federal Pacific, et al and tripping on overload isn't something it always bothers to do.
    – Ecnerwal
    Feb 6 at 18:49
  • Agreed 100%. However, an overload can occur, which the breaker then protects against. Maybe it's just a matter of semantics...
    – FreeMan
    Feb 6 at 18:50
  • It's totally semantics. The question sort of asks if the theory of it has any flaws. That doesn't belong here. This answer answers a better question: what are the most common ways for these to eventually become dangerous in residential settings? My Qn: Should they even be allowed in residential given the limited distances and only two phases?
    – jay613
    Feb 7 at 0:39
  • They serve a useful purpose; they cause essentially no problems for professional electricians and homeowners who properly educate themselves before deciding to do electrical work. The other kind of homeowner, there's nothing as simple as banning MWBCs (you'd still have every grandfathered one in exsistance, as switch loops and NEMA-10s will show you) that's going to help them, (I guess the question I was going to link to has been deleted - Our Hero replaces breaker box, doesn't hook up ground, and probably crossed up neutral and hot - sparks ensue...)
    – Ecnerwal
    Feb 7 at 1:14
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A properly wired Multi Wire Branch Circuit in a typical single phase 120/240v residential service overloaded neutrals or harmonics that cause overloaded neutral are not an issue.

It is rare in residential applications, but if you have a 3-phase service or 120/208v fed by two legs of a 3-phase service then the Informational Note in the section that details provisions of MWBC's NEC 210.4 is relevant:

Informational Note No. 1: A 3-phase, 4-wire, wye-connected power system used to supply power to nonlinear loads might necessitate that the power system design allow for the possibility of high harmonic currents on the neutral conductor.

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  • "... in a typical single phase 120/240v residential service overloaded neutrals or harmonics ... are not an issue." Thanks. I was about to point out harmonics. Glad to know it's not an issue in residentail applications.
    – Scottie H
    Feb 7 at 15:58
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Some kinds of loads take in more energy than they're going to need during part of each AC line cycle, and then feed the excess energy back into the power mains during another part. Such loads are called "reactive loads", and can be subdivided into capacitive loads and inductive loads.

If one were using an extreme capacitive load on one leg of a multi-wire branch circuit and an extreme reactive load on the other, it would be possible for the RMS neutral current to exceed the RMS current on either leg. In general, however, the loads that are most reactive tend to operate with relatively small levels of current. An LED-based night light with a capacitive dropper circuit may have 0.05A load that is almost purely capacitive, and an electric clock motor may pose a 0.05A load that is almost purely inductive, and plugging one onto each leg of an MWBC may result in a neutral current that is greater than 0.05A. On the other hand, even if the neutral current were 50% greater than the higher leg current, that would still only represent 0.075A--nowhere near enough to pose any kind of overload risk.

If one were to plug enough capacitive-dropper night lights on one leg and enough motorized electric clocks on the other, it might be possible to make the neutral current slightly exceed 15A while keeping both legs under 15A. In practice, however, such an imbalance between capacitive and inductive loads would almost never occur in any realistic scenario, and even if it does the neutral current would be only slightly greater than the higher leg current.

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  • One of my questions is all about this case: diy.stackexchange.com/q/198837/71950 Feb 7 at 4:49
  • @JosephSible-ReinstateMonica: Yup. A theoretical possibility, but not really an issue for any remotely plausible combination of loads that could be present on the wire. Thinking about it, a somewhat more plausible scenario might be one where each leg powers devices that use half-wave rectifiers. The RMS neutral current would be only 1.4x the leg current, rather than 2x, but devices that use half-wave-rectified power wouldn't be terribly obscure; I've seen a number of lamps with a three-position switch whose middle setting puts a diode in series with the line.
    – supercat
    Feb 7 at 16:33
  • @JosephSible-ReinstateMonica: If the lamps had plugs that were non-polarized, but had the cable coming out the side, it would be plausible that one might have a bunch of them plugged into two circuits, with all of the lamps on each circuit sharing the same polarity.
    – supercat
    Feb 7 at 16:35
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To get an overloaded neutral on a properly wired MWBC would take a fault somewhere outside that circuit. The way the question is worded would require a fault outside the circuit by definition, you've defined that there is nothing wrong with the MWBC wiring.

In another question there was the issue raised of a generator that bonded the to poles of the split phase into a single phase, and using a fairly common TT-30 outlet out of a 120VAC @ 30A generator to supply power to a 240VAC @ 30A L14-30 house inlet. In theory the MWBC could produce enough current on the neutral to do some damage. In practice there's likely enough safety margin to prevent this.

Another matter is how unlikely it would be to have the entire load in the house (or nearly all of it) on that single MWBC to cause an overload before any circuit breaker trips. A generator larger than 3600 watts would be unlikely to have a 120VAC outlet rated for more current, it would use a split-phase 240VAC outlet instead.

Maybe some unusual fault occurred somewhere up to the breaker panel that could cause more current on the neutral and not trip a circuit breaker somewhere.

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Are there any scenarios where a neutral wire can be overloaded on a Multi Wire Branch Circuit (MWBC) when properly wired?

Yes, there are various ways in which the neutral of a circuit with multiple hots could become overloaded. This applies to both multiwire branch circuits and potentially to distribution circuits feeding subpenls. In most circumstances they will be unlikely enough that they can be discounted, but that doesn't mean they don't exist.

  1. Faulty supply. The design relies on the legs being out of phase with each other by at least 120 degrees. If this is not the case then overloading the neutral conductor is very easily possible. This may happen if power on one leg is lost and is then backfed via a hot-hot connected load. It could also happen if someone tries to feed a split phase property from a 120V generator, or tries to feed a property wired for three phase from a split phase supply.
  2. Harmonics. This is unlikely to be an issue on split phase (180 degree) systems, but is much more likely to be one on 3 phase (120 degree) systems. The reason for this is that most rectifiers use bridge type designs, which tend not to produce even harmonics, but can produce substantial odd harmonics.
  3. Grid tie generation. If one leg has a load, while the other has a source of power (e.g. a grid tie solar inverter) then there is obvious potential for overloading.
  4. Capacitive/inductive loads, if one leg is loaded down with loads that are highly capacitive while the other is loaded down with loads that are highly inductive then the currents would start to sum in the neutral.
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  • So, based on response 1., would you believe this circumstance to be something that could occur on a power outage, or repeated brownouts ? I regularly experience short term outages from tree branches.
    – M. I.
    Feb 10 at 1:56

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