Correct, because in that case the neutral current will be the sum of the current on both circuits that are on the same leg which can overload the neutral.
Yes you would still need to have a handle tie. Electrically swapping two conductors would fix the shared neutral problem but looking at the twin breakers there might not be a handle tie available.
David, the auxiliary panel label states the tandem breaker in space #8 is used for 2 appliances, Disposure and Dishwasher, share the same NM cable, one appliance use red 120V the other black 120V, and neutral. Not allowed.
Good catch. MWBC is one of the tougher things for HIs to decipher, partly because we shouldn’t (or legally can’t) go poking around in a panel and partly, because of the misunderstandings about them.
That is NOT a multi-wire branch circuit. Others will tell you something different. Anyone who disagrees is wrong. This is one of the rare instances in which I will direct you to the National Electrical Code (Article 100) as a reference. If there is no potential between the two legs (not phases, there is only one phase), there there is no MWBC. There is only one definition of MWBC and there are no exceptions to that definition.
Tandem breakers do not have tie handles. There is no need for the two breakers to be tied together.
Tandem breakers have two poles. They occupy a single pole position. Those are two different things.
There are two circuits. Each circuit requires its own grounded conductor.
This is a good discussion. So the question is, shall this be called an improper MWBC installation, or should it be called two ungrounded conductors on the same leg improperly sharing a grounded conductor?
I have called it an improper MWBC installation the very few times I have seen it. But now I can see how that might not be the best way to note it.
It’s a 3-wire cable that was run for two circuits so it was run as a MWBC. It was incorrectly wired so it is by definition not a MWBC. Since the installers intent was to run a MWBC in the first place I see no reason why it cannot be called an incorrectly wired MWBC that requires correction. IMO any other name would be unnecessarily confusing.
Interesting discussion here! One thing I’m also wondering about here, this panel was likely installed before the code change that made this install a code violation. So it was compliant back then but not if it is a new install today.
My understanding is that there are 2 issues here right? Potential overloading of the neutral and the fact that there is no handle tie which makes it a safety issue.
If only one of the breakers sharing the neutral is turned off, the neutral will still be energized. If whoever is working on that circuit is not aware of the shared neutral configuration… bzzz, ppsssst, flash. Not good.
So what does the narrative look like? 
And what level of detail meets the SOP?
I have no idea. Sincerely asking.
No. It is two different situations.
One situation is two circuits on different phases. The other situation is two circuits on the same phase. The OP is a photo of the latter.
One situation is a potential shock hazard. The other situation is a potential fire hazard.
Ah ok! So they are in the same phase… overloading is a problem since the current will be the sum of both breaker ratings.
But if they are on opposing phases it is a correct implementation, since overloading of the neutral is no longer an issue.
But a handle tie is a requirement either way.
I’m sincerely interested since I installed many tandem breakers in the past and I got this sinking feeling I did it the wrong way 
Probably did! But I can’t remember 
No, a handle tie in the latter situation is irrelevant.
Even though there is no MWBC wouldn’t it still serve its purpose of protecting someone who opened the neutral with only one of the two circuits de-energized? The handle tie would force someone to de-energize both circuits when working on either circuit using the common neutral.
Multiple circuits sharing a neutral is so common a deficiency that I’d estimate that the problem has existed in probably half the home wiring systems that I’ve analyzed because of fires or other failures. Often, there are more than two or three circuits involved. I’ve seen installations where an electrician has routed three or four circuits through a junction box, then indiscriminately tied the neutrals to the home runs without taking care to ensure that each neutral makes a continuous run to the panel with its associated ungrounded conductor or conductors. This doesn’t happen as often when there are NM cables as when the conductors are in conduits or there are no MWBCs because it is more obvious which conductors go together and there is less likelihood of there not being enough grounded conductors when NM (or other) cables are a involved.
The likelihood of an inspector (including building code compliance inspectors) ever seeing the errors is slim. The reason they are so prevalent and rarely found is that electricians often indiscriminately tie circuit grounded conductors together in places like attics and crawlspaces.
When they are discovered, nobody ever says something like “Oh, there are mis-wired MWBCs”. It is only when the shared neutral is discovered in an installation such as the one that is the topic of this discussion, in a panel box, that anyone ever mischaracterizes the wiring error as a MWBC. Calling a neutral shared on two individual circuits an mis-wired MWBC would be like calling a bicycle an mis-built boat.
In the cases such as the topic of this discussion, there may have been a MWBC at one time, but someone may have come along and indiscriminately put a tandem breaker. Once that happened, the MWBC no longer exists. Just as a cow that has been turned into a hamburger is no longer a living animal, if someone takes a MWBC and rearranges conductors into two separate circuits, the MWBC no longer exists.
The reason so many home inspectors are confused is that misinformation spreads like wildfire on the internet and people who claim to be electricians will incorrectly make the same mistake. But, whenever I present them with the situation where multiple circuits are incorrectly tied together in a junction box, they suddenly have a different opinion even though there is no difference whatsoever between the two situations.
It’s really very simple. If there is no potential (Voltage) between the ungrounded conductors, it is not a MWBC.
Another point worth mentioning is the misuse of the term “Phase”. They are called “Single-phase” systems because there is only one phase. There is only one primary winding. We could put a dozen taps off the secondary to derive a variety of Voltages, but taps are not phases. It would still be a single-phase. In the case of residential systems, we have one tap to get two different Voltages. The number of phases is determined by the number of primary windings, not the taps.
I taught continuing education to electricians for many years. It is mind-blowing how many journeymen electricians don’t understand this fundamental concept. Here again, it’s little wonder that home inspectors are confused when even some electricians go around referring to legs as phases. One primary coil = one phase. Period!
I’m sure it would, but from my perspective, I am going to recommend that a licensed electrician correct it.
Bravo! I agree with that. Lighting Circuits routed, three or four, through a J-Box once the basement ceiling was removed to rewire circuits properly.
The said thing is once the amateur gets steady current they think that’s all there is to it.
Great post, George.
Of course they are. Everything in the panel is on the same phase. There is only one phase. It is a SINGLE-PHASE system. Single means one.
They can’t be on opposing phases because there are no opposing phases to be on. Again, this is a single phase system, not multiphase.
Who requires a handle tie? The NEC certainly doesn’t. The NEC doesn’t recognize the installation as being legitimate, therefore it is not addressed. I can’t imagine that any local compliance authority would require a handle tie on an already unsafe installation.
This is super helpful! Thanks for your knowledge here @gwells It certainly clears up a lot of knowledge gaps for me personally on this topic.
I lived in South Africa for years and everything there is 2 and 3 phase. So at times I forget that in the US we mostly are dealing with a single phase.
Funny story… in South Africa wire color codes are backwards too. White is hot and black is neutral. As you can imagine that led to some interesting situations
Here is a video from another similar thread that may help explain some basic concepts.
https://forum.nachi.org/t/water-heater-wattage-vs-amperage/232693/15
I follow a few electrical groups on Facebook and some YouTube channels from all over the world. Most of the rest of the world follows IEC standards. NEMA standards are far superior. Most Americans are unaware of how inherently safe American electrical systems are. There are wide margins for error.
Two-phase systems are recognized by the NEC, but are extraordinarily rare in the US. Most home inspectors or electricians West of the Mississippi will likely never see one.
I enjoy learning the history of electrical systems in the US. In the early days, there were discussions of having as many as five phases. Over time almost everyone agreed that one and three phase systems would handle everyone’s needs and keep things simple.
I won’t say much about color codes aside from the fact that that’s an area where we still lack much standardization and there’s plenty of room for improvement.
There has never been a time during which it would have been in compliance with the NEC. I seriously doubt that it was ever allowable by any actual code or any model code.
Bob, that’s one of the best videos I’ve seen yet explaining a single-phase system. As an instructor trying to teach home inspectors the basics, my single biggest challenge is correcting the vast amount of misinformation that home inspectors spread among themselves and that electricians who have a limited understanding of electrical concepts also disseminate. HIs, by and large, don’t seem to understand the difference between an electrician and an electrical engineer.
I spend an inordinate amount of instruction time listening to HIs who are skeptical of what I say because they’ve spent too much time on forums, including this one, accepting misinformation as fact. I tell them that they are drinking from a poisoned well.
I’ve had home inspectors get angry with me when I tell them that there are NO opposing phases because there is only one phase. I’ve heard over and over “Mike Holt said …” or “Ben Gromicko said …”, referring to “Phases” (plural). All I can do is to tell them to listen more closely, more carefully. I doubt that Mike Holt and Ben Gromicko are the ones disseminating false information. It is electricians who either went through an abbreviated apprenticeship (some are only two years) or they simply misunderstood what was taught to them.
The problem will never be fixed as long as we have electricians spreading misinformation and inspectors not considering the source.
An electrician is a mechanic, not an engineer. Electricians actually learn very little about electricity during their apprenticeships. They mainly learn how to assemble electrical systems.
Mike Holt often reminds attendees of his classes that he is NOT an engineer. He does have a support team that includes some highly qualified engineers, and he introduces concepts beyond those covered in apprenticeship programs, but his primary audience is electricians, mechanics, not home inspectors. Home inspectors who spend time on Mike Holt’s website usually come away with less, not more, understanding of electrical concepts.
As long as electricians come on to this form and continue to condone and even promote the improper use of basic electrical terms, nothing will ever get any better. Those among us who know better ought to remain vigilant and continue to correct those who disseminate misinformation. Every single time someone refers to, or implies that there are, multiple phases in a single phase system, it is incumbent upon those among us who know better to correct the misinformation.
Every single day, I read or hear a home inspector misuse common terms such as “Mult-wire branch circuit”, “Isolated”, “Floating neutral”, “Dead-front”, “Romex” and on and on. Saying things like just because there are three conductors a MWBC or a miswired MWBC exists when in fact none exists is patently absurd. There are reasons other than the intent to create a MWBC for three conductors to exist in a common assembly.
Home inspectors would do themselves a favor by going out and purchasing a few good books written by real experts on electrical system design rather than to continue drinking from a poisoned well. I’ve used Mike Holt’s materials for years in providing continuing education to electricians, but I don’t recommend them for non-electricians.
Going back to the Single-phase topic, I hope the readers of this discussion invest a few minutes to watch the video. Maybe it will finally sink in for some of them that SINGLE = ONE. Everything is on the SAME PHASE because there is only one.