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So I've stumped/annoyed most of the internet with this one. I hope there might be someone who can point me in a different direction..

I have a house built in 55 with ungrounded outlets. A space heater overloaded one and now I have no power coming from the circuit that supplies them. It's a mwbc. I have two.. one supplies the lights and the other, the outlets. I was told that the neutrals from the two separate mwbc's shouldn't be connected (but they are). I know this because when I switch the "lights" breaker on, I get a little bit of electricity going thru all the outlets (enough to power an led bulb but nothing else). Or maybe whatever happened caused them to come into contact.

I should also mention that one leg of the "lights" circuit seems to be working fine (utility room and kitchen) but the other leg can only power led bulbs as well. Most have said this is an open neutral but I don't have erratic voltage like most cases I've read about. It's as if there's just nothing going to the outlets at all (I tested 120 coming from all the breakers,btw)

The poco came out and checked their stuff and I checked all the wiring at the switches,fixtures,and receptacles, and the neutrals look fine at the panel. The space heater wasn't on the first outlet in the circuit either.

I'm the type of person that if I see it I can fix it but for all I know there could be a hidden junction box (nothing in the attic though).

The only other thing that sticks out is that when it first happened, there still seemed to be enough electricity running through the outlets to cycle everything that was plugged in at the time. Of course, the first thing I did was unplug everything (breaker wasn't tripped but I reset and eventually replaced it). After that the outlets could only make a small fan groan a little.

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  • Whem you say two separate MWBCs do you mean 1 MWBC that has 2 hot wires on it, or two MWBCs that have two hot wires each, giving four? Are there two neutrals with them? Dec 9 '19 at 8:15
  • It's two MWBC's with a red black and neutral. A 15a dp to the lights and a 20a dp to the outlets. They actually originally had sp breakers with no ties which is why I didn't even know at first that I had a MWBC.
    – kmp
    Dec 9 '19 at 9:50
  • Consider one of these DP breakers you just mentioned. Are there two handles that move independently, or are they tied together? Dec 9 '19 at 16:21
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Not a lost neutral

A lost neutral in an MWBC is easy to test for. Just remove all loads and plug in two 240V-tolerant loads, one into each half of the MWBC. (watch it - the appliances may see voltages as high as 240V!) If one leg has higher voltage than 120, and the other has lower voltage than 120, and they total to 240, then you have a lost neutral.

However, from the sound of it, that's not what you have. It sounds like you have a lost hot wire.

Phantom voltage is confusing everything

Also, you are running into a phenomenon called "phantom voltage". If several wires are run parallel for a distance (like any wires in cables, or cables next to each other), the wires will have capacitive coupling. If one wire is disconnected/floating, and a voltage will appear on it, induced from the other wires. This voltage has no strength. It can move the "needle" of a digital voltmeter; those are so sensitive they will read the phantom voltage. It cannot move the needle of an old analog meter in most cases. Apparently, it can dimly light certain cheap LEDs.

Phantom voltage is never full voltage. It is somewhat less. For instance if your mains voltage is 121 volts, and you measure 109 volts with your DVM, that's not "meauring 120V". That's phantom voltage.

Phantom voltage means the wire is disconnected between here and the source, and it runs alongside or in the cable of live wires.

It appears your lighting MWBC is inducing phantom voltage onto your outlet MWBC. That's interesting, because the two hot wires in a MWBC are supposed to be opposed, and you would expect capacitive coupling from them to be equal and opposite. In other words, to cancel each other out. **I suspect this is because one wire of your lighting MWBC has a wire break.

Tied neutrals

So far, you may be thinking "well, all the evidence I saw of a tied neutral, may just be phantom voltage.” That may well be. But a tied neutral is a very serious defect. It can result in that neutral overloading. And what just happened here? You plugged in a heavy load, and the circuit quit.

So as you search this wiring, you must separate any neutrals between the 15A circuit and 20A circuit. If you have any smart switches in the 15A circuit stealing neutral from the 20A circuit, get rid of those. You can't have that; it's illegal and it will painfully complicate troubleshooting. Normally I would say distinguish them with tape markings, but they are different sizes so that should be obvious. It is OK to tie safety grounds, but you say you don't have any.

Follow the dead hot/phantom-voltaged hot back to the panel

Let the phantom voltage be your guide. The wire break is either at the first failed outlet in the chain, or last working outlet. I'm using the royal meaning of "outlet" to include any point of use, including switches.

One more thing. The single likeliest cause of problems like this is backstab connections. Those are uninspectable. The only way to inspect them is destructive. Push the release tab if you can, and then twist and pull the wire out. (Never cut. Wire length is precious, but our entire point here is to inspect the side of the wire for spatter and spallation marks from arcing).

If you found tiny arcing marks, you probably found your problem. Welcome to backstabs.

Like we say, don't use backstabs at all; but if you're tempted to just jab it back in, don't. The backstab's tiny spring is now weakened, and if used again, will only cause problems like this. The backstab has 4 springs and cost 50 cents, what do you expect from it? :) Use the side screws.

Priorities

I would focus on fixing the lighting circuit, patrolling for tied neutrals, and then, once lighting is back, focus on the outlet circuit.

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  • So once the lighting circuit is back to normal, how do I diagnose the outlet circuit?.. all the outlets are dead. There are a total of 9 in the circuit and the sh was on the 5th one. Is it most likely a break from outlet 5 to 6 and bc they're all tied together, none of them are getting voltage?
    – kmp
    Dec 10 '19 at 8:23
  • @kmp I have a feeling you'll find both problems together. Dec 10 '19 at 21:41
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The neutral is common to both circuits in a MWBC this is correct.
If you have 2 neutrals it’s not a mwbc and the 2 circuits should not be tied together. When the home was built the handle ties were not required that came later for MWBC’s. The problem you have is a bad connection, small heaters cause this all the time on branch circuits . If I read everything correctly your lights work but not the outlets , we usually find the failed connection at the last working outlet or the first non working, it could be the hot or the neutral. If backstabs are used on the receptacles (push in connectors) this can and usually is the cause but if the screw terminals and wire nuts are used look for a broken wire close to a connection or pulled out of a wirenut. You have already checked at the panel so it is ok other than the statement neutrals are ok, a multiwire branch circuit uses 2 hot and 1 neutral on adjacent handle tied or common trip breakers. If you have 2 neutrals they were tied together and they should not be (now you have a tie it would be ok but I would want to find out where the 2 circuits are tied because this is not correct with multiple neutrals and part of the reason you are having a hard time getting an answer. So go back to the last working outlet on that circuit, if not there it will be at the first non working outlet done this probably a thousand times and it is 1 of 3 places 99.5% you have checked the panel.

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