Branch Circuit, Multiwire, NEC 2020 - [210.4], (19min:38sec)

Multiwire branch circuits are a cost-saving wiring technique that shares one common neutral conductor between multiple branch circuits. This technique saves wiring costs but introduces a handful of dangerous side effects, including potential confusion, safety, and equipment damage risks. Multiwire branch circuits require each phase conductor to be on different phases. Doing this guarantees their shared neutral will never need to carry a higher current than any of the phase conductors, which is why it is possible to use the same ampacity for all the current-carrying conductors of the multiwire branch circuit. Preventing the need to use a larger neutral conductor, is where the cost savings happen, so this is only permitted by the code when the phase conductors are on different phases.

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[OwensandCompany]

I was working at a hospital, a maintenance electrician was working nearby. He opened a junction box and unbundled the neutrals while the circuit was energized. There was a nice POP A few minutes later the Director of the Laboratory walked out into the hallway and asked why the power to a section of his lab just went out. At the end of the day, there was $120, 000 worth of damaged equipment...It was a multi-wire branch circuit.

[visiblepulse]

Last year I opened a ceiling gang box to tap in for a doorbell transformer. When I removed the wire nut from the neutral, the home run white lifted. In a nearby study, a 6-outlet adapter with surge protection started sparking and smoking. I could not pull it out of the outlet as it was screwed in. I reconnected the neutral and was then able to remove the adapter and drop it outside while it continued to melt. The shared neutral condition had presented the surge protect circuit with over 120v.

[neilbrookins]

Great video. Very helpful. One thing that may cause people to connect multi wire branch circuits wrong is the use of tandem breakers. If you connect the two hots to two tandem breakers you will over load the neutral because tandem breakers are same leg. But many people assume that any adjacent breakers are always opposite leg. Tandem is an exception to that.

Also I was surprised that no one mentioned that shared neutral is not compatible with gfci. That’s a good reason not to do it.

[PowderMill]

👍🏼 Thank you Mike and Company !!
As a firefighter/paramedic who relies on your videos for extra knowledge, I must say you are a real lifesaver.

[crowlsyong]

Mike Holt gets me excited about electricity. Man I just love his vibe. Have a good day!

[TGUlricksen]

I love how Mike handles these people. I wish I could do this gracefully. This panel of people are cough They’re everywhere.

[neutrodyne]

What a great video. It explains in detail what every electrician should know about Multiwire circuits.

[bigtuck]

3 mins in and my head hurts. Salute to you guys

[nyinfamousk]

This guy is awesome, I would have paid much more attention if all my teachers were these good.

[SwingboyPA]

If you are just beginning to learn this stuff, I want to add 2 things:
1). At 1:01, Mike does not finish the sentence: The equal voltage must be between each phase conductor *and the neutral*. If you're measuring voltage between 2 phases (as in the example), there is no "equal" to be had.
Also worth mentioning:
2). Single pole AFCI and GFCI circuit breakers will trip if you put them on either or both poles. I've seen people try to pigtail the neutral to go to each of 2 SP breakers; and that won't work. You must use a double pole breaker -and here's the best part: if you're working in an old house, you might have an extra hot on that circuit that trips the DP breaker. Like a hot sharing a neutral with another hot ...that is already sharing a neutral. This is serious and must be investigated because that means that at least 2 of the hots are on the same phase.
I've also seen a GFCI receptacle where someone tried to spit the neutral at the GFCI or after it (instead of before it). The load side of the GFCI must use only the load from one phase and all the neutral current must be on that phase. That wasn't part of Mike's talk, but new folks have to know that this is also an important part of understanding the ramifications in a GFCI and AFCI world.
I use AFCI and GFCI protection on shared neutral (multiwire branch circuits) knob & tube all the time as a diagnostic tool to find extra neutrals all the time ...because sometimes a neutral doesn't get shared where you can see it; it happens in a junction box somewhere else in the building.

All that said, Mike is an awesome teacher and we're lucky to have him!

[NipkowDisk]

My house (built in 1978) has two MW circuits. Neither of them had handle ties for the breakers (they weren't even next to each other although they were on separate legs) AND the circuit to the laundry room used the device yoke for dividing it up!!. Needless to say, I fixed that very quickly.

Neutral integrity is an absolute with MW circuits. Period.

Reckon I'm showing my age here, but I also refer to these as "Edison" circuits.

[surfride]

I have wired a dozen or so homes ages ago for my father a gc. I messed up several jobs by overloading the neutral with breaker/leg misplacement. It was eventually caught, no damage done and corrected. Neutrals are anything but neutral, it should be called something else, like return, neutral minimizes the danger, much like interchangeably using grounding and bonding; that too is a deadly mistake. Just ask the families of the dead burned alive in the enclosed metallic bus stops with metallic grated benches... On a three phase panel, the 220 leg is orange and labeled stinger; a novice should head the messages; dont F with this. I digress; too many improperly trained, improper supervised and lazy inspectors create deadly hazards; like I did. Appreciate you!

[rafaelcastro]

Thank you Mike for all the technical support, information-Advice that you share! 👏

[commercialsparky]

The legend himself with another amazing video

[_fun_facts]

Ran into this today. Two separate breakers, one neutral.

[DellAnderson]

I can vouch for the Murphy's law of multi-wire circuits. Had a variant of such a circuit in a 240/120V all electric motorhome. Some 120V receptacles had 3 volts, others had 208 volts. Finally found an open neutral in the opposite end of the 40 foot vehicle (it was a loosened critical neutral crimp connector!), but too late to save the $2000+ RV fridge. :-(

[timrxn]

NOW I understand why I got zapped after killing a breaker and going into a box. Multi branch circuit but no handle-tie. I’m a diy’er but trying to learn what is ACTUALLY going on and not be just a hold-my-beer hack. Thanks for your excellent videos

[kahlil]

It's not only the panel you have to pay attention to but the phase as well. Some guys will land the ungrounded conductors on single pole breakers on the same phase which would make the unbalanced load additive on the neutral. So if you shut off one breaker thinking you are safe on that neutral you might have another hot wire out there still sending power back on it as well.

[dannylee]

I'm currently doing a remodel on a house. In two different locations I isolated the breaker and turned it off. But I was still reading a small amount of voltage 24 volts I believe. And one case I was able to find the other breaker and completely turn it off. Looks like this is a multi-branch but the breakers are not connected with each other. Very dangerous.

[tedlahm]

Exactly. Kirchhoff’s voltage law for series circuits. The load with the HIGHEST resistance (ohms) will have the highest (proportional) voltage drop.