Neutral Imbalance in Multiwire Branch Circuits (Two Hots, One Neutral)
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- Äas pĆidĂĄn 6. 03. 2022
- In the latest episode of Electrician U, a question came in from a viewer that needed some help understanding a topic. The question centered around sharing a neutral between 2 circuits. As this is a common practice in the construction industry, Dustin came prepared to drop some knowledge on the subject in an ongoing effort to better our viewers understanding!
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First, lets discuss Multiwire Branch Circuits. This is where we have multiple hot conductors paired with a single neutral. This installation can be in the form of Non-Metallic Cable (with a black, red, white, and grounding conductor) or even with MC Cable/standard pipe & wire method. Under normal circumstances, this is a perfectly acceptable practice, provided (among other things) that you have connected the hot conductors to TWO SEPARATE phases. One hot to A phase and the other hot to B phase for example. This way you donât overload the neutral with return current from the same phase. Another thing that the NEC requires us to do is make sure that the breaker handles for the multiwire branch circuits have a common handle. This can be accomplished via a two (or three pole, if it is a three phase multiwire branch circuit) pole breaker OR other approved breaker handle tie. By doing this, if you shut off one breaker, you should not have any return current from any of the other circuits in the multiwire branch on the shared (or common) neutral conductor.
To make it a bit easier to understand, imagine, that while the black hot conductor if pushing the current thru the wire/load, the white conductor is pulling it thru. And if the red/white set were doing the same thing, then the current that is on the white conductor of a multiwire branch circuit would cancel it out and we wouldnât have any issues. It would be the same, in essence, as a 240v 2 pole circuit with two hots. Now, this would only be true if the exact same load was on each circuit. Letâs say 10 amps. What would end up happening is the 12 ohms worth of resistance (120v divided by 10a = 12 ohms. Basic Ohmâs Law) would be doubled across double the voltage (12 Ohmâs + 12 Ohmâs on a 240v circuit now). The current is still able to make the complete loop from source, thru load(s) and return to source, therefore making a viable electrical circuit, and everything operates as it should.
Letâs get back to the statement of the exact same sized load being on each circuit and unpack that for a minute. We have discussed what would happen if the loads were balanced. But what if the loads were Imbalanced? Say 10a on one circuit and 15a on the other. Well, the neutral conductor (grounded conductor) would carry that imbalance of 5a back to source. This doesnât necessarily pose any problem other than the fact that there are 5a of current flow on something other than the hot conductor. If we were to look at it from two single branch circuits (with the same amount of current on each), in lieu of a multiwire branch circuit, it would look much the same. 10a flowing out on the black wire, thru the load and returning on the white conductor, and the same with the red/white set. But the point where those neutral conductors came together (at the neutral bus bar) it would become 0a worth of current flow as they would cancel themselves out. So as an overall picture, the neutral is only carrying the unbalanced amount of current!
A big issue that can arise is when we do not use hots from different phases. Instead of each carrying 10a worth of current working in tandem (one pushing the other pulling for example) they are now both trying to push that 10a of current since they are both traveling the same way. So, while the breaker may be fine (10a worth of current on it) the neutral, at the point they came together, would have 20a worth of current on it since there was nothing to carry the other 10a away!
#electrician #electrical #electricity
![Branch Circuit, Multiwire, NEC 2020 - [210.4], (19min:38sec)](http://i.ytimg.com/vi/Qynp92rIlYg/mqdefault.jpg)
Iâve been asking this question for the entire 4 years of my apprenticeship and no one has been able to answer it. Thank you for taking the time to explain this đđœ
Now you further understand the importance of pigtailing grounded conductors as well, I'll bet đ
Your apprenticeship program must really suck. This is super basic.
A very complete and on target explanation. A minor correction in terms. A residential electrical system is single phase. The red and black are at different polarity, not different phases. Red is positive polarity (peak sign wave) and black is negative polarity (trough of the sine wave). That's the push/pull effect on the single-phase ac system. [yes, the sign waves flip back and forth, but red as positive polarity and black as negative polarity is how the NEC describes them in 200.6 & 201.5]
When I was a first year.. almost 6 years ago.. a Journeyman asked me which wire was the most dangerous or requires the most caution. I of course said... well it's your hot.. it's bringing power through your circuit.. I feel like this video is an amazing example of why your neutral CAN be the most dangerous. It's something to keep an eye out for before you start jumping into your project. Very nice video. đ
Good stuff. đ
For multi wire branch circuits,
I've been Using breaker handle ties that allow for independent tripping but simultaneous switching Because I believe that's right, right?
If you haven't already you should make a video about do's and don'ts of multi wire branch circuits.
Should make for a lot of content hence my comment is so f****** long
like If 2 single branchers are in one Box, you don't just tie all the whites together.
And why you might find a neutral wire in the panel has turned a darker tan color compared to the other white ones. Like what can happen if you disconnect neutrals at the panel Without killing the breaker(s) it belongs to. Pig tailing neutrals at devices. And if you find Kitchen outlet Receptacle with black and red on one side, Note the tab is broke off, Separating the terminals.
Labeling.
How you can ruin plugged in equipment by Being really cool and working on a multi wire branch circuit while it's hot ( Or The other breaker is still on) and disconnect the neutral.
Biggest shock of my 50 year career was a neutral. Service drop, (overhead) was torn off the side of the building, laying on the ground. I had cut & taped off both ungrounded conductors. I then separated the neutral ACSR cable(overhead) from the SEU that should have run down the house. I was unable to let go. Still not sure how long I was stuck there, it seemed a very long time.
@@earthtones1138 i believe thats right? how you ever get your licenses?
@@williambeauregard5440 50 year career. and you working hot circuit? how you make it that long?
the most dangerous wire is the one you think is dead.
Love your videos. You should mock these up as examples in a lab environment, use a clamp meter and show us the values.
Why?
@@mikeiver I guess I just like to see these things in a lab environment, or in actual application. It's fun to see, and when I studied electrical in college, I preferred lab classes over theory classes.
@@cachapma99 Fair enough, some can only learn by seeing and doing rather than reading. this video is more geared to the beginner rather than the seasoned tech I should expect. there is always something to be learned though.
I'd like to see that as well. He's saying on a 20 amp two pole breaker if you hook 12-3 and run two separate 120v circuits off it, you actually have 240v coming back on the neutral?
@@schowers Nope. If you have a pair of breakers, one on each phase and utilizing a common neutral between them, that only the imbalance current is returned on the neutral. So lets say that you have a pair of 120V loads, one on each phase, with each drawing 10 amps from their respective phase. The actual imbalance current on the neutral would come out to ZERO. Each load would drop 120V and the current would return from one phase to the other. Now if we leave the load on the A phase at 10 Amps and reduce the load on the B phase to 7 amps we have a total of 7 amps from the A phase being returned thru the B phase and the balance of the current being returned thru the neutral, that being 3 amps. Get it?
Now there are other things in play here that I will not go into such as the possibility of excessive induced currents in a shared neutral should the loads be motors. IE: Power factors of less than 1. Mainly because I don't understand the underlying math and theory enough to speak on the subject with any authority. But the above should provide enough to help you understand.
This by far, is my favorite electrical knowledge channel. You explain everything to complete understanding and do it with the perfect blend of wit and humor. Thanks for all that you do!
THIS is the video I really wanted to see. The issue is significant for me. My house was built in 1938 with a two-wire 110 volt service which fed the entire house through two fuses. Wiring is all in conduit and was all routed through ONE 4" square box. Entire house. About 30 years ago service was upgraded to 240 volt. The electrician routed a couple of additional circuits but left the rest as is. I have untangled some of it, especially after breakers started tripping. There are definitely shared neutrals remaining. They did try to save money on wiring and conduit in this post-Depression house. And residential loads back then aren't what they are now. It is great to know how this works, as I continue to unravel the wiring in this house.
Dustin , I am older than you , But had to make a career / job change . I have only been a electrician for 8 years now . Learning Residential , Commercial , Industrial , So much to learn . Everyday is a learning experience . Love the way you teach . Thank you .
This is gold man! Saving me years of class and helping me understand exactly what Iâm doing out in the field as a helper. Thanks
Great explanation. MWBC is what got me interested in how residential electrical works. I got shocked on the neutral when trying to change a light fixture, because my breakers were installed before the handle tie requirement, and I only had one flipped off. Then I did a bunch of research to figure out why that had happened, and along the way became very interested in how this all works.
I tried to change a light fixture and assumed turning the lightswitch off would disconnect the power. I felt a weird tingle but proceeded anyway. In my defense, I was about 13 or 14 and didn't know to turn off the breaker.
Never assume the power is off; always use a tester to verify! I then brush the wire with my hand as an additional check before grabbing it.
@@MrGatlin98 Tesla did a much better job at making sure you understand differentials, and that all current eeeks to seek a balanced state, and that means retruning to ground.
I once got shocked by a neutral because a 3 way switch on the circuit had neutrals from a different circuit in one of the 2 switch boxes.
â@@mmoser9483 Not always , two hot legs would choose to run most all current in between them rather than going through the ground ,current preffers to equalise the highest of all differentials .
As mentioned below a few times, there are some NEC regulations, including that both circuits on a MBC must be on a single 2-P breaker at the MAIN panel and and all sub/intermediate. The biggest risk I've seen is where Handyman/ Homeowner Joe moves one of the breakers in the chain not knowing it's part of a MBC and suddenly you have an overload on the neutral that neither breaker detects. The risk of hot neutrals is also a real thing too if you've ever been buzzed by one. That really makes for a fun troubleshooting experience in an unknown location.
As soon as any imbalance happens that amount of current flows on neutral.
I was surprised when I replaced some outlets in my mother's apartment and got shocked by the neutral when I thought I had the power completely off. It turns out that the NEC did not always require the use of a 2 pole breaker for an MBC and the electrical contractor used two separate breakers on two separate phases instead. Now I know that I have to shut off both breakers when replacing any outlet on either leg of the MBC.
Dustin, I came back to watch this again today because you gave me so much to think about my mind was going crazy today while driving to work and thinking about this. You are a truly awesome teacher and I really thank you for everything your teaching in these videos.
I've learned more from you than I have in school lol. Thank you for what you do. I wish I could work side by side with you for just a couple months.
Once again you have taken something that in my days of schooling seemed hard to grasp and explained it simply and perfectly. Keep it up.đ
Thanks for the video, as always you explained it in a way that I understood. I just started working as an electrician and your videos have helped me a lot.
Why do all of your outlets look look like they're extremely worried? đ This is a great explanation of these situations. It gets confusing when you add another phase when you don't understand how phases work. I've learned a tremendous amount from your videos. Some of the things I already knew what would happen but never knew why. Thank you for your time and effort to educate us.
Dustin, Multi wire branch circuits in dwellings can be with 2 pole breakers or handle ties manufactured by the same as the panel. The reason NEC started requiring them is not for overcurrent protection. It is for maintenance work to ensure disconnection of all hots on common neutral.
Really good video! Keep it up! Respectfully, Kevin
Wouldnât it be for both?
@@twestgard2 If you are sayin both disconnection and overcurrent. A two pole breaker will trip both. One for overcurrent and other nuisance trip. The handle ties will disconnect both, but overcurrent on one side will only trip one. That is because it was not the reason.
Makes sense... once you disconnect one side of the run, the other side will operate just like a standard 2-wire run. I was wondering about this as my home has 3-wire runs but they are not tied together. I suspect because the work was done before this was part of the NEC (added the rule in 2008 I believe?) so it's nice to know it wasn't for operational safety, but for service tech safety.
@@KevinCoop1 I agree with everything you said, however, requiring a handle tie on single pole breakers means that you're basically forced to have the two breakers right next to each other. This ensures that the breakers are on different hot legs, which prevents the neutral from being overloaded. Without the handle-tie requirement, one could put the two single pole breakers in any random spot in the panel, and could possibly put both breakers on the same hot leg, which could overload the neutral. So in that sense, requiring handle-ties kind of DOES prevent overcurrent (on the neutral).
@@jonclark1288 I agree with you that it does stop neutral overload from being on the same line, but my statement was the reason it was put in NEC was not because of your instance, it was originally for commercial work with multi wire branch circuits feeding portable office partitions. I looked up the in writing proposal to the panel and also talked to a person on that panel that I used to work with. A maintenance worker got shocked while working on the furniture. From there, they decided the next cycle to require handle ties or multi pole breakers for all multi wire branch circuits. So again, the reason why it got put in NEC was not overcurrent protection.
Respectfully, Kevin
Perfect explanation!! I had a situation at a convenience store where they used a single neutral on two 20 amp circuits on the same phase, it melted the neutral wire at the wire nut inside a junction box. The customer complaint was the smell of burning plastic near the cash register. Traced it back to a Junction Box underneath of the counter. The neutral wire was glowing orange at the wire nut and all the insulation was burned off. đ
someone moved breakers around in the panel and didn't know to check for shared neutral/phase.
yeah that's reallly funny... -sarcasm
Coming through clutch with the super hero moves that day! Hope you were well rewarded for saving the business from burning down. đ„
@@throttlebottle5906 so that circuit cant be on the same phase? It has to be a 240v circuit on separate phase?
@@gerardotorres7904 correct, if it's on different phase's the current is mainly flows across through the two devices and balances out, with any remainder going back on the neutral conductor.
if only one device is powered on then all of it's current will go back on the neutral. say it's a 20 amp circuit, then only up to 20 amps max will go back on the neutral.
now if it's wired on the same phase, and both 20 amp breakers, the current will not balance across the devices and you'll end up with max of 40 amps trying to go back on the neutral! that would obviously be double what the conductor is rated for, overheat and start to burn. I've ran into many times over the years.
it's now NEC code to have a two pole breaker on any shared neutral circuit, that keeps them on opposing phases and ensures the whole circuit is dead when breaker is off, that way you wont get shocked off the neutral, nor overload it.
This is a huge help, I've been struggling to figure out how this all worked in school right now. Thanks!
I listen to your videos all day at work and now I found one to listen to to help me with my slump during homework!
Man i love these videos ... keep them coming .. so helpful and educational
Love your videos! Please keep it up you and your crew do amazing!
You need an oscilloscope. Easy to see how current cancels with opposite sine waves.
Exactly !! Also, if you pause the video at, lets say (8:42), and have a look at the two pink arrows, if you can visualize the top pink arrow, with the arrow head at the neutral wire, you can see that when both arrow heads are at the neutral wire, they point in opposite directions, meaning the black wire is at +90 and the red wire is at -90, and their currents are at (+10A) and (-15A), respectively. And that is how you end up with (5A) on the Neutral.
@@griffmustard thank you for your contribution!
@@mattbaker6587 Matt, you're very welcome !!!
And if you yanked the neutral connection with uneven loads, one load would burn brighter (see higher voltage) than the other (the higher resistance load).
When equal loads, they work fine with no neutral at all.
Doesn't help when most electricians don't know what the X & Y axis is or how a sin wave pertains to electricity. They just parrot back what they see on YT without understanding it. A cartoon would be better.
Such a good video about something you come across every so often and also end up having to do for simplicity sometimes.
Tons of great extra info in the very knowledgeable comment section too.
Thanks everybody!
Your videos are the best electrical system content Iâve seen on CZcams, Iâm subscribed! I wired two âback to backâ bathrooms (they share a common wall) on a 12/3 circuit to ease the long run in a very low attic space. My old (1952) house has a shocking number of loads on each old 2-wire cloth covered romex circuit, and I am breaking them up while replacing the wiring as I redo each room. After watching your video, I went back and verified each circuit in the 12/3 was on a different power leg (they were), so Iâm feeling good on that one. But now, considering the potential for doubling the load on the neutral if miswired, I think Iâll just stick to single cable runs where the neutral is just carrying the one circuit. I was ok, but your video made me go and make sure I was, and thatâs the definition of worthwhile content. Excellent work!
If it's hard to access and both circuits need seperate neutrals, you could use 12/4,
I show the Black phase as a sine wave and the Red phase as an inverted or minus sine wave (-). The current for the neutral is the addition of the two. In no case is the addition of the two greater than one phase and if they are balanced they perfectly cancel out leaving a flat line. If one phase is 5A and the other 10A then you will get peaks of +10A and -5A leaving a 5A current in the line.
I really appreciated this. I had a few run ins with this situation recently before I seperated the supply lines. I actually have a bigger problem at my nephew's house. He has 3 or more breakers with a common neutral. I wonder if you could help me explain to him and his father how scary of a problem this is, since me telling them "this is really bad" isn't helping.... they seem to be of the mindset that" well, it's working, so why me worry?" Maybe you already made a video of it. Thank you so much!
Great explainer! This becomes especially important if you have tandem breakers in your panel. The house I just bought at the end of last year has a number of them and most were OK but a couple were doubled onto a MWBC. It's so easy to avoid by simply not sharing a single tandem breaker for a MWBC (should be split and tied) and yet someone did it anyway. I've found a few other "Why would anyone do this!?" surprises along the way. Yay for old houses and shoddy work!
RESPECT! Thanks for ALL the years of sharing simple ways to understand complex problems. Your doing a Great Job! Thanks for sharing your Journey through the trade! Best of Wishes Brother
Shouldâve talked about what happens if you lost the Neutral in the multi wire circuit with the imbalance loads
Doesnât it act in series causing the potential for one load to be burned out due to kirchoffs law of current pretty sure the possibility of one load not receiving enough amps because source is supplying lower current for the lower load also exsist just depends which is connected closer to source
Year one sparky here so could be so so wrong â ïž
A friend of mine has this happen to his entire house cause the neutral was lost at the transformer supplying his home. Nearly every piece of sensitive equipment in his home died, including the brand new central AC system as all of the computers in those things need 120.
Woooo!! Thatâs bad!! Anything connected to those circuits, get 240⊠erâthing gone.
It might help to include waveforms in conjunction with push/pull to show the addition of two phase waveforms adding up to zero in one case or 5 in the other case.
I ran across this exact situation in my house. The hots were on red/ black phases on the panel but weren't combined with those little breaker locks so i was getting phantom current on my neutral at an outlet and i was like WTF. This explained it so much more deeply than i figured out on my own. I gotta thank you . Been a long time watcher but this was so exactly what i was up against and explained so in depth that i gotta say hats off on this one.. well done and thank you!
Nobody explains this stuff better. Thanks professor Dustin !
Kirchhof's first law. The sum of currents flowing into a junction must be equal to the current flowing out of it
Yay. Opposite phases of equal current sum to zero, thus no current in the neutral.
Kitchen counter plugs are usually wired with a 14/3 wire, the tabs between the outlets is snapped off. Red wire one side, black on other side and the white does both. So you can use a toaster oven and a coffee maker at the same time on the same plug. How is that any different than this, isnât that an unbalanced system? If it is then why do they even make 14/3 wire?
@@huejanus5505 The neutral will still only carry the imbalance back. Of course if only one of those is operating then that will be the full current...
@@huejanus5505 As long as the black and the red are going to different phases on the panel this is ok
Yes that makes sense, especially when he showed the diagram with the resistive loads. Then you can calculate the current flow.
Overall this is a very helpful video. But simply labeling that node with arrows showing 10A in and 15A out means the extra 5A must be coming in on the neutral would have saved a lot of hand waving
A diagram with "10 amp loads" was shown at one point , but that is a bit vague and misleading. it suggests you might be able to just look at the labels of each device. In practice that gets you in the ballpark of what the balance is, but to really understand what is happening, and why, you would have to know the effective resistances. That diagram with resistances makes more sense to me.
Great information. I just have a caution to add.
This is a split single phase system (often referred to as 2 phase.) The secondary winding on the transformer puts out 240 volts from end to end. There is also a center tap (a wire connected to the center of the secondary winding.) From either end to the center tap is 120 volts. The power company grounds the center tap. That is the neutral. It is a grounded conductor, but not a negative wire & not a safety ground. The push and pull is making reference to frequency. In the USA, the frequency is 60 Hertz (cycles per second.) So, 60 times per second, the red & black wires cycle from 0 volts to 120 volts. When the red leg is hot, the black leg is not. That allows 2 circuits to share a neutral at full capacity as long as each circuit is connected to a different leg. As mentioned, the neutral carries the difference if the 2 legs don't have exactly the same load (and in 120 volt circuits, they rarely do.) That neutral serves one more purpose. If one leg becomes open, then the neutral will carry the full load of the other leg.
However, Multi Wire Branch Circuits (MWBC) have a danger you should be aware of. Regardless of the load on either circuit, you DO need the neutral wire, even if it carries no load. It's the neutral wire that renders two 120 volt circuits from the 240 volts (just like it does at the transformer.) Every MWBC must have a bonded neutral connected to the neutral of each load at the point where the 240V circuit is split into two 120V ckts. That creates two parallel 120V circuits. If the neutral wire becomes unbonded (disconnected) from the breaker panel, those two parallel 120V circuits become two 120V circuits in series which creates a single 240V circuit & 240 volts will pass through both circuits and everything connected to them. That's fie because the 2 circuits are resistance in series. But if the resistance of leg "A" is increased by turning something off, without a neutral, leg "B" would require more current than leg A. That current would be pulled through leg A since there is no neutral. When resistance is increased, but the current from leg B remains the same, the result is a voltage increase on leg A. That will result in a surplus of voltage on the neutral of leg B. (The neutral will have more voltage than the hot can cancel.)
If that happens, damage to devices connected to both circuits will occur and it's likely to be beyond reasonable repair. Always turn off the appropriate breakers before loosening the neutral of a MWBC. (Think of two AA batteries. If you connect negative to positive on two 1.5 volt batteries, you get 3 volts. If you connect the positives together and the negatives together of two 1.5 volt batteries, you still only have 1.5 volts. A MWBC is exactly the same, except it's AC, not DC so there is no negative and positive, and don't even think about connecting the 2 legs together. I know, but I had to say it just to be sure ...)
Unrelated, but safety is always worth mentioning: The biggest danger of getting shocked by 120 volts occurs when a hot wire falls and rests on you or you fall onto or into something hot (such as an open bus). If you bump it, you can fall away from it but if either you or "it" comes to rest while you're in contact with it, you're in trouble. Don't fool around with hot panels alone or push a metal fishtape into a panel.
Well said!
My electrician told me that I had a multi wire branch circuit wired house. I found this video and it made it all so clear. Thanks.
Love this one. Best description of why you must split passes if you share a neutral
Excellent explanation of phase relationships that unfortunately most people have trouble understanding!
Would love to see you make a video on balancing out a panel between the two legs of power
Love your videos. Did this because it made sense in my head when I built my home, I'm so relieved it won't burn the place down. đ
This is great, Iâve been asking this question and no experience electricians can answer this. Iâm very technical with questioning everything and no one can answer. Great job Dustin
Great vid. Would have been good to add: if no loads are attached to the opposite leg it acts like a normal circuit, the dangers/behaviours of an open neutral in these circumstances, and using a split recep as a visual
I lost two computer monitors and one PSU to an apartment with bad neutral. One leg could dip to 90-100V L-N and the other surge to 140-150V measured at the breaker box. The situation improved significantly (125V max, 110V min) after the power company changed the transformer a few months later. The transformer must have had a broken neutral and the building still have a loose neutral between the transformer and unit panel.
I actually have this in my home. They are mostly on the circuits that feed the upstairs probably because they were to lazy to fish proper wire to feed everything. My house was built back in 1907 and originally had knob & tube wiring. When it was rewired, they fished this type of wire to the attic to feed the upstairs rooms.
There's nothing lazy about this, it's a perfectly fine way to wire things.
thank you for your videos. it helped me chase down a problem with with an open neutral where two branches were shearing the same neutral. The master bedroom outlets and bathroom lights were out, found the problem in an outlet in the kitchen where a 12/3 conductor came in from the main panel. This is where the circuits branched off. The neutral from the effected branch was loose from a bundle of neutrals wire nutted together. Connected neutral and corrected problem, lights and outlets working again.
Thank you.
Awesome explanations !
Thank you for using correct terms with regards to âVoltage acrossâ, âCurrent throughâ, and âResistance ofâ.
Hey Electrician u , can you make a video on how to read wiring diagrams (if you havenât already) Iâm having trouble with that in school thanks!!!! Love your vids
Justin, this was a real good lesson for sure. I ran a multi wire branch circuit some time ago sharing a neutral between a lighting circuit and a dedicated microwave circuit. They are two separate phases but I used two breakers that operate independently of each other. Now I recognize the danger: I could open a breaker for the lighting circuit but the microwave circuit would still be feeding that circuit through the shared neutral. I think I need to abandon the the sharing idea and pull a separate cable. Your thoughts?
Just add a handle tie connecting the two breaker handles. That way, you trip both at the same time.
Once I figured out that a common neutral will only carry the maximum load of an imbalance, which is equivalent to the maximum load of either hot, it all made so much more sense. I understood this before this video, but "one pulls while the other pushes" is a great way to explain this to anyone else so I still learned something useful for watching. Thanks!
Thank you for this. Our 1976 house is loaded with 12-3 shared neutrals.
I'm not an electrician, but I play one with my home and equipment, and have for 50 years. I try to understand and follow code and best practices and your explanations help a lot with techniques and how and why. I recently had a licensed electrician looking at my panel to give me an estimate on some extensive work. I have one 12-3 w/g cable connected two 20A breakers on opposite phases feeding 2 different circuits, just as you've described here. He called this an Edison circuit and said that Edison circuits are no longer allowed. After seeing your presentation, I'm wondering about his statement; are these circuits no longer code?
Thanks for doing Electrician U!
While I'm not an electrician, as far as I can tell in the current NEC code a multibranch circuit (Edison circuit) is permitted so long as both hots are connected to a Tandem Breaker. This ensure that the hots are on different phases, and that if one circuit flips it takes out the other circuit that's sharing the neutral. There are also rules on the device side that the neutrals must be pigtailed together before the devices, so that the neutral is not dependent on the device. (No daisy chaining, must have the neutral split into independent wires to the device after you split your multibranch circuit back into two 120V circuits with the shared neutral.) Now, you're state may have different rules that supersede the NEC published rules, as well as sometimes local municipalities have ordinances that further restrict wiring.
@@joatamonk you cannot connect a multi-wire branch circuit to a tandem breaker. I think you meant to say double pole breaker, which is entirely different. Multi-wire branch circuits must be connected to a double pole breaker OR to two single pole breakers that are handle-tied together. This ensures that 1. The breakers are on different phases so as not to overload the neutral, and 2. To ensure that if you have to turn the breaker off to work on the circuit, both hot legs are dead. Now if you're using 2 single pole breakers tied together, if one breaker trips, the other will NOT necessarily trip (a breaker can trip without the handle physically moving). This however, is NOT a safety concern. It's totally ok to have the other phase powered even if the other one tripped. If there was a problem with the other phase staying energized, then that breaker would trip as well. That's actually a benefit of using single pole breakers handle-tied over using a double pole breaker.
@@joatamonk Aha! Tied-together breakers and no daisy-chaining make good sense. Thanks!
Missed an opportunity to do what you have done well on other videos that helps make sure things are done correctly - reference code and talk about what needs to be done to make sure code compliant and not creating an issue inadvertently. My experience has been that what you spent only a few minutes talking about at the end - issue with MBC on same phase - is what's typically done and hence create the potential for too much current on the neutral. Nice to know the theory but it would be beneficial for a simple explanation of what should be done to properly wire a MBC,
Never stopped those videos only one thing to say excellent
I'm a electrician who is a real visual person when I listen to somebody else, & so at around 14:36 I got a visual from listening to you that helped me figure out how to explain to somebody who asked me why you can't have two hots on the same leg sharing a neutral, & the visual I got was: picture two people on the same side of a lightweight wall (calling each side or wall a leg) pushing with 20 lb of pressure each (20amps) on the wall versus.... one person on one side of wall and one person on the other pushing 20 lb of pressure each side....& then the thin wall just safely spins round & round with them pushing, BUT the wall with two people pushing on the same side... the wall falls over & they fall on flat on their faces with the wall!!! đ I don't know I can't help what visuals I get when I hear great teachings!!! đ
Hi, Dustin.
Could you make a video about phase displacement in transformers? I've been a bit confused about it in class, and we've been connecting transformer banks in parallel, thankfully all of my connections have worked and nothing blew up.
But I'd love to see this topic in depth.
IF you really want to fully understand phase displacements in transformers, it is best to study phasor algebra. Using geometry to explain how the sine waves react is not going to be enough.
@@randomlife7935 I'm taking a technician's degree, I asked my instructors and they're never gonna teach that to us in depth, just basics. So, I'm trying to find out a dumbed down version of it, basically.
@@randomlife7935 I think there actually is a fairly sound geometric representation using triangles. I will revisit this tonight and see if I can pull it together in words. Or link to an image/video.
This is also known as the Vector Group of a Transformer, and it details the Phase Displacement/Shift between the Primary and Secondary terminals of a Transformer, connected in (Delta-Wye) or (Wye-Delta). In any displacement situation, you will always have a Lagging or Leading condition between the Primary and Secondary. On a Transformer Data/Name Plate, you might see (Dyn1), (Dyn11), (Yd1) or (Yd11). This is indicating a Shift of (+30 degrees) or (-30 degrees) between the Primary and Secondary. Often times you might see the face of a Clock being used to show the Phase angle. When the Long Hand or the Short Hand of the Clock (I forget which one) shows (11), it indicates a (+30 degree) shift, and when it shows (1), it indicates (-30 degree) shift.
Its not a hard concept to understand, it just takes a little researching of the topic, to put it all together, and to have it make sense.
No, what he is talking about folks is connecting transformers in parallel...it's very easy. He's talking about the direction of the windings and the direction in which the magnetic field is flowing.
The direction is typically indicated on a circuit diagram by a dot placed at one end of the coil symbol.
Now on a single transformer, they will wind both the primary and secondary windings in the same direction, so that a positive voltage applied across the terminals will drive a current through the primary and magnetically induce a voltage in the secondary coil which is of the same polarity, such that a positive voltage in gives a positive voltage out on the secondary.
Now, when you start connecting transformers in parallel and in series, you need to ensure the same principle is observed. Otherwise you are going to be in trouble.
Look for a polarity identification mark on the transformer to indicate which way the coil is wound. It might be on or near to the connection terminal for the winding.
Electricians remind me of Thanos. As with all things there must be balance.đ€Ł
Thanks for this video. This is especially pertinent because people are now getting 120/240 split phase interlocks or transfer switches installed and then using 120V generators to power them, often just bridging the two hots. So they have potential of overloading the neutral when using these generators.
You are a master Dustin !
Thank you for help
Love from đšđŠ
Love your stuff dude I'm leaning so much and I use it in my field all the time
After much googling, this is the Best video ive seen to explain this. Thank you!
Easy to understand from a Working Electrician and Smart Person. Thank you Man.
That was great, thanks for answering my question; almost never hear someone speak of this situation.
Very nicely done. Presented a concept that most have never even though of.
Very nice explanation. As a young apprentice without knowing theory, we were always taught to balance the panel. Always had to make sure all 3 conductor cables from panel had each hot on a black and red when neutral was shared. This will clarify for many young apprentices. The push and pull is something you don't realize. Most may think of branch circuit behavior more like DC. Mike Holt doesn't have Jack on you. LOL
As always, very well explained. I was going to make a comment regarding the fact that a combined or split neutral return path on a center tapped 240V connection has no impact to the overall electrical circuit, but you got there at the 12 minute mark. I like that you demonstrated the danger of the single phase common neutral circuit as well. I think there is more here that could be explained to help people understand why they get a poke off of the neutral, but it might be a bit much for one video. Thanks again for an understandable and well spoken video that will make sense to the general electrical worker public
Thank you for sharing your knowledge. Great videos
Great explanation of MWBCs. i have an older house and wanted to install an appliance in the basement on an existing MWB circuit. I wanted to properly calculate load on the circuit and get an idea of how the circuit would handle a possible unbalanced load on a shared neutral. Mentioning how it is similar to how two single circuits terminate in the panel except the neutrals are tied together before the panel and how the different phases on the two legs cancel each other out on the neutral and the unbalanced difference is what flows through the neutral was also a big help. The best explanation i have come across.
You need to go further with this scenario.
If you lose the neutral to the cabinet that is doubled to the outlets and you have a separate load going to each outlet, then you will have 220 volts divided into each outlet, depending on their amp draw!
That is why I never use this circuit unless it was a 220 circuit.
I have also worked on 110 lighting circuits that shared the return circuit in which I isolated the one circuit, thinking it was safe to work on, but in fact the neutral was hot from another circuit's return.
I have also run into the same situation!
It was on two, 277 3-phase lighting circuits sharing a single return.
Always great videos! Thanks for all you do.
Multiwire Branch Circuits NEC 210.4 and Multiple Branch Circuits NEC 210.7 sound the same in the code, and have the same breaker requirements but are different. Good job on showing and explaining Multiwire Branch Circuits. thumbs up.
The operative function here is that both phases are 180° out of synch with each other. Any imbalance is resolved in the neutral, and is not the addition of the two currents rather the only current resolved in the neutral is the result of the lower current subtracted from the higher current that cancels out the other leg.
Our home in SF was gutted and completely redone top to bottom. They installed subpanels and all outlets and lighting (except bedroom outlets using ARC breakers) are MWB circuits. So at least some people are still installing them as of 2015, I assume to save on copper.
I think sometimes I overthink this stuff and make it more complicated than what it is but you did a great job of laying this out it's kind of help me put it together thanks
Mighty impressive that Dustin invested into this whiteboard or touchscreen device that greatly helps visual learners understand the concepts. It really is a shame that one of the largest community colleges in the U.S. cannot have these resources for it's instructors. Instead I had my instructor refer me to Electrician U channel. And the name of the college is Houston Community College...
One hot , one neutral !! Correct balance!! Thanks đđœ
Good presentation, I would like to see a lab test with a current hook-on and some power meter. I like the way you simplified the issue, The analysis is as if it were a DC circuit. Would be interesting see both fans working with the neutral removed.
Thank You. Now I understand how the garage that was added many years ago to my 1957 house that I just bought is wired. I couldn't understand why there are so many red wires in the junction boxes in the garage. They ran two multi-wire branch circuits from the box in the house to the garage.
This answers my question I was wondering now that I'm getting into more electrical stuff. I just wired an l14-30 to 2 220 30a welding plugs and a 110 20a plug
Awesome video! Thanks Dustin! Question, at the top of the video you mentioned that the multi-wire branch circuit, when wired correctly using both phases, should use a double-pole breaker (both must trip at same time). Is that always the case, even when your two hots are going out to separate receptacles, powering different equipment (as in your example)?
Thanks for this video. I have this situation where I had a 12/3 on two separate breakers that did not have a common shut off handle. The 12/3 gets split in a junction box to service two separate circuits. I am now going to rectify the fuse box issue by swapping the two separate 20amp breakers for a 20 amp breaker with one common service handle.
Your always on the spot with your explanations. :) I appreciate that. :)
I learned this using sign wave. You positive and negative wave cancel each other out. Love the reminders though. Basic electronics, love it.
Hereâs a complication related to that configuration which came up when I had some recent kitchen work done. I wanted the GFCI function moved to the panel and my electrician said he had to use a double breaker for the two kitchen circuits. The catch is that he wanted to put an AFCI on the circuits as well, but there is no dual function double breaker for my panel. We went with just the GFCI and the inspector passed it, so Iâm happy.
They werenât new circuits, but once you touch an old circuit you are supposed to bring it to new standards. That isnât always easy.
Love your white board. Sure makes it easy to draw & explain.
If those two legs were different phases, you wouldnât add 120 plus 120 to get 240. Youâd multiply 120 times 1.73 and youâd get 208 from phase to phase. They are both the same phase!
14;12... We did this w/ a few lighting circuits. We had to make sure the red & black was on the same 120v side of the load ctr. The bulbs were different wattage and were sharing unintended current when needed were acting as a light dimmer to the other bulb.
Code calls for bar tied breakers for MWBC runs. Seen guys that failed to identify such, open the neutral and burn up electronics and other loads as a result. Here in the north eastern United States it is somewhat common to find. Also common are the hots both coming from the same phase and overloading the neutral. Homeowners and handyman are rather good at this one! This is also common in allot of commercial electrical installations as well. Cube sets fed with a "full boat" are a good example. Most have each breaker as a separate rather than on a 3 pole breaker. Women and their heaters take those out all the time so the maintainance worker swamps them out to prevent the whole cube set from being brought down. Ah, good times! Very good video.
DamnâŠ.. this clears things up ! đ thanks for the videos!
Iâve been thru multiple classes in votec for hvac electrical and you teach excellent . These are great refresher courses and Iâm learning and understanding so much more every time. Thank you so much. Electricity is so fascinating.
My company does multiwire branch circuits for most of their home runs. Holy shit thatâs a lot more complicated than I thought. Thanks for the vid
Thank you so much. I could not find an answer to that question. Itâs been driving me crazy for a long time.
THANK YOU! No one has been able to explain this to me where it makes absolute perfect sense to me like you have. That unresolved itch of not understanding which has been irritating my brain for two years, has been scratched! Ahhh....resolution... it feels purple.
I'm not an electrician...but my dad is...n i was taught this long time ago that the most dangerous or problematic wire can be a neutral...1. a open neutral 2. shared neutral from two different circuits 2 circuits sharing neutral đ..3. depending on how someone wired a switch ur white wire will be carrying power..4..tandem breakers will share a neutral many times because electrician or diy dont add a second neutral through there conduit...but can be confusing if u dont mark with different color tape...i was taught to respect electricity always work safely even when breaker is off, to work as if breaker is not off..
Iâd love to see a video about ringing out your neutrals because Iâm back feeding a solar system and backing up loads in house with a battery back up system.
Amazingly simple, now. Thank you!!!
People just donât understand. I recently had a service call where this was happening. The existing panel was an 8 space 120/240v single phase panel. This was in a small beauty salon. Beauty salons require a lot of circuits. So whoever did the buildout, decided it would be a good idea to use tandem (skinny) breakers rather than replace the panel. That by itself isnât a problem really, the problem was they used each side of the tandem breaker as a multi wire breaker. So each neutral, potentially carried 40amps of current on it. The call I got was an emergency service call stating that the panel was hot to the touch, there was a smell of weird smelling smoke and a âsizzlingâ sound coming from the walls. What I found was it had likely been cooking for a while bc the isolators on the neutral bus bar had completely melted away. The insulation on the #12 whites were all melted away into one large lump, the insulation on the feeder neutral which was now carrying potentially 4x 40 amps on #6 wire, also melted. Some bad bad ju ju right there. I donât even consider a guy an electrician if he doesnât understand this lesson right here. BTW, itâs a close similarity to what a 3 phase multi wire system would look like. The numbers may vary, but the basic function is the same. Neutral Carries the balance of the load back to the source.
As an added bonus, I would like to request a video explaining: ungrounded, grounded, and grounding. A.k.a. âHotâ, âneutralâ, âgroundâ respectively. A lot of my new apprentices think the âneutralâ doesnât have power and get shocked on it because they donât understand that the neutral is also a conductor. Thank you for the videos. I use them as training tools for my guys. Keep up the good work brother. đđŒ
I did this exact thing a few years ago in my garage, there was an unused 10/3 wire bringing 240 volts, but my garage didn't have any outlets so I tried to figure if I could split it. I'm not an electrician, I just do diy stuff. I know a lot more about electricity now so I know I did it right, but I had a lot of difficulty trying to find accurate information at the time. I wish I had this video back then!
Amazing. This explained it so well. Thank You.
I see this all the time, and I was guilty in the beginning, using a split breaker with a 3 wire, thinking I was getting two circuits to an attic, so I learned that was a no go
Really good! Now I need to change something in my house to correct what I did 20 years ago. This old Pushmatic panel does not have alternating A and B phases on each row of breakers. Example, every breaker on the left hand side is on A phase and every breaker on the right hand side is phase B. Unfortunately I did not realize this when I added the multi wire branch circuit.
Hell yeah man I'm glad you're on here doing this this is what's up dude anybody wants to learn listen to this man he's going to teach you the right way shout out from priority1 electric Sacramento CA 916 keep those videos coming bro I could definitely give you a couple of good subjects to talk about like three phase rotation Delta or why transformers Delta high legs isolated grounding receptacles
Three phase 2000 amp breakers with arc suppression the history of zinsco and if they're so bad how come they're still running 60/70 years from now why newer panels are not built like old ones why Federal Pacific zinsco and stablock bulldog are not ul listed anymore
totally nailed what I was wondering about. And it makes sense, thanks,