Branch Circuit, Multiwire, NEC 2020 - [210.4], (19min:38sec)
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- čas přidán 16. 08. 2020
- 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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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.
Oh my gosh, this is the number one story on this topic! I'll share this in my next class.
@@MikeHoltNEC when, during this video, Laboratory’s were mentioned specifically, I just laughed because of how practical the information was to me because of this experience. It’s a very real issue with that circuit. Edison didn’t have to worry about electronics in the 1890’s.
Why not just flip the breaker to the off position before disconnecting the neutral on any type of branch circuit, multi-wire or otherwise?
If not you always run the risk of having an energized neutral situation. I’ve seen energized neutrals on regular branch circuits where the power would come through the load and into the connecting neutral.
120K is pretty bad but I got you beat. Working as a locomotive technician, we had a mixup on current software. Manager had me put the wrong control package on a locomotive and we burned up $750,000 worth of equipment.
@@defytyrantsofmississippi2198 The point is that you don't necessarily know that a neutral is being used by another circuit unless you put an amp meter on it after you turn off the circuit that you want to work on.
Even worse (though VERY unlikely), before you turn off the breaker you work on, you see 10 amps on the hot and 10 amps on the neutral ...BUT this is a neutral that is also being fed by 2 other circuits that are on opposite poles and also have 10 amps on each. The 10 amps on opposite poles cancel each other out so that your amp meter would read 0 amps after you turn off the breaker that you want to work on. Not much you can do about that other than measure amperage on the neutral while turning off each of the other breakers ...and nobody is going to do that.
I work extensively in old houses and I have seen a few examples of more than 2 hots using a single neutral. I've seen things like 5 hots using 3 neutrals through various junction boxes. This sort of thing happens because an 'electrician' just randomly connects circuits in an old fuse box that gets turned into a junction box without maintaining the paired status of each circuit's hot(s) and neutral.
THIS IS JUST ANOTHER REASON THAT AFCI AND GFCI PROTECTION IS SO IMPORTANT.
Thank you for the great instruction and positive outlook on health and life! They are equally important in my opinion!
Thank you Mike for all the technical support, information-Advice that you share! 👏
You are most welcome.
@@MikeHoltNEC Hello can you please recommend multiwire branch circuit quiz worksheet and answer book? Also can you please recommend a tutor who can teach circuits and conduit?
👍🏼 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.
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.
I guess if the 2 loads combined don't exceed the amperage of a single breaker, it wouldn't be an issue. An example would be a refrigerator and a dishwasher on a 20 amp tamden breaker. If the dishwasher pulls 10 amps and the fridge pulls 5 amps, the combined load on the neutral would just be 15 amps. But in that case the 2 could have been combined on a single 20 amp breaker and not a mwbc.
@@jeffhenson5471 Not OK. The same argument could be made for ANY two wires on the same phase. Don't ever do this. The reason that you don't do it is because you don't know what the potential load is.. for example, what if someone plugs in a toaster oven with an extension cord going to the same receptacle the refrigerator is plugged into because there weren't enough counter receptacles?
I'd like to reword this. If you connect the two hots to the two circuits on the same tandem breaker, then the neutral will be on the same leg (phase). However, if you use one switch from each of two separate tandem breakers, each hot would be on a different phase.
That said, you still have not met the requirement that the two circuit breakers be tied together with a handle tie.
There are devices called "quad breakers" which are essentially two tandem breakers bonded together in one case with the two inner breakers connected with a handle tie and the two outer breakers connected with the handle tie thereby allowing for (2) separate 240v circuits.
In this case, if my description is poorly worded remember that "a picture is worth a thousand words", so simply Google "quad circuit breaker" and click on 'images'.
In a full panel that accepts tandem breakers, these can be really handy when you're adding something like an electric dryer because they can come and configurations like 30 amps / 20 amps. :-)
Not trying to be mean or arrogant here, but for new folks, this might be surprising information that is really helpful. :-)
@@SwingboyPA yes tandem breakers can be very useful when used correctly. Your example of adding a dryer circuit is helpful. The problem is I’ve seen too many older houses with tandem used incorrectly by someone who didn’t understand what they were doing. A thorough inspection will often find various electrical problems even when the work was done by a professional.
@@neilbrookins8428 yes, I run into that all the time. Almost all of my work is residential in houses over 100 years old ... So layers of work over layers of work. :-p
I am constantly using double pole AFCI circuit breakers -literally as diagnostic tools- to find *additional* shared neutrals on circuits that are already shared neutral circuits.... Like I'm making sketch plans of people's houses with color coded circuits for shared neutrals and shared hots. LoL
It's always nice to 'meet' another electrician who is thorough and thoughtful Neil. :-)
This guy is awesome, I would have paid much more attention if all my teachers were these good.
Thank you, have a nice day and God Bless
i sceptical of most people on youtube till they prove to be creditable but first time i watch his video i believed everything he had said. he knows code book forwards and backswards.he covers too much material for my old brain but thats on me
This helped thank you Mr.Holt!!!
Mike Holt gets me excited about electricity. Man I just love his vibe. Have a good day!
Thanks, but I'm probably more excited than you are when I'm teaching!
Yes he does. No way could anyone could get bored in his sessions.
watched the whole show... excellent
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.
The legend himself with another amazing video
Thank you, it's you guys that drive me to excellence.
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.
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. :-(
I've had the most problems with open neutrals caused by the utility company. It's like some of the linemen don't think it's as important to securely tighten the split bolt on the neutral as they do on the hots.
3 mins in and my head hurts. Salute to you guys
Suck it up... watch it again1
The Murphy's law thing made me lol, thanks for that.
Ran into this today. Two separate breakers, one neutral.
Brilliant
What a great video. It explains in detail what every electrician should know about Multiwire circuits.
Thanks!
Thank you for your videos, very helpful!.
You are most welcome, God Bless.
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
I’m sure the guy with his manager buddy appreciates the shout out for their insurance fraud at the Best Buy lightning incident!
Glad I’m not the only one that caught that 😂😂😂
Lol 😆
I learned a lot at first it was hard to understand your English but I did catch on with the pictures I didn't even know that existed that answers a lot of questions really awesome video
Fascinating forum!
I love the Murphy's law calculation
Great video contents! This is where the handy electric circuit phrase comes. Before you should a gun, you have to know which end to place against your shoulder. Before you do anything electricity, you must understand what a circuit is…
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!
Thanks for the video =)
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.
Learned to tie the groups together when traversing a box. Thanks.
I don't understand your comment.
@@MikeHoltNEC 14:32 210.4d Probably my poor English.
Exactly. Kirchhoff’s voltage law for series circuits. The load with the HIGHEST resistance (ohms) will have the highest (proportional) voltage drop.
crazy people! I am glad u are ok and safe.
Thanks for verifying something that I knew all along. I found one of these miswired circuits in my house and fixed it, but didn't relate it to the code.
I've always wire the Multi-wire branch breakers as " Utilization equipment" only and saved myself headaches.
I'd like to add a note to Graphic 2. This rule is easily looked over when dealing with 120/240, 120/208, or 277/480. Where this becomes and issue is a High Leg Delta panel.
I have made it a habit to place a current clamp around any neutral conductor before disconnecting or lifting the neutral conductor from its connection when working in commercial or industrial settings. The best practice is to trace the neutral and be aware not every circuit you are encountering was installed to code!
100% good practice!
I learned to call these network circuits.
Is this correct terminology? Not an electrician by trade but know more about electricity than most electricians I work with. + I never see these breakers even if on opposite legs tied together that would trip or turn them off together!
If a multi wire branch ckt has equal load on each, you will read 0 amps on the shared portion of the neutral. So, be careful. The current cancels, which is why the whole thing works when using different sides of the split phase. If you share a neutral for two hots on the same phase, the current will add together.
That is good thinking. Good way to trace down a neutral too.
@@paaao Yeah, but if the hot measures (say) 5 amps and the neutral measures anything else (including 0 amps per your example), you'll know there is either a low grade short or a shared neutral. If the amperage changes when you turn off the breaker for the circuit you want to work on, you'll know it's a shared neutral. :-)
Greg's method is a good one and most folks don't bother.
We have used this on lights. 6 lights, 3 on one switch, 3 on another
That makes me wonder how many tandem breakers i see are actually 20 & 15 amp multiwire branch circuits that are wired with 12/3 or 14/3
As a home owner, any time an electrician works on my house...I unplug Everything. Not practical in larger commercial buildings, but I'm not losing my 80 inch TV.
Multiple branch circuit should be tags somehow as a MB circuit. In aerospace industry we of course used wire diagrams that were consistent and you were trained how to read & understand them, and tag outs used when working with any circuit. Most overloads and shocks came from In-Shop repair were we worked on components on the bench.
MB ckts are no longer allowed in commercial bldgs
Need to see a video on 3 phase and high or wild legs in a panel. Seen a lot of electricians give 240 to a lot of 120 equipment!
They were called appliance circuits years ago. Cheap way to run one cable to a kitchen to get two circuits. Much easier to understand when they were both in one double gang box with two receptacles, or one split receptacle, as opposed to all over the place.
And all that DANGER just to save a neutral wire run.
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!
Wow... what an interesting idea... to use the GFCI as a diagnostic tool to determine shared neutrals! Do you have some sort of "kit" you built that plugs in, or do you wire that up each time?
@@marcfruchtman9473 You don't need a kit; I do it in the panel with a GFCI or DF circuit breaker.
Keep in mind that if you try this 'locally' (like at a particular receptacle location) you might miss whatever happened *before* the location that you are testing. So it's always best to use a circuit breaker to test the entire circuit.
I just had an issue with a damaged receptacle on a 3 wire circuit, the receptacle was being use as a juction to feed the other receptacles on the other rooms, when I disconnected the neutral , the other room receptacles increased the voltage to 180 v. And the neutral going to the load was backfeding 108 v intermittently . Got me thinking about how easy one can damage equipment with this wirings Wich I guess it was normal in the old days.
common in the older houses ( per-1990's ) iv live in sadly, and yes it can start fires 🔥 and or damage equipment or hurt someone
I think multiwire circuit is not planned for that kind of charges, but for apliances working as a whole like a panel for control of pumps, specific machine, or even small subpanels, or similar loads as suburban light distribution, for residencial distribution as in this video is better and marked individual monofasic circuits.
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.
yep common in older houses 🏡 as i have the same experience as you and it can start a fire 🔥 and damage equipment or hurt you ie 40v to 260v but not stable at that voltage as it was grounding funny ect. and its also common to see little to no safety bonding/grounding or improper neutral bonding
I absolutely hate this rule. But I do understand the importance of if.
14:20 Line-to-line loads are permitted if the breakers are common trip 210.4(C) Exception 2.
You are correct.
Where can I find more info/detail about the phase cancellation
really fascinating how the alternating phases can cancel out and not overload a neutral
Are multi branch circuits good for to replace audiophile grade dedicated circuits
there are times I need 240 volts here. So I run DP Breakers for those circuits. But I still run two neutrals and mark them. Because my 240 circuits do not need the neutrals but my 120 volt outlets do.
Does a multiwire branch make sense for the audio file system to reduce noise??
I have observed in a wood shop 3 phase machinery where to save money in electricity the owner wired each phase to a different shop.
Looking at a the definition of article 100 we could infer that a 3-wire delta circuit and a 4-wire delta circuit are not MW circuits because in the former there is no neutral, and in the latter there are no L-N voltages. Yet a corner grounded delta could be considered a multi wire circuit because it has a balanced L-N voltages. Food for thought.
Hum... never considered this angle, but you are right!
This is such a great video. I just happened upon it because I was looking to help someone who was interested in multi-wire branch circuits and their dangers.
As someone else commented below, these Multi-wire branch circuits are not compatible with GFCI... which thankfully means that over time, we will see fewer and fewer multi-wire branch circuits in the residential world as the NEC increases the requirements for more GFCI in the home.
True, although any multi branch circuit must be put on a DP circuit breaker ...and I always suggest using a GFCI or AFCI DP breaker.
As far as I know at this time that I'm writing, there are not yet Dual Function double pole circuit breakers ...but I predict there will be.
Remember that a grounded receptacle (ie 3-prong) on a non-grounded circuit must be BOTH AFCI and GFCI protected. For now, with a shared neutral (multibranch) circuit, that means a double pole AFCI circuit breaker and a GFCI at the specific location.
Given the many old houses have hard steel work boxes with beveled corners, I hope the industry will save us from installing GFCI receptacles in to overly stuffed wallcases and eventually provide double pole DF breakers.
@@SwingboyPA I am not sure how it would be possible for a standard GFCI breaker to work in this configuration. Wouldn't any significant unbalanced load trip the GFCI?
@@marcfruchtman9473 If by "standard" you mean "single pole", then yes, you are correct: it would trip. That's why you must use a DP (double pole) AFCI circuit breaker on a multi-branch circuit if you want to protect the entirety of both circuits. Same goes if you wanted to use GFCI.
Make sense?
@@SwingboyPA I have been trying to find a manual / users guide (for any 2 pole GFCI breaker) that states that it accepts / allows multiwire configuration. So, if you happen to have a link to a manual...
@@marcfruchtman9473 You don't need a manual, it works because the breaker looks at both hots and justifies that against the neutral. AFCI or GFCI double pole breaker on a multiwire circuit...it will work. I've been an electrician for about 35 years and work almost exclusively in old homes in Philadelphia, PA. I use AFCI, GFCI, and Dual Function circuit breakers all the time on knob and tube circuits ...which are often shared neutral circuits. I'm not trying to be arrogant, but troubleshooting and remediating old wiring happens to be my specialty because I do it all the time. The guy who trained me is the same; we both really enjoy the niche of working in old houses so this is something I know very well. :-)
If it doesn't work, there is something else wrong. ...And again, this is why I use AFCI and GFCI circuit breakers as a tool as much as a protective device.
Found a violation of 210.4D at my parents place when I upgraded the panel. 2 20A breakers feeding the garage sub panel. Not good!
Ive smoked a few pieces of equipment before i figured out what i was going on.
10:39 From a contractors stand point mbc’s are what’s most common
Yes!!
It seems a multi branch circuit involving two 20 amp outlets with a shared neutral going back to a double pole breaker is not really too dedicated circuits. I guess because the first outlet affectively has 120V going across it and the flow continues across to the other circuit in the neutral receives no current. Not sure I understand how 240 V is involved here. So basically the second outlet is Sharing power with the first outlet where the returning circuit off the second outlet flows back to the Second Circuit breaker to complete the loop.
Hi Mike, this is Daniel. I was wondering why I can't see your video of August 18? , It says it's private. By the Way You Guys Are Awesome! 👍
Watch tonight at 7 pm EST August 25th, 2020.
Just a quick comment at 12:48 the wire is labeled 12-2 With ground however its clearly a 12-3 with ground. Just a typo in your graphic.
You are 100% right! Wow, probably been that way for twenty years... Thanks.
She noticed it, I'm sure they'll change.
I used to run them everywhere... but my reading of the AFCI requirements make that prohibitively expensive for residential work. You have to either buy a 2 pole AFCI breaker (100-250$ per breaker or run EMT all to "save" a single 12g or 14g THHN wire....) Not a huge fan of handle ties b/c they seem to get removed when the grouped single pole breakers inevitably get moved. If there is a way to run them that makes sense (cost wise) and follow the AFCI requirements i would love to know.
210.12 (A)(4) says that you can use a AFCI rated receptacle at the beginning of a branch circuit, up to 50ft for 14g and 70ft for 12g. Hope that helps.
@@simmcowaPang122 I wish that was an interpretation I could hold I could be wrong (please let me know if I am)
but 2019 was the last time I did heavy research into it.... So if its outdated PLEASE correct me
I know there are jurisdictions that are allowing this (including my home jurisdiction). But other then using the single pole "Eaton Combination" I just don't see it actually satisfying 210.12(A)(4).
Eaton have a listed "standard" breaker/AFCI 1st receptacle that when used in combination DO satisfy the requirements but Im not clear how ANY standard breaker/NMC/AFCI receptacle could.
I believe (with strong evidence) that any "standard" breaker/NMC/AFCI 1st receptacle does not satisfy 240(A)(4) and allowing it is a miss interpretation on behalf of my jurisdiction and others.
My reading/interpretation of the exceptions that would satisfy code would fall under sections 210.12(A)(4)... The only UL listed product that would meet the poorly worded code that are commercially available to allow use under this section (Specifically under 240(A)(4)) would be the "Eaton Combination" ...
The Eaton combination's UL listing meets the requirements of 210.12(A)(4) as a "System Combination". In other words only when a specific Eaton only (non afci) breaker and the Eaton only AFCI device tested and listed as a combination as a listed system qualifies.
I only install Square D for a number of reason so for me its not an option... Even though it would MAY pass inspection... and I would generally have to argue it up the chain... which makes the "cost savings" moot. I dont agree with the code wording and think the AFCI requirements are gov't mandated corporate welfare.. But I follow the code as written (in my interpretation) b/c in order to hold this strong ANTI-Post 1997 Code changes turned mfg sponsored marketing nonsense position and advocate for changes I think its a moral imperative to.
We need to CHANGE the code its nonsense not find workarounds that will pass but DONT (in my interpretation) actually satisfy the code... The code is the problem and needs to have common sense safety not corporate sales as its primary purpose and that unfortunately has been the evolution of the code since about (at the latest) 2008... and I would argue 1997 was the last pure "safety" focused code. Its been downhill corporate shilling since.
We used to run a standard 2pole breaker on 12/3 romex to GFCI 1st receptacles on the A leg and B leg and it worked great, was efficient use of materials and was less cost. in materials/labor and saved customers money... It worked well, safely and was highly efficient .. that changed when the AFCI requirements came into effect for Small Appliance Branch Circuits...
In my opinion these code changes were made to sell MORE copper 2x 12/2s and 2x 10times more expensive AFCI/GFCI Combo breakers. There is no good argument for requiring AFCI protection on Small appliance circuits that already had GFCI requirements... its was all about SALES in the name of safety and is total corruption.
Ohio for instance SPECIFICALLY EXCLUDES small appliance branch circuits from AFCI requirements b/c the justification for requiring them is ridiculous. The Qty., placement and amperage requirements for Small appliance branch circuits and the UL listing requirements for small appliance already address the supposed "dangers" used to justify AFCI requirements in the 1st place.
Ohio has the exception to specifically allow the use of MWBCs to meet the requirements to provide the large numbers of separate Small appliance branch circuits in a safe and efficient way and to cost effectively allow installations that are arguably safer due to provided many separate 20amp circuits that will likely have large inductive and resistive loads simultaneously operated while connected to them.
Arguably NMC(romex) does not require AFCI protection from the 1st receptacle to the service panel when installed properly according to codes as old as '95... . the only good argument is "at and after"the 1st receptacle.... so that crappy electronics and extension cords plugged into garbage quality back stabbed $0.99 duplex receptacles dont burn your house down...
That was the ENTIRE stated premise for AFCI in the 1st place... It was not to protect branch circuit wiring from the panel to the 1st outlet. But of course now the code panels position is that "the tech exists so now you need AFCI to protect that "dangerous" NMC from undesired effect of arcing" Nmc that didn't become dangerous when properly sized and installed until 2008....
Its not NMC (Romex) its the devices including and running from the 1st downstream receptacle and the equipment plugged into them are the problem..
The problem IS NOT properly sized and installed Romex from the 1st outlet to the service panel... The code panel is making that argument now in the name of selling product as corporate shills.
Unless the two hot goes in the same terminal box with share neutral
"Do not get creative."
Are you saying that in order for two hots to share a neutral they(the 2 hots) have to land on the same breaker? Like a 2 pole breaker?
Nope, watch the video again. Two single-pole breaker with 'identified' handle tie are permitted, see 240.15(B).
but the voltage between the 2 hot wires must be 240vm diff phase?
I can't remember at what minute it was but on of the guys said something about multiple neutrals under same screw is there a code on that? Thanks for your mind and time
Look under 408.41
Thanks for the reply
I think maybe they're referring to 300.13(B)
It's amazing how few who work with electrical wiring actually understand this phenomenon of the Edison 3 wire system. To the point where when I wish to communicate my skill set I just say, "did you know when both branches are balanced the neutral current gets nulled out, down to zero." It wasn't until I was 40, looking at a panel and saw three 10 gauge wires heading to an apartment and it hit me. Why isn't the neutral bigger, won't it have twice the current? It was at this moment my teacher from RETS (radio-electronics-television-service) stopped by. I spent 3 days wrapping my mind around what's happening literally inside the wire nut where it goes from 3 wire to 4 wire.
WOW, a RETS grad. I went to RETS in Nutley and graduated in 1979. We did a lot of partying in the parking lot, but I learned a lot and did very well. Retired now. It's a small world.
Same here. Catonsville, MD. 1999
In 3 phase Wye multiwire branch circuits with nonlinear loads, one can create high harmonic currents in the neutral. Can anyone elaborate on this?
This is no longer an issue in todays wiring because 'tuning filters' have been added to equipment. But do a search for harmonics to gain more information.
How do you find the shared neutral wire after the fact when they weren't grouped initially?
Post your question on MikeHolt.com/Forum.
I use an AFCI circuit breaker as a diagnostic tool.
1). When a new AFCI trips, I leave everything connected and turn off all circuit breakers except the one that is tripping. If it stays on, I suspect another breaker was feeding current to that (now AFCI protected) circuit. I then turn on breakers 1 at a time to see if additional current trips the AFCI breaker by adding current to the neutral. Using an amp meter, I can measure changes in the neutral as well.
2). Often the problem is 2 (or more) hots sharing 2 (or sometimes more) neutrals.
Now I identify the (new) AFCI neutral in the panel and put an *amp meter on it* instead of leaving it open since the neutral will still conduct electricity from the other circuit(s) even when the AFCI is in the off position. I turn off all breakers, and then turn them on 1 at a time to see if I get any current on the neutral -thereby showing that the neutral is shared with whatever circuit I just turned on -and thereby also avoiding an open neutral.
(At this point obviously I can use a double pole AFCI breaker to protect both circuits with a pigtail to the (2) separate neutrals)
Computers have electronic boards and are non linear loads, technically the code prohibits the use of MWBC on circuits where the majority of the load is non linear. Code doesn’t care if your going to lose a bid. Do it right.
Interesting stuff! As a "DIY guy" I see 12/2 is half the price as 12/3 (for example), why not run all 12/2?! I suppose less labor. Are MWBC's typically used in residential applications?
tough to use in residential these days because the nec has lost their damn minds with GFCI/AFCI being required basically everywhere.
I’ve been doing this 30 years, never seen a multi wire in a residential environment, doesn’t mean it doesn’t exist somewhere though.
@@suspicionofdeceit I had them in my residential home. But that's because I hired a commercial electrician to do the work :)
@@suspicionofdeceit my house in Seattle is a 2001 build and there are 5 general lighting/recep circuits. 4 of the 5 are actually paired MWBC. So probably the only reason the 5th isn't also a MWBC is because it's an odd number remainder.
@@suspicionofdeceit they are around everywhere. Including residential easpecially if home had upgrades over the years
Nice
Thanks
I have a question about my inverter off grid.. and I am also connected to utility open ground on the inverter standard of the inverter question is about grounding
Please contact a local Solar contractor.
Do not get creative. Thanks for that quote.Have homeowners to so called engineers get creative. Best one was on a Friday afternoon and project engineer said o yea you guys have to hurry up and get a 50 amp line ran for a tech coming in Monday morning to start up a new machine. Told him that would require triple time to have it done on Sunday. He said just grab power from the 3 hanging 20 amp cords hanging from ceiling. Say what. He said 20 + 20 + 20= 60 amps. Broke his heart when we told him you can not parallel #12 wire. 3 of us made 3 days pay for working that Sunday. Told him if you have the money we always have the time.
Your offering a grantee by adding the extra neutral selling point
Pardon my ignorance but what's the difference electrically between this and the neutrals all connected to the neutral bar in the main panel.
You need to watch the video again. A multiwire branch circuit always starts at the panel.
I don’t think you understand that the L1 and L2 are 180 degrees opposite on the current side. That’s what AC alternating current stands for. So IF you are doing this type of circuit for design/build reasons you are sharing a neutral. Depending on loads you may or may not be seeing current go back to the panel. If you are not going to do this you are running another home run in 2 physical places. I used this in my own home to supply power to back to back side opposite wall that 1 leg is microwave and gas stove top loads and other is TV entertainment system. Used a 10/3 cable instead of 2 12/3. Easier and it was all the space I had in panel. Breaker is a double pole 20amp. Heavier wire is due to length and less voltage drop
I had the same thing happen to me at a condo I bought and the two breakers weren't tied together like they should be, that was when I found out the whole place was MBC
Will using a GFCI at start of each branched circuit avoid the open neutral issue?
Nope
Jjjaaa don't get creative
The GFCI measures the current leaving on one of the legs and returning on the neutral. If they don’t equal then it flips. With two hot legs feeding the shared neutral there will always be an imbalance. That’s what the problem was with grounded neutrals on GFCI generators connected to houses, where the current would have two paths back and would flip off the circuit each time turned on because of the imbalance with the neutral.
I came here because I have a critical loads, 120v sub panel, where I want to bring just ONE more circuit over to the subpanel, but it's part of a multiwire branch circuit. And the second circuit, I don't want to bring to the sub panel - nor could I anyway, as it's a 120v panel, thus on a single phase. I'm dead in the water over this last circuit unfortunately.
So the only issue is when removing a neutral? If you were to relocate or add a outlet or light off a 10/2 12/2 you will be ok just be sure not to mess with the bundle of commons?
Correct.
I have my fan running and my LED recessed lights on. When I turn off the fan, on it's own switch, my LED lights also turn off momentarily (and then come back on). Why??? I have a switch for my fan, and a switch next to it for my LED recessed lights in my bedroom. Any insight would be appreciated.
I suggest you try different LED lights, not the same brand.
The MWBC is unsafe. Period. This is because it cannot pass a one component failure analysis. A robust electrical system must be able to tolerate at least a single failure without catastrophic results, a single failure includes a severed or shorted wire.
What is troublesome is that this brings the entire NEC Code into question: what other approved methods are dangerous?
That's an instresting perspective... excellent.
@@MikeHoltNEC I mean the NEC is all about nail plates, conduit, etc., devices and methods to protect and make the system survive attacks. And then NEC allows MWBC. Ironically, 2 neutrals in the same slot is safer than the MWBC - at least with 2 neutrals you know _Danger._
But this video and situation reminds me of a perplexing situation in a small, old home with knob and tube wiring - you may find it interesting and may already know what the problem is.
The house had 3 circuits done in K&T.
Evidently back in the 30's, the house was wired on a single leg and they must have gotten creative.
Balancing the circuits between legs on a new breaker panel would have lead to.... fire - without tripping a breaker.
I have no theory how they could have wired things for this to happen.
NEC was not meant to be used as design material, it was written to prevent fires
I have a tricky multiple choice question, hopefully someone with more knowledge can help me out
A White/grey wire in a feeder circuit ________:
A)Be bonded or have a circuit breaker
B)Is the largest wire in that circuit
C)Should not have a voltage to ground
My notes:
On the main panel, neutral is connected to the bar that is bonded with the panel, the green grounding wire is also connected to the same bar. (so does that mean the answer is A?)
In the sub panel, the neutral wire is connected to the neutral bar which is isolated from the metal box with plastic between them, the Green grounding wire is separated and is connected to its own bar that is bonded to the sub panel box.
In the circuit that this question applies to,
--------- “The neutral wire is going to be sized larger than the Green grounding wire”
(But that doesn’t mean it’s the LARGEST in the circuit right? Because the two hot wire could be the same size)
--------"The neutral should have no potential difference(voltage) to the ground" (does that mean the answer is C?)
(But that doesn’t mean it’s the LARGEST in the circuit right? Because the two hot wire could be the same size)
@mike holt
Thanks!
But by putting a clamp on meter to test the neutral in MBC won't help much cause there won't be a load...
If a even load..
A MBC neutral will always have current if the load are on.
@@MikeHoltNEC of they are perfectly balanced??
15 on 1 15 on 2, 0 on neutral..
@@mechelb4158 I understand the 'theory' but let's get real, this is never going to happen.
@@MikeHoltNEC possible!
Somebody runs 2 heaters on a mbc...
Possible to pls make a video on linear vs non linear.
Thanks!
@@mechelb4158 What percentage of the time do you think that when loads are on, the neutral conductor would show '0.00A?' I give it almost zero, but I'm interested in your thoughts.
"the people on the TV will start moving very fast...." LOLOLOLOL
'for a very short amount of time...' LOL
When he says 50% further with a MWBC is that only with DC?
Is there any current code that confirms this when doing voltage drop
Example Line 1 - 10A, Line 2 = 10A, Neutral = 0A. This is not Code, this is electrical theory, and doesn't matter ac or dc.
@@MikeHoltNEC 8:08 When talking about Edison and how he "invented" the MWBC.
So I understand the the theory of neutral carrying current.
What's throwing me off is the voltage drop and Edison being able to go further due to a mwbc.
@@anotherreview5586 Let's have two 2-wire circuits carrying 10A and each conductor has a resistance of .5 ohms. The voltage drop of each conductor is E = IxR, = 10A x 0.50 ohms = 5V per conductor. Assume 120V circuit less 10V (5V x 2 wires), the load for each of the two circuits operates at 110V.
If multiwire circuit Line 1 = 10A, Line 2 = 10A, then N = 0A. In this case, there is only 5V drop on the phase for circuit 1 and 5V drop for circuit 2 (since there is no neutral current). So each circuit operates at 115V. This is covered in my Electrical Theory program (MikeHolt.com/Theory).
Switching power supplies, which have become more and more common these days, tend to produce odd order harmonics that dont always cancel the way a nice symmetrical sine wave does.
True, but with ‘tuning filters’ in the equipment, we don’t have the issues we had 20 years ago.
Confused, you say the code states its ok to use multiwire branch circuits for office furnishings but nec code 605.9(d) states there not permitted? Am I misinterpreting this? Thanks
I never discussed Office Furnishing, but 605.9(D) only applies to the internal wiring of Freestanding-Type Office Furnishings, Cord- and Plug-Connected. Freestanding-Type Office Furnishings, Cord- and Plug-Connected is wired by the manufacturer, not the electrician. So your comment is a moot point. Right?
@@MikeHoltNEC Thank you for the reply, sorry for the confusion, it was Erik that was talking about office cubicles. And your response answered my question , I did not know that code reference only applied to the internal wiring. Thank you again!
What if the GROUNDED conductor in the field is taking off. Is it the same if it's taken off in the panel???
What? Please just use the term 'neutral.'
Why can’t u remove the neutral on load first? Cause it’s load and ur voltage it’s on without a return?
That is a different topic, not related to the discussion of a multiwire neutral circuit.
I'm just learning about these multiwire circuits because I ran into one this week in our 1970s commercial building and I wanted to understand the code. Boy, the wording of the code is so confusing and non-obvious. When first learning this, a general introduction would be so useful, but can't be found in the code. A general intro should start something like this:
Multiwire branch circuits are a cost-saving wiring technique that shares one common neutral wire 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 branch to be on different phase legs. Doing this guarantees their shared ground will never need to carry a higher current than any of the branch hot wires, which is why it is possible to use the same ampacity wires for all the current-carrying wires of the multiwire branch circuit. Preventing the need to use a larger neural, is where the cost savings happen, so this is only permitted by the code when the branches are on different phase legs.
Excellent! I'll add to my book and CZcams intro.
The thing about the NEC is that it isn't meant to be an explanation of electrical techniques. It's a reference book for people who are already trained professionals, not a "how to" book. I'm not trying to be an ass, it's just important for people to understand. Mike (and others) have produced materials on "how to" that refer to techniques that you can use to comply with the NEC.
On the visual with one duplex above the other at 11:51 the 12/3 cable is marked 12/2. Just saying...
You think... thanks
seems that it be safer to keep each line of multi branch circut on the same phase..or leg....then not runing 220 v through one cable. thought it is not permitted to bring 220 into a switch/outlet box . i say run that extra neutral.........saving money can cost money or a life
Anyone have a good idea of why the inspector failed me on:....older house service upgrade
service upgrade to 200a. No re wire. no 6 Range wire landed on 2pole 50a.
Keep in mind older wire so the jacketed cable has 2 current carrying conductors and 1 stranded al ground...HE will not pass till I send a picture of that stranded al ground tapped white... Why would I indicate that a non insulated wire is an insulated a neutral???
Post your question on MikeHolt.com/Forum.
Can this be corrected to two individual circuits?
You'd have to run a new line to the point where the 2 circuits 'split'. If you are a homeowner reding this, make sure your multi wire branch circuits (colloquially called "shared neutral" circuits) are on a double pole breaker. Preferably a double pole AFCI breaker. If it trips, you may have another hot feeding that neutral or the neutral may be connected to another neutral somewhere in the building.
@@SwingboyPA Thanks
For those who were stumped by the math. 12 ohms + 24 ohms = RT= 36 ohms
IT = 240V / RT 36 Ohms so 240V / 36 = 6.666666667Amps
Now to find the individual voltage drop of then 12 & 24 ohm resistors
V=I x R
Since I is the same throughout the entire series circuit as long as the resistance and voltage value remains the constant:
6.666666667A * 24ohms = 160VD
6.666666667A * 12 ohms = 80 VD
VD= Voltage Drop
Thank you....
I know Mike made the joke about how the most expensive piece of equipment will get destroyed. In this case would the hair dryer be ok since it has a lower voltage drop than before?
@@jacksadowski2382 If it can be fried it will be fired. Current is tricky, it could bypass all of the expensive stuff and hit the microwave or something like a phone charger and in other cases fry everything and leave the TV's working. Current is freaky.
Why is this service referred to as single phase when you repeatedly refer to two phases?
Single-phase is different than a reference to 'two phase conductors.' Watch MikeHolt.com/Systems
NEC should require shared commons tagged or marked as such in panels.
Well, the hots for that said nuetral should be on a 2-pole breaker or locked breakers so you should easily figure it out. If you can't you shouldn't be there in the first place.
@@vlad1889 Nothing should be left for "figuring" when it comes to these types of circuits - the risk/danger is too high. An NEC Official red label or tag would make it crystal clear. NOTE: Don't forget, it is an assumption that the panel is wired correctly.
That would be top perfect. And most O see do not have the 2 hots on two breakers common tripping. And some panels minies can be installed that the 2 in one breakers come off just on leg!
On way too many where diy electricians do to add on circuits and they think all the neutrals are OK to tie together anywhere 🙄
At 6:50 your numbers are not right. The hair driver is a resistive load the TV is not a resistive load as it don't use a transformer in side for the power drop it is a switch mode PSU. If L1 and L2 wired threw a hair driver or coffee pot as there using resistive load elements it will act like just a resistor So L1 threw the 12 to 20 ohm like a wire wound resistor then the TV. I am a spark in the UK and also a AV engineer my first work was in the TV trade. Strapping as you use as most of your 2 or 3 wire is from a center tapped transformer. To give you your 110 120 for you to get 230 240. We in the UK use most of the time TNCS. We're are neutral is grounded at the star side also at the home.
We work at the museum write know: that building was built at 1920... with remodeling in 1930, 1950, 1970, and so on... imagine what a mess you have??! So: we move a neutral wire in the j’box, and in the 5 foot 8 inch pipe, the neutral wire is terns in wire nut, (!!) and that sheared neutral is discontinuing the circuit that was connected to it, and that cost Musium a $2,500 copy machine... just because there was a 120V outlet that was connected to that light circuit!! If you guys don’t believe me, I have a video about it! That’s a great show! I can tell you that!
Please email me (Mike@MikeHolt.com) the video!
So on these circuits are they all ran with 3 wire or 2 wire and then they are able to split off into their own 2 wires…
Please watch the video again.