# How to best explain why we shouldn't splice different circuit's neutrals together



## Bluenose for rent (Nov 6, 2020)

Splicing neutrals from different circuits together has the likelihood of overloading them. Neutral is a current carrying conductor. On a 2wire, 120vac circuit the neutral carries the same current as the hot, and it is un-fused. Splicing neutrals from the same phase together in a three phase system (or from the same leg in a single phase system) is a fire hazard because these conductors then carry the return current of the 2 hots in parallel, meaning each neutral will carry a portion of the total current of the 2 hots, in proportion to each neutral conductor‘s impedance. So if you have 12 amps on cct1 and12 amps on cct7 and you splice the neutrals together you’d have 24 amps returning on those two neutrals. If one of them has 1/2 the impedance of the other (like if it’s half as long) then that lower impedance conductor will carry 18 amps. Remember neutrals are un-fused. There are other reasons not to randomly splice neutrals together, but this is the one that’s a fire hazard.
.


----------



## splatz (May 23, 2015)

Of course your making troubleshooting more difficult and maintenance potentially hazardous, if someone turns off one breaker to work on the circuit not knowing the neutral is tied to another circuit, zap. 

You're paralleling conductors which in addition to the potential overload @Bluenose for rent mentioned, could also cause inductive heating.


----------



## Almost Retired (Sep 14, 2021)

_the best way to explain it to others ...._

explain the shock hazard (as mentioned above) while trouble shooting, or any other maintenance 
and how that neutral will bite when it is disconnected
ask them if they want that to happen to them someday?

then continue with the O/L issue and the paralleling issue while not in the same conduit (1/0 min for parallel)
then continue with the over heating issue and the resultant fire hazard

ask them if they want to burn up somebodies baby ?
then go to court in handcuffs, and eventually prison for a multi year stretch ?

if they arent ready to change their ways after that i would report them to the boss for simple ignorance of code and then stupidity for refusing to learn


----------



## SWDweller (Dec 9, 2020)

I was working under a foreman who did not have the experience I did. I needed a job and he immediately set me into the switchgear room which was covered in about a foot of branch and feeder conductors. We were leaving one day and my boy Richard was up 15 feet or so on a metal scaffold. He was splicing in a new string of florescent fixtures, hot. I put down my tools, asked if I should go and shut down the temp lighting circuit. Richard explained to the crew how he was a journey man and could work hot. Missed the blue wire nut by that much and got nailed I caught him before he hit the ground. Then I went over and shut down the circuit. I never said anything nor did I challenge his authority. Job finished up he got laid off and I was promoted to foreman on the new job. As soon as I could I hired Richard back. I liked the guy, but he is a bit lacking in the electrical knowledge department. One day he asked me to explain what happened, he was unaware that the neutral carried current.


----------



## backstay (Feb 3, 2011)

It’s amazing how many electricians don’t understand basic electricity, let alone something like kirchhoff's circuit laws.


----------



## Almost Retired (Sep 14, 2021)

my only schooling in electrical is a 2 yr degree in Electronics tech (long story)

the best thing it taught me was basic Electricity
volts, ohms, amps, watts, reactance,capacitance, inductance
and of course all the various laws and concepts

i still use that knowledge daily after 37 years
it has made me a Much better electrician


----------



## Bluenose for rent (Nov 6, 2020)

Almost Retired said:


> my only schooling in electrical is a 2 yr degree in Electronics tech (long story)
> 
> the best thing it taught me was basic Electricity
> volts, ohms, amps, watts, reactance,capacitance, inductance
> ...


I never did any of the apprenticeship training here either, highschool physics and the school of hard knocks, they gave me some nasty letter in my sixth year as an apprentice telling me if I didn’t go to community college they’d take away my apprenticeship card, so I said “how about you let me write the redseal exam instead?” They said oh nobody passes like that but if you can show 10000 hours and get to journeyman signuratures we’ll take your 750$ and let you fail, so I winged it and got it first go. The training seems to be hit or miss, some guys get a lot out of it and others don’t seem to know anything even after passing all the tests. 🤷


----------



## Ronan (9 mo ago)

backstay said:


> It’s amazing how many electricians don’t understand basic electricity, let alone something like kirchhoff's circuit laws.


I come across this on every job site, with every company I've worked at. A couple apprentice buddies of mine; one doesn't know what the neutral does, the other can't explain voltage. I would bet $10 that none of the guys I work with, even the journeymen, have any idea what KVL is, much less what series & parallel is. I go out of my way to learn stuff, and this specific issue only came up after I wirenutted two neutrals together from different circuits and a little voice in the back of my head kept sounding off that it didn't "feel" right. So I went & looked it up. Took me a while to find it. These are the only three sources I could find on the subject. Ancient video by Karl Riley, article by Karl Riley, & Windheim EMF Solutionos video.

I get the impression that this is a much more common issue than some might think.


----------



## Ronan (9 mo ago)

Bluenose for rent said:


> Splicing neutrals from different circuits together has the likelihood of overloading them. Neutral is a current carrying conductor. On a 2wire, 120vac circuit the neutral carries the same current as the hot, and it is un-fused. Splicing neutrals from the same phase together in a three phase system (or from the same leg in a single phase system) is a fire hazard because these conductors then carry the return current of the 2 hots in parallel, meaning each neutral will carry a portion of the total current of the 2 hots, in proportion to each neutral conductor‘s impedance. So if you have 12 amps on cct1 and12 amps on cct7 and you splice the neutrals together you’d have 24 amps returning on those two neutrals. If one of them has 1/2 the impedance of the other (like if it’s half as long) then that lower impedance conductor will carry 18 amps. Remember neutrals are un-fused. There are other reasons not to randomly splice neutrals together, but this is the one that’s a fire hazard.


Sorry, I'm not familiar with the term "un-fused". A Google search on Bing just mentions burying conduit. And thanks for the highly detailed answer. I'll be studying it over lunch.


----------



## Bluenose for rent (Nov 6, 2020)

Ronan said:


> I come across this on every job site, with every company I've worked at. A couple apprentice buddies of mine; one doesn't know what the neutral does, the other can't explain voltage. I would bet $10 that none of the guys I work with, even the journeymen, have any idea what KVL is, much less what series & parallel is. I go out of my way to learn stuff, and this specific issue only came up after I wirenutted two neutrals together from different circuits and a little voice in the back of my head kept sounding off that it didn't "feel" right. So I went & looked it up. Took me a while to find it. These are the only three sources I could find on the subject. Ancient video by Karl Riley, article by Karl Riley, & Windheim EMF Solutionos video.
> 
> I get the impression that this is a much more common issue than some might think.


For sure man, I can’t tell you how many times I’ve heard “electricity takes the fastest path to ground” from electricians, ticketed journeyman eletricians. And when I tell them there’s a few things wrong with that statement they glaze over and never talk about theory around me again.


----------



## Ronan (9 mo ago)

Bluenose for rent said:


> For sure man, I can’t tell you how many times I’ve heard “electricity takes the fastest path to ground” from electricians, ticketed journeyman eletricians. And when I tell them there’s a few things wrong with that statement they glaze over and never talk about theory around me again.


When I gently try to mention, "it's going back to source...the transformer," and I get corrected where they say, "...no, it's going to ground," I just shut up and nod my head. I won't get anywhere arguing with senior level journeymen. This is the exact reason I quit ABC apprenticeship and study on my own.


----------



## Bluenose for rent (Nov 6, 2020)

Ronan said:


> Sorry, I'm not familiar with the term "un-fused". A Google search on Bing just mentions burying conduit. And thanks for the highly detailed answer. I'll be studying it over lunch.


Un-fused just means no overcurrent protecting the conductor like a breaker or a fuse.


----------



## CMP (Oct 30, 2019)

Besides the overcurrent and backfeed problems mentioned earlier, you can also have trouble with GFCI and AFCI devices, on any of the circuits that have their neutrals shared with other circuits.


----------



## BleedingLungsMurphy (10 mo ago)

Ronan said:


> see a bunch of neutrals in a box and splice them together, or to cut corners and just splice neutrals from circuits 8 & 10 together in a 4-square. I hadn't seen any formal training to this specifically and had to research it out on my own.


Have you learned 3 phase theory yet? Circuits 8, 10, 12 can share a neutral.


> I just happened to be in a situation with two 4 & 5th year apprentices who were about to splice two circuits together where I mentioned that it's not supposed to be done, due to net current issues where there's an imbalance between the hot & neutral, but they didn't really seem to understand what I was saying. They pretty much poo-pooed what I said, but then (for whatever reason) decided against splicing the circuits together.


How do you know that you were right? I'm getting the impression that they were right, and you didn't understand why you were wrong.


> This is the exact reason I quit ABC apprenticeship and study on my own.


Take it with a grain of salt and stick with your apprenticeship.


----------



## Ronan (9 mo ago)

BleedingLungsMurphy said:


> Have you learned 3 phase theory yet? Circuits 8, 10, 12 can share a neutral.


As I said in the second line of this post, I'm not talking about a shared neutral. I'm talking about neutral-neutral shunting.



BleedingLungsMurphy said:


> How do you know that you were right? I'm getting the impression that they were right, and you didn't understand why you were wrong.


From all the responses here, it's clearly shown that you don't want neutrals from different circuits spliced together. The apprentices I was talking to were not right and had no idea what I was talking about, even though they have a good 4 years of "school" under their belt. I am more than clear on the subject we were talking about. Take a look at Karl Riley's video here, and you'll see what I'm talking about. Again, read the comments in this post.


BleedingLungsMurphy said:


> Take it with a grain of salt and stick with your apprenticeship.


I am still doing my apprenticeship. I'm just not part of any organization apprenticeship school like ABC. Every single student I've asked, "so have you learned anything from apprenticeship school?" has responded with, "F-CK NO! It's a waste of time." So I learn on the job and through my studies. I have no time or patience for institutions that literally waste my time and my employer's money. After a year with the only "lab" consisting of less than one hour of pipe bending, I quit. Bought my own benders, and learned on the job as well as at home with Home Depot pipe.


----------



## BleedingLungsMurphy (10 mo ago)

Ronan said:


> As I said in the second line of this post, I'm not talking about a shared neutral.


Have you learned 3 phase theory yet?


> From all the responses here, it's clearly shown that you don't want neutrals from different circuits spliced together.


You can not apply this logic to all circuits. You seem to lack the basic understanding of 3 phase theory.

For example; a 4x4 box is fed with EMT from a panel and contains circuits 8 and 10.

This box also has 2 armored cables. One for the device on circuit 8 and a separate armored cable for device on circuit 10.

In this scenario all of the neutrals will be spliced together. This is not cutting corners. It sounds like you are confused by branch circuit neutrals.


> each circuit shall be identified or grouped to correspond with the ungrounded circuit conductor(s)


Now, if you had circuits 8, 10, and 11. You would splice the neutrals for 8 and 10 together, and 11 would be separate from 8 and 10 because it's a different set.


----------



## Bluenose for rent (Nov 6, 2020)

BleedingLungsMurphy said:


> Have you learned 3 phase theory yet?
> 
> You can not apply this logic to all circuits. You seem to lack the basic understanding of 3 phase theory.
> 
> ...


Too make this crystal clear for the OP, ccts 8 and 10 can absolutely share a neutral, if there’s one line neutral heading back to the panel. If there are separate line neutrals each must be connected to it’s corresponding load. They cannot be connected in parallel.


----------



## emtnut (Mar 1, 2015)

CMP said:


> Besides the overcurrent and backfeed problems mentioned earlier, you can also have trouble with GFCI and AFCI devices, on any of the circuits that have their neutrals shared with other circuits.


That's what I was thinking as I read through this.

Worthwhile to mention for the OP, that many of the AFCIs have now removed the GFP from them.
(Arguably the only thing the AFCI ever did)


----------



## Ronan (9 mo ago)

Bluenose for rent said:


> Too make this crystal clear for the OP, ccts 8 and 10 can absolutely share a neutral, if there’s one line neutral heading back to the panel. If there are separate line neutrals each must be connected to it’s corresponding load. They cannot be connected in parallel.


I'm not sure where my confusion is. My scenario, for example, has circuits 8 & 10, both 12/3 12/2 with their own neutral, each with their own load. This isn't sharing a neutral; it's neutral-neutral shunting. If @BleedingLungsMurphy is saying that those neutrals can be tied together, then I'm missing something completely. If he's talking about 8 & 10 serving a dryer with one neutral, I'm not talking about that & this post isn't about that.


----------



## HertzHound (Jan 22, 2019)

I think you mean 12-2. Grounds don’t count unless your talking about SJ and similar cord.


----------



## Almost Retired (Sep 14, 2021)

Ronan said:


> Take a look at Karl Riley's video here,


EXCELLENT VIDEO
VERY INFORMATIVE

i always knew those things were wrong, but i didnt know why exactly, now i do


----------



## CMP (Oct 30, 2019)

[ am still doing my apprenticeship. I'm just not part of any organization apprenticeship school like ABC. Every single student I've asked, "so have you learned anything from apprenticeship school?" has responded with, "**** NO! It's a waste of time." So I learn on the job and through my studies. I have no time or patience for institutions that literally waste my time and my employer's money. After a year with the only "lab" consisting of less than one hour of pipe bending, I quit. Bought my own benders, and learned on the job as well as at home with Home Depot pipe.[/QUOTE]

Learning on your own can be a valuable way to go. Don’t let the naysayer’s belittle you to tell you it’s just wrong. When I was at the point that your at now, I was working with a master that got his license from a box of cracker jacks. He was the most ignorant person I have ever encountered in the trade. But it turned out to be a good learning experience, what I learned from him was, everything not to do in the trade. He was just a substance abuser and just used his business as an excuse to try and hide his bad habits from his wife.

Bending skills was a good example, about the only thing he could bend accurately was romex. I was forced to figure it out myself. I learned how to do it with high precision, with many failed attempts in the process. Once I became proficient, I became aware that the layout and bending methods that I developed, I later discovered that nobody else uses. I found that out when I had to hire a contractor that had hydraulic push bender to bend a pile of 2” into a bunch of saddle bends to go around a bunch of pilasters on a remote job site. When we got done the guy that owned and operated the bender wanted to know where I learned that method, as he never seen that method before, and every four point saddle measured to the drawings and there was not one piece of scrap.

If your self motivated and really want to learn, you can do it by paying attention and self study. You just have to want it bad enough. You can learn a great amount just by observing how others do things, some bad some good. But you need to cultivate that interest of being the most knowledgeable and talented in the work you choose. I t took me a decade to learn enough to be highly proficient in all areas that I practice in, to go out on my own and hang my own shingle for industrial work. It can be done, you just have to want it bad enough. There are several members here that have done it already. Don’t let the naysayers tell you otherwise.


----------



## Bluenose for rent (Nov 6, 2020)

Ronan said:


> I'm not sure where my confusion is. My scenario, for example, has circuits 8 & 10, both 12/3 with their own neutral, each with their own load. This isn't sharing a neutral; it's neutral-neutral shunting. If @BleedingLungsMurphy is saying that those neutrals can be tied together, then I'm missing something completely. If he's talking about 8 & 10 serving a dryer with one neutral, I'm not talking about that & this post isn't about that.


Normally 8 and 10 would make up one 12/3 red would be 8 black would be 10 if it’s three phase.


----------



## Ronan (9 mo ago)

HertzHound said:


> I think you mean 12-2. Grounds don’t count unless your talking about SJ and similar cord.


Oopsies. Correct. I meant 12/2. Thanks.


----------



## Ronan (9 mo ago)

CMP said:


> [ am still doing my apprenticeship. I'm just not part of any organization apprenticeship school like ABC. Every single student I've asked, "so have you learned anything from apprenticeship school?" has responded with, "**** NO! It's a waste of time." So I learn on the job and through my studies. I have no time or patience for institutions that literally waste my time and my employer's money. After a year with the only "lab" consisting of less than one hour of pipe bending, I quit. Bought my own benders, and learned on the job as well as at home with Home Depot pipe.
> 
> Learning on your own can be a valuable way to go. Don’t let the naysayer’s belittle you to tell you it’s just wrong. When I was at the point that your at now, I was working with a master that got his license from a box of cracker jacks. He was the most ignorant person I have ever encountered in the trade. But it turned out to be a good learning experience, what I learned from him was, everything not to do in the trade. He was just a substance abuser and just used his business as an excuse to try and hide his bad habits from his wife.
> 
> ...


This is exactly how I do things. I watch what everyone does, the good and the bad. I listen to everyone complain, and keep my mouth shut. When the foreman is giving info to others, I go join them so I can learn what they're talking about. I organize supplies and am the first to unload **** off the trucks. I have all the books, more than is healthy for me, including the Mike Hold DVD collection (which always puts me to sleep). I've even resorted to reorganizing the NEC based on type of job, so I have all the residential code in one place. It's already proven handy as I brought up the fact that none of the dishwashers nor garbage disposals had GFIs in the apartment units we were building...they were soon changed after I mentioned it. I HAVE to make this work. It's not just a job for me.


----------



## Ronan (9 mo ago)

Bluenose for rent said:


> Normally 8 and 10 would make up one 12/3 red would be 8 black would be 10 if it’s three phase.


In this instance, it didn't. 8 went to lights in a bathroom. 10 went to lights in the hallway. Completely different homeruns.


----------



## Bluenose for rent (Nov 6, 2020)

Ronan said:


> In this instance, it didn't. 8 went to lights in a bathroom. 10 went to lights in the hallway. Completely different homeruns.


Sounds like a weird numbering scenario, but yeah home runs should never have the neutrals in parallel


----------



## Ronan (9 mo ago)

Bluenose for rent said:


> Sounds like a weird numbering scenario, but yeah home runs should never have the neutrals in parallel


I quit shortly after this one job site. The company had many issues.


----------



## Bluenose for rent (Nov 6, 2020)

Ronan said:


> I quit shortly after this one job site. The company had many issues.


I think they all do though…


----------



## BleedingLungsMurphy (10 mo ago)

Ronan said:


> circuits 8 & 10, both 12/3 with their own neutral, each with their own load


Is this 3 phase commercial? Now I'm assuming this is 3 phase commercial, not residential, because you mentioned 4 square boxes being spliced by multiple apprentices. I don't see this happening on residential jobs.


> If @BleedingLungsMurphy is saying that those neutrals can be tied together, then I'm missing something completely.


3 phase theory is my guess. In Canada we do not learn anything about 3 phase until 2nd year schooling.










> Completely different homeruns.


I got pretty cocky in my first year. I thought I had picked up the lingo. I was installing light and I asked if I should bring the "home run" from the first light to the switch box. My Jman laughed and said that is a switch leg.

Please don't take this the wrong way, but, are you a first year? I see this too often. A little bit of misguided knowledge can do a lot of damage, or seed doubt in more experienced employees who will now question their own knowledge. Generally, it is not your job to teach 4th or 5th year apprentices how to do their job or question their work. If you think something is wrong you should go directly to your journeyman or foreman and ask them. Apprentices correcting apprentices is how you got into this situation where you don't understand what you're asking, or how to explain something.


----------



## paulengr (Oct 8, 2017)

In a 240 V circuit as an example we are used to visualizing all the current carrying conductors and each one passes through a breaker/fuse. There is no confusion that the breaker/fuse is there to protect the branch circuit.

If we share ungrounded phase conductors we are connecting them across multiple fuses or breakers. It should be obvious that this makes it easily possible to overload the phase conductors or prevent the breakers/fuses from protecting the load(s). This is a good way to start a fire.

With grounded phase conductors we rely on the fact that they are grounded for safety reasons. So we can’t put a circuit breaker pole or a fuse in the grounded phase conductor. The overcurrent protection in the ungrounded phases conductors are doing “double duty” protecting both themselves and the grounded phase conductors. So the grounded phase conductors look different because they land at the neutral bus bar (on non-GFCI/AFCI breakers) but other than landing directly on the bus they are just like any other phase conductor. Shorting them together has the same effect as ungrounded phase conductors.

Notice that I used the term that NEC uses. NEC calks it a grounded phase conductor. I purposely did not use the term “neutral”. A “neutral” is a phase conductor that is electrically the same voltage from all the other phase conductors. It’s the center of a 3 phase wye or a center tap in a split phase system. Neutrals are not always grounded (480/277 can be floating). A “neutral” is basically a transformer or generator term. But “grounded phase conductors” is a mouthful. We all just use the word neutral as a slang term.

So at this point it should be obvious why “sharing neutrals” is a bad practice. Other than the fact that a Voltmeter will read close to zero Volts to ground, a neutral is just like any other phase conductor.

Visually it’s easy to see why sharing neutrals may seem harmless. All the grounded phase conductors are connected directly to the neutral bar. In the MDP the grounds and neutrals may even land on the same bus bar. Electricians often make thd mistake that the neutral conductors are like some kind of drainage system since it measures zero Volts t ground. They assume neutrals just collect “spent electrons” returning them to the transformer. Nothing could be further from reality. For one thing it’s AC power. Electrons just vibrate back and forth. They don’t actually move very far.


----------



## Quickservice (Apr 23, 2020)

splatz said:


> Of course your making troubleshooting more difficult and maintenance potentially hazardous,* if someone turns off one breaker to work on the circuit not knowing the neutral is tied to another circuit, zap. *
> 
> You're paralleling conductors which in addition to the potential overload @Bluenose for rent mentioned, could also cause inductive heating.


That first happened to me for the first time about 20 years ago! Not fun! Have put the Tic-Tracer on both the hot AND neutral ever since.


----------



## BleedingLungsMurphy (10 mo ago)

I want to expand on this diagram a bit. In a balanced 3 phase system the home run neutral carries a negligible current and basically acts as a reference to ground. 3 individual devices can make a balanced 3 phase circuit. This is the same concept that allows reduced neutral feeders. This is also the reason why you do not want multiple home run neutrals for 3 phase circuits. For example; (8 + n, 10 + N, 12 + N) to the same box. By using a separate neutral for each phase you are now sending the current all the way back to the neutral bar to balance between the neutrals. It will not overload anything, and others have already mentioned the issues this would create when troubleshooting.

If you cross neutrals between sets the circuit may appear to operate normally while the loads are balanced. When loads are turned off the unbalanced current can overload the neutral, and backfeed other devices.


----------



## Ronan (9 mo ago)

BleedingLungsMurphy said:


> Is this 3 phase commercial? Now I'm assuming this is 3 phase commercial, not residential, because you mentioned 4 square boxes being spliced by multiple apprentices. I don't see this happening on residential jobs.
> 
> 3 phase theory is my guess. In Canada we do not learn anything about 3 phase until 2nd year schooling.
> View attachment 167084
> ...


It wasn't three phase. It was single phase. Each circuit was a 120 circuit with their own homerun and their own neutral. Again, this isn't a shared neutral scenario. It's neutral-to-neutral-shunting where each circuit has it's own neutral. Every time I post, I try to qualify and clarify what I'm saying so there's no confusion.

It is my job to clarify and point out safety issues no matter if it's the foreman who's making the calls. Just because someone has years under their belt, is a journeyman or 4th year apprentice doesn't automatically mean they know the code or understand theory.


----------



## BleedingLungsMurphy (10 mo ago)

Ronan said:


> What's the best way to explain to others the hazards of splicing different neutrals together?


What hazards? Other than code violations or troubleshooting issues....there is no hazard or increased danger from parallel neutral home runs in the scenario you described. Yes, it's a bad idea, but it will not cause the wire to "overload" with some mysterious current from the ether.


> It was single phase


Well then it would be impossible to over load parallel neutrals in this scenario using circuits 8 and 10.....even if one of the parallel neutral home runs was lost, the circuit would operate as a multi wire branch circuit. Now, if it was 8 and 9 or 10 and 11 then, theoretically, you could overload a neutral.


> It is my job to clarify and point out safety issues no matter if it's the foreman who's making the calls


Debatable. If you are trying to apply 1st or 2nd year knowledge to 4th year theory you will never understand why you are wrong. Have you ever heard the phrase, "Too many cooks in the kitchen"? Jobs would never get anywhere if every apprentice went around with your attitude trying to correct the whole world.


> When I gently try to mention, "it's going back to source...the transformer," and I get corrected where they say, "...no, it's going to ground," I just shut up and nod my head.


Once again, you are misunderstanding. The neutral on the transformer is grounded. That is the reference point for 0v. Saying it goes back to "ground" or "grounded neutral conductor" or "the transformer"  is essentially saying the same thing and you are trying to argue over semantics. I get the impression that you are more concerned with being "right" than actually learning how to do your job.

It's a long way down to the valley of despair, my friend, good luck.


----------



## splatz (May 23, 2015)

BleedingLungsMurphy said:


> What hazards? Other than code violations or troubleshooting issues....there is no hazard or increased danger from parallel neutral home runs in the scenario you described. Yes, it's a bad idea, but it will not cause the wire to "overload" with some mysterious current from the ether.


If you tie all the neutrals from different circuits together in a box, you've effectively combined the neutrals onto a set of parallel wires that will carry the combined neutral current of the circuits. If they are on the same leg, there will be no cancellation. The individual wires in the parallel set will all carry the same current if they're the same impedance. If they're all about the same type of wire and the same length and no bad splices, they'll all have about the same impedance. (Not a coincidence these conditions are in the rules for paralleling wires.)

So let's say there's two 20A circuits on the same leg, wired with #12 awg, carrying 14 amps of neutral current, together in the same box. Let's say you splice all the neutrals together, effectively combining the neutrals on a parallel set - what happens? If the neutral wires are routed together back to the panel, so they're about the same length and no bad splices etc., they'll have about the same impedance, and they'll each carry 14 amps, no overload.

But what if the circuits are routed differently, and the neutral wires are much different lengths? That's not unusual at all. Let's say one neutral is 10 feet long, and the other is 100 feet long. The long wire has 10 times the impedance of the short wire. In parallel, the lower impedance path carries more of the current. I did the math and if I am figuring right, the short wire will carry 21.8 amps of the combined 24 amp load, and the long wire will carry 2.2 amps, overloading the short wire.

That's if you do this in just one box. If you just did this in mulitple boxes with multiple combinations of circuits you'd quickly have a math problem I am not going to do.

(I think there is a tendency to hand-wave this without actually thinking about the math, because we are used to thinking of wire resistance as negligible when figuring things - and it usually is negligible as long as you don't get creative and do things like combine neutrals for multiple circuits onto a parallel set.)


----------



## emtnut (Mar 1, 2015)

BleedingLungsMurphy said:


> What hazards? Other than code violations or troubleshooting issues....there is no hazard or increased danger from parallel neutral home runs in the scenario you described. Yes, it's a bad idea, but it will not cause the wire to "overload" with some mysterious current from the ether.


Main hazard: Someday if someone is moving breakers around to make room for a DP, when they see 2 SP breakers each on 14/2, it tells you they can be placed anywhere. That is a serious hazard.
For our US friends, we don't require handle ties on 'most' MWBCs. And many of your older installs were done without handle ties as well (not sure when NEC changed that).

Secondary hazard is turning off the circuit to work on it, you can have neutral current running through what you expect is a dead circuit. Even if you test it, doesn't guarantee that someone in the house won't turn something on connected to that other circuit.


----------



## Bluenose for rent (Nov 6, 2020)

BleedingLungsMurphy said:


> What hazards? Other than code violations or troubleshooting issues....there is no hazard or increased danger from parallel neutral home runs in the scenario you described. Yes, it's a bad idea, but it will not cause the wire to "overload" with some mysterious current from the ether.
> 
> Well then it would be impossible to over load parallel neutrals in this scenario using circuits 8 and 10.....even if one of the parallel neutral home runs was lost, the circuit would operate as a multi wire branch circuit. Now, if it was 8 and 9 or 10 and 11 then, theoretically, you could overload a neutral.
> 
> ...


I know I’m risking an eye roll emoji but it’s not semantics. Ground and source are not the same thing. Return current can and does flow through ground but it’s supposed be minimised and directed safely. Understanding the difference between these terms matters a lot.


----------



## paulengr (Oct 8, 2017)

BleedingLungsMurphy said:


> I want to expand on this diagram a bit. In a balanced 3 phase system the home run neutral carries a negligible current and basically acts as a reference to ground. 3 individual devices can make a balanced 3 phase circuit. This is the same concept that allows reduced neutral feeders. This is also the reason why you do not want multiple home run neutrals for 3 phase circuits. For example; (8 + n, 10 + N, 12 + N) to the same box. By using a separate neutral for each phase you are now sending the current all the way back to the neutral bar to balance between the neutrals. It will not overload anything, and others have already mentioned the issues this would create when troubleshooting.
> 
> If you cross neutrals between sets the circuit may appear to operate normally while the loads are balanced. When loads are turned off the unbalanced current can overload the neutral, and backfeed other devices.
> View attachment 167090


In a balanced system you get capacitive coupling which induces a current. It’s proportional to the size of the transformers and shielded (medium voltage) cables but I’ve seen 20-30 A in severe cases (100 MW of connected load).

With power electronics neutral currents are normal. The power components turn on and off in the middle of a cycle which isn’t balanced and leads to neutral currents.


----------



## GladMech (Sep 18, 2020)

Imagine having to explain this to a Mexican electrician. (circa. 1986) An industrial factory electrician was having a very difficult time understanding why he needed THREE wires to complete a circuit. This is because residential power sometimes didn't even require TWO wires! Take a tennis shoe, tie *one* wire to it, and throw it over the line at the street, and the light bulbs and refrigerator come on! 
(not hearsay. observation.)


----------



## Ronan (9 mo ago)

BleedingLungsMurphy said:


> What hazards? Other than code violations or troubleshooting issues....there is no hazard or increased danger from parallel neutral home runs in the scenario you described. Yes, it's a bad idea, but it will not cause the wire to "overload" with some mysterious current from the ether.


I strongly suggest you watch *Tracing Magnetic Fields EMF by Karl Riley*.







BleedingLungsMurphy said:


> Debatable. If you are trying to apply 1st or 2nd year knowledge to 4th year theory you will never understand why you are wrong.


I always speak up at least once, if I see a code violation. There is a risk that I might be wrong, but it's not that often. Worst case scenario is that I, and those around me, learn something.



BleedingLungsMurphy said:


> Once again, you are misunderstanding. The neutral on the transformer is grounded. That is the reference point for 0v. Saying it goes back to "ground" or "grounded neutral conductor" or "the transformer"  is essentially saying the same thing and you are trying to argue over semantics. I get the impression that you are more concerned with being "right" than actually learning how to do your job.


I am not going to argue with this last point you made.


----------



## Almost Retired (Sep 14, 2021)

BleedingLungsMurphy said:


> What hazards? Other than code violations or troubleshooting issues....there is no hazard or increased danger from parallel neutral home runs in the scenario you described. Yes, it's a bad idea, but it will not cause the wire to "overload" with some mysterious current from the ether.
> 
> Well then it would be impossible to over load parallel neutrals in this scenario using circuits 8 and 10.....even if one of the parallel neutral home runs was lost, the circuit would operate as a multi wire branch circuit. Now, if it was 8 and 9 or 10 and 11 then, theoretically, you could overload a neutral.
> 
> ...


i will also strongly suggest you watch that video


----------



## wiltavares135 (Sep 25, 2019)

Ronan said:


> What's the best way to explain to others the hazards of splicing different neutrals together?
> *Note:* To minimize confusion, I'm not talking about a shared neutral, like with a dryer.
> 
> I recently learned about why we are not supposed to splice neutrals from different circuits together. I think it's easy, especially for apprentices (even those ready to take their journeyman's test) to see a bunch of neutrals in a box and splice them together, or to cut corners and just splice neutrals from circuits 8 & 10 together in a 4-square. I hadn't seen any formal training to this specifically and had to research it out on my own.
> ...





Ronan said:


> What's the best way to explain to others the hazards of splicing different neutrals together?
> *Note:* To minimize confusion, I'm not talking about a shared neutral, like with a dryer.
> 
> I recently learned about why we are not supposed to splice neutrals from different circuits together. I think it's easy, especially for apprentices (even those ready to take their journeyman's test) to see a bunch of neutrals in a box and splice them together, or to cut corners and just splice neutrals from circuits 8 & 10 together in a 4-square. I hadn't seen any formal training to this specifically and had to research it out on my own.
> ...


When you run wire for a 208/120 system, say you pull #12 wire for 20amps with phase A,B&C which is black red and blue with one neutral. you then have 3 microwave ovens, one for each circuit sharing that same neutral. Then if you splice together another neutral from another circuit that has its own neutral, and each microwave oven is running at the same time the first shared neutral we spoke of will burn up or catch on fire because there is no way that neutral can take any more of a load. In a wye system such as we are speaking of, the neutral is used to balance out the current on each phase, when tying together neutrals from other branches, all you are doing is make some of those neutral wires to overload with current that doesn’t belong returning back on that wire/s. Dedicated neutral is a hot with its own neutral, balanced neutral is a shared neutral that has more than one hot returning on it no more than the phase of the system you are using. If you are using a 240/120 system that means you are using a single phase system, so that means phases A & B. so you will share black , red with one neutral. 208/120 is a 3 phase system so it will be shared like I spoke of earlier. I hope this helps To simplify it.


----------



## GladMech (Sep 18, 2020)

If you have 3 phase with a neutral to a subpanel with lots of single phase loads, the neutral may even have to be larger than the phases, in case all the loads on only one side of the room were on, i.e., all of one phase and part of another.


----------



## Almost Retired (Sep 14, 2021)

GladMech said:


> If you have 3 phase with a neutral to a subpanel with lots of single phase loads, the neutral may even have to be larger than the phases, in case all the loads on only one side of the room were on, i.e., all of one phase and part of another.


what is your formula for determining the neutral current for an equally loaded A, B, C MWB
also how do you determine the neutral current for an Un-equally loaded A, B, C MWB


----------



## GladMech (Sep 18, 2020)

Equally loaded should be zero by definition. That would be all three phase motors and nothing else. Probably never exactly zero.
It's too late in the day to do the math and also... I don't want to. Three phase math makes my head hurt.  But an obvious case of unequal loading would be all single phase devices on phase A and nothing else on, in which case the neutral current would be equal to the phase. For normal loads, that is probably the maximum.
If there are VFDs involved, then the neutral may be upsized because of harmonics and other electronic considerations that are beyond a mere mechanical engineer/electrician.


----------



## kristopherk1985 (Oct 26, 2021)

SWDweller said:


> I was working under a foreman who did not have the experience I did. I needed a job and he immediately set me into the switchgear room which was covered in about a foot of branch and feeder conductors. We were leaving one day and my boy Richard was up 15 feet or so on a metal scaffold. He was splicing in a new string of florescent fixtures, hot. I put down my tools, asked if I should go and shut down the temp lighting circuit. Richard explained to the crew how he was a journey man and could work hot. Missed the blue wire nut by that much and got nailed I caught him before he hit the ground. Then I went over and shut down the circuit. I never said anything nor did I challenge his authority. Job finished up he got laid off and I was promoted to foreman on the new job. As soon as I could I hired Richard back. I liked the guy, but he is a bit lacking in the electrical knowledge department. One day he asked me to explain what happened, he was unaware that the neutral carried current.


Yeah, I always connect ground, neutral, then hot. He probably got hit after he connected the hot first and the lighting load was returning on the fixture neutral.


----------



## splatz (May 23, 2015)

Almost Retired said:


> what is your formula for determining the neutral current for an equally loaded A, B, C MWB
> also how do you determine the neutral current for an Un-equally loaded A, B, C MWB


current on neutral is a vector sum ... the formula 

given
current on A phase = A
current on B phase = B
current on C phase = C

neutral current =


----------



## Aldo Moretti (Aug 29, 2011)

Ronan said:


> What's the best way to explain to others the hazards of splicing different neutrals together?
> *Note:* To minimize confusion, I'm not talking about a shared neutral, like with a dryer.
> 
> I recently learned about why we are not supposed to splice neutrals from different circuits together. I think it's easy, especially for apprentices (even those ready to take their journeyman's test) to see a bunch of neutrals in a box and splice them together, or to cut corners and just splice neutrals from circuits 8 & 10 together in a 4-square. I hadn't seen any formal training to this specifically and had to research it out on my own.
> ...


Sharing a neutral on a round house (3 phase 240/120) is fine, the same with single phase 120/240, but one has to make sure not to simply grab a neutral splicing it to another circuit, basically only use the neutral which is part of the circuits that are existing. 
This is extremely important for safety reasons that other people have mentioned, and also because if you deviate, your ideas will not pan out and what you are trying to do won't work.
For instance, I had a customer with a long path light system with two three way and a four way switch in the middle. At the bottom of the system was a second unit with switches inside. 
The customer requested that their tenant at the bottom that while after activating the switch on the top at the street could turn off the lights at the bottom with the 3 way switch, but when the owners get home and turn on the lights outside their unit with all the other lights on the path, the tenant did not feel free to turn of all the path lights from the bottom, the light bothered the bottom tenants and they needed to turn the system from above, but wanted to turn it off from below and also turn on and off the light at their own discretion whether the 3 way system was on or off.
I went home for lunch and thought about it and came up with a solution, went back to the job and (luckily) had a 3 way switch in my shop and here's what I did, I took one traveler from the 3 way path system, and one hot from a lighting circuit in the bottom unit, and ran the common to the outside light. That means the 3 way switch inside the bottom unit could either pick a traveler from the path system or a constant hot from the inside of the bottom unit. Since it was only one traveler, and there was still the existing 3 way switch inside with an additional 3 way with a constant hot, the tenant was able to have the bottom light come on with the 3 way on top, and turn it off at the bottom. It might hard to get a grip on, but it did solve a complicated problem which most people might know how to solve it.
Back to the neutral, I knew that I had to use the same neutral that came down from above in the path system and not use the neutral that was in the bottom unit lighting system. I did not even try it, I know it wouldn't have worked.
On another job nearby up on Mt. Tam where I live, a customer needed a lot of work done on an old house and part of that was replacing a sub-panel that basically fed the whole house, it was all knob and tube and all the wires were black going into the panel, even the neutrals were black ! What had to be done is for me to carefully label each wire and make sure that every neutral was with the same existing hot, and color code the old wires too when landing them in the new panel. That job was not inspected by the City of M.V., but I have done other jobs in S.F. where the inspector can mandate that all circuits going into a new panel must be phase coated even if all the circuits are 12/2 NMB, meaning that all the hot wires are black.
There are many other instances where I have been called to solve flickering and pulsating lights connected to modern dimmers after new work was done, every time I found that the electrican simply "grabbed a neutral" in a box and "called it bueno" and left the job.


----------



## Aldo Moretti (Aug 29, 2011)

BleedingLungsMurphy said:


> Have you learned 3 phase theory yet?
> 
> You can not apply this logic to all circuits. You seem to lack the basic understanding of 3 phase theory.
> 
> ...


Unless it is 3 phase known as a round house, then it's fine to share the neutral.


----------



## paulengr (Oct 8, 2017)

GladMech said:


> If you have 3 phase with a neutral to a subpanel with lots of single phase loads, the neutral may even have to be larger than the phases, in case all the loads on only one side of the room were on, i.e., all of one phase and part of another.


How?

If you put 20 A on all three phases the neutral will be zero. If you have 1 phase at 20 A the neutral will be 20 A. Two phases to neutral will partly cancel each other once again leading to under 20 A. Visually if I have 3 phases at 20 A and shut one off the most the neutral could be is 20 A.


----------



## paulengr (Oct 8, 2017)

Almost Retired said:


> what is your formula for determining the neutral current for an equally loaded A, B, C MWB
> also how do you determine the neutral current for an Un-equally loaded A, B, C MWB


On truly balanced systems the neutral current is close to zero.

On unbalanced systems, NEC has a formula but like most others we just run a full size neutral and don’t play around with it.


----------



## paulengr (Oct 8, 2017)

GladMech said:


> Equally loaded should be zero by definition. That would be all three phase motors and nothing else. Probably never exactly zero.
> It's too late in the day to do the math and also... I don't want to. Three phase math makes my head hurt.  But an obvious case of unequal loading would be all single phase devices on phase A and nothing else on, in which case the neutral current would be equal to the phase. For normal loads, that is probably the maximum.
> If there are VFDs involved, then the neutral may be upsized because of harmonics and other electronic considerations that are beyond a mere mechanical engineer/electrician.


A VFF draws 3 phase relatively balanced power on the line side. It does not have a neutral connection, only L1, L2, L3, the front end is a 3 phase rectifier.

It draws current only when the line voltage is higher than the voltage on the DC bus (capacitor). There are 6 diodes so as you expect we get 6 pulses of current, 2 per phase. So we get “rabbit ears” for current. There is ground ((not neutral) current but it is negligible as long as there is a good ground between the drive and the motor. 



http://www.gozuk.com/image/vfd-input-current.jpg



On the load side it looks considerably worse. On that side you normally get significant common mode (ground) current by nature. This is relatively harmless by itself but often it induces currents into the bearings that exceed the bearing current limits. When this happens you get fluting. On small motors, typically 10 HP or less it’s due to high frequencies and easily removed. On large motors it’s due to circulating common mode currents and gets more severe with motor size. On a 900 HP fan motor with a particularly bad setup I measured 90 A arcs hitting the bearing. That sounds like a lot but FLA was close to 1000 A and the bearings were big enough to handle up to about 10 A.


----------



## GladMech (Sep 18, 2020)

paulengr said:


> How?
> 
> If you put 20 A on all three phases the neutral will be zero. If you have 1 phase at 20 A the neutral will be 20 A. Two phases to neutral will partly cancel each other once again leading to under 20 A. Visually if I have 3 phases at 20 A and shut one off the most the neutral could be is 20 A.


Yeah, I realized that after I wrote it. I conflated three phase with VFD considerations. See 2nd post.


----------



## splatz (May 23, 2015)

splatz said:


> current on A phase = Ia
> current on B phase = Ib
> current on C phase = Ic
> 
> current on neutral is a vector sum


If you have trouble visualizing the neutral current cancellation, try this, I made it on desmos.com - it shows the vector sum of the neutral current on a 20A three phase MWBC. You can slide around the values for neutral current A, B, and C from 0-20A and you'll see you can't get the result (orange endpoint) out of the 20A circle.

https://www.desmos.com/calculator/bohsbpmem9


----------



## splatz (May 23, 2015)

LGLS said:


> Yeah wonderful unfortunately we all know that, and you’re posting this because it amazes you because it’s a new discovery. I’ve known it for 40 years. Sleep tight kiddo


No new discoveries in three phase theory here for me, although I guess I still a little amazed that reality works out the same way the equations work out. 

I definitely find the desmos.com graphical calculator amazing, free and available on the internet for anyone to use. Anyone that had a Texas Instruments graphing calculator in the 1980s would agree. I am still discovering new features in Desmos. 

The only discovery for me, and it's not amazing but it is a relief, is that I could figure out the equations to build that graph without googling anything, cracking a book or otherwise looking something up. I put that stuff on a shelf 35 years ago but it's still in there somewhere. Seems to do the brain good to trot it out once in a while, you should try it 👍


----------



## GrayHair (Jan 14, 2013)

splatz said:


> Seems to do the brain good to trot it out once in a while, you should try it 👍


I do that ever morning. 
Who am I? Where am I? ...


----------



## LGLS (Nov 10, 2007)

wiltavares135 said:


> When you run wire for a 208/120 system, say you pull #12 wire for 20amps with phase A,B&C which is black red and blue with one neutral. you then have 3 microwave ovens, one for each circuit sharing that same neutral. Then if you splice together another neutral from another circuit that has its own neutral, and each microwave oven is running at the same time the first shared neutral we spoke of will burn up or catch on fire because there is no way that neutral can take any more of a load. In a wye system such as we are speaking of, the neutral is used to balance out the current on each phase, when tying together neutrals from other branches, all you are doing is make some of those neutral wires to overload with current that doesn’t belong returning back on that wire/s. Dedicated neutral is a hot with its own neutral, balanced neutral is a shared neutral that has more than one hot returning on it no more than the phase of the system you are using. If you are using a 240/120 system that means you are using a single phase system, so that means phases A & B. so you will share black , red with one neutral. 208/120 is a 3 phase system so it will be shared like I spoke of earlier. I hope this helps To simplify it.


No not really. In your example all three microwaves are running. So the neutral has no-load as each of the three phase is a perfectly balanced and therefore there is zero current on the neutral and what is coming back from that fourth circuit unrelated to this multi wire branch circuit would be a piece of cake for the neutral to handle.


----------



## mpcxl (Dec 9, 2015)

I think current on different phases cancel each other and is not cumulative, so your neutral will not overload


----------



## CAUSA (Apr 3, 2013)

splatz said:


> If you have trouble visualizing the neutral current cancellation, try this, I made it on desmos.com - it shows the vector sum of the neutral current on a 20A three phase MWBC. You can slide around the values for neutral current A, B, and C from 0-20A and you'll see you can't get the result (orange endpoint) out of the 20A circle.
> 
> https://www.desmos.com/calculator/bohsbpmem9


I did not even know that this software existed. 

This would make such a fantastic teaching tool.

I’m to used to using the Vector summation on the calculator.


----------



## joe-nwt (Mar 28, 2019)

I've been following this thread since the beginning. In my opinion, anyone who doesn't understand the consequences of willy-nilly tying neutrals together shouldn't be tying them together in the first place. If it's apprentii doing this, where are the journeymen that are supposed to be supervising them? 

Even if someone's math or understanding is not up to snuff, all in the trade have been told it's a code violation and not to do it.


----------

