Need help with multiwire branch circuit serving both 120V and 240V loads

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milwaukeesteve

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Milwaukee, WI
Thank you George for those replies. I was wondering when you would weigh in on this one.:D


I agree with everything you said in your second reply. Everything. I will now expand on the fantastic points you made in that reply.


If we look at the original rule again, 210.4(C), we see, as you stated, that the MWBC SHALL supply ONLY line-to-neutral loads. There, we have covered line-to-neutral. Especially when it uses the words SHALL and ONLY; those are very strong words. (Kind of like the word ALL when dealing with SA recepts on a kitchen countertop, but that is another humdinger of a topic).
Now the exceptions would be exceptions to using ONLY line-to-neutral loads. (I use the word ONLY there because I am following the rule's use of ONLY.) Therefore, the exceptions would be for NON, and I repeat, NON line-to-neutral loads. Again, if the original rule wasn't so emphatic with the SHALL and ONLY, maybe I would consider including line-to-neutral loads within the exceptions, but somehow I can't.


I said I agreed with everything you said in that reply and I did. You even helped me point out my arguement even better:D. Near the end of your reply though, you showed how you thought that Mike came up with the 'group of loads' concept. Again, I agree with that, but 'group' does not mean 'dual voltage' or 'combined' loads; it just means more than one.



As for the Range/Dryer arguement: first those 120V loads (including the convenience outlets on old ranges) are internally fused. And they are UL listed and approved for that. These are also manufactured items, and not field installed, 'combined load' MWBC's.
 

Rockyd

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Nevada
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Retired after 40 years as an electrician.
Congratulations George on your test!

As for the code telling me what I can do on the average...it's more about providing guidance when in doubt, on an issue. 90.1(A) reads to me as an attempt to tell us to go forth and do safe work. Is it always pretty? No, and this is where the code comes into play. On complicated jobs, we are installing things that may be cutting edge technology. We don't have to understand how it works, we just have to install it so that it does the objective in a safe way (if it falls under jurisdiction) as determined to this point in history.

Per the NEC handbook the code is a living document, and as such, anyone can submit a proposal for change or public comment, and all proposals and comments are subject to a rigorous public review process.

I think, the code, after going through committee, is the best effort, for that moment in time, to lend proper guidance, when needed, by those who are qualified, on past empirical knowledge. Past that point, it becomes a free for all. In the field, the code book is a valuable tool to justify our engineering and installations (provided that it is followed). We reach for the code when we have to march our efforts, past those who may not understand, that our project does meet standards for which it was designed, in a safe manner. Even with it's imperfections, The Code does help keep the Electrical Industry to a standard of reasonable safety, and chaos to a minimum.

Great thing about this forum, is that there are people far sharper than myself, frequently here. When in question on an issue, this is a great place to come for gathering opionions, and various sources of information. I learn from various posts, and, in turn, offer code articles, and various other sources, along with an opinion to others.
 
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George Stolz

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milwaukeesteve said:
Thank you George for those replies. I was wondering when you would weigh in on this one. :D
Kind of like waiting for the dentist to walk in after you've sat down, isn't it? :D

I agree with everything you said in your second reply. Everything.
I don't think you do, because you're coming to a different conclusion. Hee hee hee... :D

Again, I agree with that, but 'group' does not mean 'dual voltage' or 'combined' loads; it just means more than one.
Can you define what you mean when you say "dual voltage" and "combined"?

As for the Range/Dryer arguement: first those 120V loads (including the convenience outlets on old ranges) are internally fused.
Are you sure, or are you making an educated guess?
 

milwaukeesteve

Senior Member
Location
Milwaukee, WI
Hey George,
In reference to your last reply, a couple of things.

I did state that I agreed with everything in that one reply you had, because you just stated eveything, even down to the 'group of loads' at the end. That is why I agreed with you, because you stated everything as is. However, what you didn't state was a conclusion, or where someone takes what you stated to mean what everybody has been arguing. There wasn't anything that took you from 'group of loads' to being allowed to putting both a 120V and a 240V load on the same MWBC.
That is the other thing I need to clarify. When I said 'combined' and 'dual voltage', I am referring to this principle or idea that we are allowed to combine both 120V and 240V loads, within a MWBC, protected by the same breaker or breakers with handleties, as shown in Mike's Graphic and Description. My arguement is that Exception 2 does not even allow us to consider 120V loads, because the rule itself addressed 120V loads (SHALL ... ONLY). 210.4(C)

As for the Ranges and Dryers, "I believe" were the words I used. But that is irrelevant, because this is not a field wired MWBC with 120V and 240V. Yes, it is in effect the same thing, but the NEC does not care. Exception 1 says it is ok to use a MWBC to feed that dryer or range, and the units are manufactured and UL listed pieces of equipment. The NEC is satisfied with that. So the arguement of the Range/Dryer is in principle the same thing, but according to the NEC, it is not.

----clarification only---
I am referring to 120V and 240V because that is what the original topic is about. Line to Line can refer to other voltages and phasing, but to keep the argument streamlined, I will stay with your standard house panel and voltages, in regards to the original question.
 

George Stolz

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milwaukeesteve said:
HThere wasn't anything that took you from 'group of loads' to being allowed to putting both a 120V and a 240V load on the same MWBC.

Maybe this will clear up where I'm coming from:
210.jpg

There are three MWBCs.
The 1st ( 2, 4 ) represents the main rule.
The 2nd ( 6, 8 ) represents exception 1.
The 3rd ( 10 , 12 ) is exception 2.

Each circuit is unlike the other. How do you envision the third MWBC according to the text?

My arguement is that Exception 2 does not even allow us to consider 120V loads, because the rule itself addressed 120V loads (SHALL ... ONLY).
From the circuit breaker's perspective, is it supplying a 120V load? Or is it serving a mix as shown in my diagram?

I will stay with your standard house panel and voltages, in regards to the original question.
No argument from me. :)
 
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milwaukeesteve

Senior Member
Location
Milwaukee, WI
Hey George, I like your diagram. Is this one your own?

This diagram shows exactly how I read the code rule and exceptions to be.

1) the rule itself states only for line to neutral loads which you show in a 3wire MWBC serving 2 different 120V loads, sharing a common neutral

2) the first exception states only for 1 utilization equipment, which you show again with a 3wire circuit that feeds only one load.

3) the second exception, with all line conductors simultaneously disconnected, allows for multiple loads, as you show by a 3wire MWBC feeding 4 seperate loads.

I love the diagram, and will emphatically agree that what the code rule reads is exactly like your diagram.


Now for the monkey wrench.
What this whole thread is about, is not whether you can have multiple loads (as shown in your diagram with the 4 seperate loads), but rather that one of those 'multiple' loads be instead a 120V load, not another 240V load. So imagine a 240V baseboard heater with an installed convenience recept, and putting that on a 12/3 MWBC, protected by a 2pole 20A breaker (or 2 separate breakers handletied). If you look at Mike Holt's Graphic on this and compare it to your Graphic, you will see that Mike's will show a 'combination' of both 120V and 240V loads.
This is where I have used the words 'combined' or 'dual voltage' as opposed to 'group' or 'multiple'.

What I don't understand from Mike and the others that have argued this topic in this thread is that how I can read this code rule and see your graphic, and they can read this code rule and see Mike's graphic.
 

George Stolz

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milwaukeesteve said:
Hey George, I like your diagram. Is this one your own?
Yes. I still have the original, so it can be tweaked if the conversation requires it. :)

What this whole thread is about, is not whether you can have multiple loads (as shown in your diagram with the 4 seperate loads), but rather that one of those 'multiple' loads be instead a 120V load, not another 240V load.
Looking at my diagram, did you notice that
  • Appliance 1 was a 120/240 load (MWBC)
  • Appliance 2 was a 120V load (L-N)
  • Appliance 3 is a 240 (L-L) load
  • Appliance 4 is a 120/240 load (MWBC)?

This is where I have used the words 'combined' or 'dual voltage' as opposed to 'group' or 'multiple'.
To keep us speaking the same language, can we call an appliance similar to a range a "MW" (Multiwire) load, and the others by their voltage?

What I don't understand from Mike and the others that have argued this topic in this thread is that how I can read this code rule and see your graphic, and they can read this code rule and see Mike's graphic.
I don't see the difference between Mike Holt's graphic and my third circuit.

Is the difference you're seeing because his two appliances are nestled up close to each other?
 

milwaukeesteve

Senior Member
Location
Milwaukee, WI
Now that we are on the right track, and we are arguing apples and oranges correctly:), I will again state my arguement.
The wording in exception 2 as written in the CODE book, does not allow wiring like this. The wording as written in the HAND book, DOES state that this is accepted wiring.

So, is the HAND book stating what the hand book thinks the CODE book is 'implying'? Then that is an interpretation.

If the code rule doesn't say that you either CAN or CANNOT do something, who's call is it to say whether you can do it or if you absolutely cannot.

My reasoning as stated earlier, is that the rule itself is for LINE to NEUTRAL loads, given the severity of the language.
Just because the exception states if all ungrounded conductors are simultaneously disconnected, that suddenly it allows for all kinds of funky combination loads, seems like a big leap for me.
I did like your statement about 'groups of loads', because in my mind, that is exactly what this exception means, but not a 240/120v and a 120v and a 240v load.
Imagine that they made mini, individual use close dryers, maybe for like a dormitory. Say they only had loads of 8Amps. Now you want to run a single circuit, or say that one dryer circuit was existing and you wanted to reuse it. You put 3 of these dryers on the same 30A dryer circuit. That is how I read this exception. This single circuit is for more than one utilization equipment(exception 1), and all ungrounded connectors are simultaneously disconnected (exception 2).

Granted this may not be the best example, and I did not research Appliances before I wrote it, but this is what I could come up with. I really can't think of anyplace I have ever come across or installed anything where I have used this exception.
 

DesertRat

Member
I've been watching this thread intently, lots of good info and interesting debate. Finally I believe I will chime in.

Steve, I do not understand your argument, on a purely semantic level.

Yes, 210.4(C) restricts a MWBC to strictly line to neutral loads on it's face.
But reading further we come to exception 2,

"where all ungrounded conductors of the multiwire branch circuit are opened simulatneously by the branch circuit overcurrent device."

So lets presume we've installed a two pole common trip breaker on our MWBC. That would meet the required criteria of exception 2, thus excepting(which is to say eliminating) the requirements of the rule itself (by definiton this is what an exception does.) Exception 2 has now absolved us of the requirement of the rule, but has not stated any new restrictions that we must adhere to (except for the simultaneous opening that we have already met.) Therefore we can do essentially what we want, as the rule no longer applies (since we've met the criteria for exception.) What stops us from having a 120V and 240V load on our MWBC? Certainly not 210.4(C) as it has been excepted and no longer applies. Certainly not exception 2 of that rule, because it states no specific restrictions in it's text.
 

milwaukeesteve

Senior Member
Location
Milwaukee, WI
Thank you for chiming in.

What your arguement states, and very well if I may add, is my problem with this arguement.

Again, I have not said that exception 2 doesn't allow this. It is rather, a question of should it. The exeception doesn't specifically state whether you can or cannot do this, rather just allows it because it just addresses the simultaneous disconnection.

Should it be allowed. I have a problem with the protecting of the circuit, and the safety side of this. I have stated that this is my opinion. I am not judging this rule based solely on my opinion. Then what do you do if you have a substantial 120V load, do you increase the size of one of the breakers? There is something that is wrong with that (opinion).

I feel that accepting this as something we can do can be dangerous.
The code rule doesn't tell me I can't leave my screwdriver laying across the phase bars in my switchgear, but I don't.

Again, I feel that 120V loads are addressed in the rule itself and that jumping to the assumption that exception 2 allows you to introduce 120V loads again, just because it doesn't say so is a leap.
 

DesertRat

Member
milwaukeesteve said:
Thank you for chiming in.

What your arguement states, and very well if I may add, is my problem with this arguement.

Again, I have not said that exception 2 doesn't allow this. It is rather, a question of should it. The exeception doesn't specifically state whether you can or cannot do this, rather just allows it because it just addresses the simultaneous disconnection.

In practice, our concern should be what code does or does not allow. To deal with what code should or should not allow we delve into the world of making proposals to change that code. Certainly an acceptible option.

milwaukeesteve said:
Should it be allowed. I have a problem with the protecting of the circuit, and the safety side of this. I have stated that this is my opinion. I am not judging this rule based solely on my opinion. Then what do you do if you have a substantial 120V load, do you increase the size of one of the breakers? There is something that is wrong with that (opinion).

If you have a substantial 120V load, substantial enough to require a larger breaker, then you have a bit of a design issue on your hands. Though, your ability to upsize the breaker will depend on if you can meet the criteria of 210.20(c) (or 210.21 if your 120V load happens to an "outlet device") I agree, things could get a bit dicey here. But at some level, we have to count on ourselves (and the engineers who design the systems we install) to provide good design. IMO code cannot be expected to cover all possible contingencies of the human mind. It is not a design manual, nor is it a babysitter.



milwaukeesteve said:
Again, I feel that 120V loads are addressed in the rule itself and that jumping to the assumption that exception 2 allows you to introduce 120V loads again, just because it doesn't say so is a leap.

I fail to see where there is any leap of logic here. If we meet the criteria of exception 2, the text of 210.4 no longer applies. You can't use that text to read anything of a requirement once you've met exception 2, as exception 2 "excepts" 210.4 in and of itself. In this case, you have to step away from the rules laid forth in the NEC and apply to them, the rules of English usage (as is essentially true of all written and verbal instruction.)

Try this. If I tell you:
"You are required to stand on your head and sing"
"Exception: if you are an electrician"

Will you be standing on your head? Will you be singing?
 

milwaukeesteve

Senior Member
Location
Milwaukee, WI
Why did it take so long to jump in on this discussion?
Again, well done.

I see yours, and George's, and Charlie's and all else that was part of this, point in all of this. I didn't mean to disagree, just to disagree. I didn't mean to disagree either based on my opinion.

As I stated in the beginning of this thread. This was one of the issues that frustrated me last time. I conceded last time that I wasn't correct. Again, I will state that I am not correct.

Thank you.
 

jim dungar

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I quess it is my turn to stop being silent.

How can you have a MWBC, under exception #2, without 120V single pole loads?

If Line to Neutral loads can be handled only under the base rule, 210.4(C), then why have any exceptions?

Your concern seems to be with the protection of a very large 120V load. 1) My assertion is that general branch circuit OCPDs are not intended to protect loads, but rather only the circuit conductors.
2) If you assume that general branch OCPDs are intended to always protect the load then how do you address an 11A vacuum cleaner plugged into a duplex receptacle on #12 wire protected by a 20A breaker?
 

DesertRat

Member
milwaukeesteve said:
Why did it take so long to jump in on this discussion?
Again, well done.


Mostly I was just reading along. Besides, this is a code forum, so I let discussions of code, as it were, take precidence. My point was linguistic in nature. Not that that effects relevancy, but it doesn't lead one to dig through the code book or look into materials and opinions in support of or opposed to an argument. The beauty of this forum, and of the disagreements and debates that occur here, is that we can all learn a great deal from them. Code debates make for a good 6 or 7 page thread, semantic arguments do not :) So I refrained, and followed along.
 

BarryO

Senior Member
Location
Bend, OR
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Electrical engineer (retired)
jim dungar said:
2) If you assume that general branch OCPDs are intended to always protect the load then how do you address an 11A vacuum cleaner plugged into a duplex receptacle on #12 wire protected by a 20A breaker?

240-5(B)(1)

In other words, since the vacuum is listed, in its supplied configuration with a 15A plug, it is considered safe to be protected by the branch circuit overprotection device of any circuit that supplies a 15A receptacle (a 15A or 20A branch). That's what UL tests it to.

This is one reason why it is not permitted to install a 15A receptacle on, say, a 50A branch circuit.
 

George Stolz

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DesertRat, welcome to the forum. :)
DesertRat said:
The beauty of this forum, and of the disagreements and debates that occur here, is that we can all learn a great deal from them.
Agreed wholeheartedly. I have never thought about this section as much as I did, when thinking about this thread. :)

DesertRat said:
Code debates make for a good 6 or 7 page thread, semantic arguments do not
What is a code discussion besides a debate of semantics? :D

milwaukeesteve said:
I conceded last time that I wasn't correct. Again, I will state that I am not correct.
You think you're getting off that light? Now, that we've got you on a downhill run, are going to exhaust you by trying to get you comfortable with the idea. :D

My opinion is, the option is there, but I daresay I may never make use of it.

In closing, let me say that I hope this discussion doesn't send you on another hiatus. This was a very civilized, good discussion, IMO. You're an asset to the forum.
 

jim dungar

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BarryO,

You missed my point entirely> Given the same ampacity breaker, if the appliance is protected on a line-neutral only circuit what makes it unsafe now that it is on a MWBC along with a line-line load?
 

BarryO

Senior Member
Location
Bend, OR
Occupation
Electrical engineer (retired)
Jim,

My bad; I have to admit I haven't followed every single post on this thread.

if the appliance is protected on a line-neutral only circuit what makes it unsafe now that it is on a MWBC along with a line-line load?
The case that Exception 2 deals with is if the breaker on the appliance's line conductor trips, and the appliance (and all the line-to-neutral outlets on that conductor), ends up getting fed from the other line conductor through the line-to-line load. A line with a tripped breaker is still energized, and the 120V and 240V loads both have potentially damaging, unknown voltage potentials across them. Thus the Exception 2 requirement that an OCPD trip both poles simultaneously; i.e, common trip not just handle ties. I believe that's the origin for the Exception 2 requirement(?)

One thing I'd say about Steve's line of argument (somewhat philosophical, somewhat symantic) is that taking a step back and reading 90.5, and maybe even the NEC Style Manual, might be beneficial.
 

George Stolz

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BarryO said:
The case that Exception 2 deals with is if the breaker on the appliance's line conductor trips, and the appliance (and all the line-to-neutral outlets on that conductor), ends up getting fed from the other line conductor through the line-to-line load.
I don't see how that can happen. A line-to-line circuit would be broken, and the line-neutral circuit would either be energized from it's supply or disconnected from it's supply.

How could power bootleg onto the other phase?
 
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