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

[roger]

Then your MWBC is no longer a Branch circuit but a Feeder.

That was not a very good hypothetical answer.

Exactly, until we can see something that could be served with MWBC we can't have good answer.


Your branch circuit would have to meet the requirements of 430.42 or 430.52 for the motor protection, so unless the filter and motor were a factory assembly, it would be hard to serve.

Of course in any event I could always use a breaker similar to the one below if I had different loads.

Roger

 

[milwaukeesteve]

http://www.mikeholt.com/code_forum/showthread.php?t=59403&highlight=dual+voltage

Above is the link to a thread that we did a year ago. In it we are all saying the same thing as now. Including others like Bob/Iwire and Jim Dungar who haven't been in on this one. It is all the same thing that we are arguing this time.

I just don't see how we can make an assumption when something is not stated.
I don't use the handbook, I use the Code book.
What I see in your arguements are not arguements, but stating what others are stating (interpretations) and using them as absolutes.

What the Code says is not what you are saying in your replies. What the handbook is saying doesn't mean anything to me because that is not the Code Book.


In that picture that you show there, I see that the 100A handle is part of a 100 A 2 pole breaker (notice the words common trip). The 35 Amp breaker is a single pole breaker that is handletied to it. There is nothing wrong with this if the situation calls for it. But this is not a picture of a 120/240V MWBC.


IF you don't like my Well Pump example, imagine it being a 240V fixed electric heat, and some other 120V load.

 

[allenwayne]

Steve not sure how many years in trade you have,I assume alot.I have learned over the years that all electricians think at some point that they are the only ones right and the best there is out there.There are many points in the code that I feel need addressed and changed.That they are allowing a substandard install.IMO.When I get in that sitation I go for the utmost install that makes me comfortable.Like a MWBC I use an internal trip 2 pole breaker and not a handle tie.A handle tie is allowed but I prefer that both ungrounded conductors on the MWBC be opened together and trip together.

Years ago when I was a baby electrician I caught the neutral load from flourescent lighting on a MWBC and when I got off the floor after falling off the ladder I decided that a 2 pole breaker was needed.

 

[jsharvey]

Thank Goodness

Thank Goodness

Not the #8 alarm clock!!!

Well at least it wasn't a #3 alarm clock, that would be disasterous

 

[jsharvey]

Say What?

Say What?

What the Code says is not what you are saying in your replies. What the handbook is saying doesn't mean anything to me because that is not the Code Book.

Ummm Excuse me ?? I'm sitting here with a 2005 NFPA 70 NEC handbook published BY NFPA. And you're saying that isn't the code? please explain since the handbook is what I used to take and pass my journeymans test a few years ago?

J.S.Harvey

 

[don_resqcapt19]

I'm sitting here with a 2005 NFPA 70 NEC handbook published BY NFPA. And you're saying that isn't the code?

The handbook is not the code and the opinions expressed there in are not any more valid then those posted here. The following statement will be found in the handbook.

The commentary and supplementary materials in this handbook are not a part of the Code and do not constitute Formal Interpretations of the NFPA (which can be obtained only through requests processed by the responsible technical committees in accordance with the published procedures of the NFPA). The commentary and supplementary materials, therefore, solely reflect the personal opinions of the editor or other contributors and do not necessarily represent the official position of the NFPA or its technical committees.

Don

 

[jim dungar]

The NEC Handbook contains the NEC, commentary and supplementary materials. Just be careful that you are citing the NEC sections (in black/white) and not the other material (in blue/green).

The handbook contains the complete text of NFPA70...

 

[peter d]

Why do I get the feeling of deja vu?

FWIW, I agree with Roger 100%.

 

[jsharvey]

Thanks

Thanks

The NEC Handbook contains the NEC, commentary and supplementary materials. Just be careful that you are citing the NEC sections (in black/white) and not the other material (in blue/green).

Thanks Jim and Dan, I was worried for a second that I had missed something somewhere. Knew the Code was in the text and about the "other" parts. I was just thrown for a minute by the "handbook isnt code" statement.

J.S. Harvey

 

[LarryFine]

I'm confused as to why there is confusion here. When I think about a MWBC supplying L-to-L and L-to-N loads, I look at it as having to meet the requirements of ANY circuit with L-to-L loads first. Thus, the internal-common-trip requirement; a handle-tie's purpose is simultaneous manual operation.

I hope it's understood that an internal-common-trip breaker will trip both lines from a line-to-neutral (or -ground) fault even if the handle tie is removed. Drill the long rivets from a 2-pole breaker some time: there is a piece of material that couples the two trip mechanisms; typically a small metal rectangle.

Second, there is a difference in the code when it states "Common Trip" and "a Means shall be provided".
Common trip is very specific in what it needs: a multipole breaker with and internal common trip.
A Means shall be provided is also specific that you should provide a means for simultaneous opening, and that a Internal Common Trip is NOT allowed, because it is not "provided". How can something be "provided" if it would already exist?

Isn't this a bit of a stretch? The way I see it, using an internal-common-trip breaker IS one method of "providing a means" of simultaneous manual operation, as is an approved handle-tie. You really take "provided" to mean that it must be field-installed and not part of an assembly?

My point is that a MWBC that supplies L-to-L and L-to-N loads needs to meet the more stringent requirement(s) when there is a difference between the rules for each type of load. L-to-L requires an internal-common-trip; the presence of the L-to-N load doesn't alter that.

As for shared-neutral accidental openings causing voltage imbalancing, it's certainly a problem, but that's not a good reason to outlaw the method. There are many other wiring schemes that depend on integrity of the wiring system. But like other topics, if you don't like the practice, don't do it.

Of course in any event I could always use a breaker similar to the one below if I had different loads.

Roger

Sure, as long as the common-internal-trip mechanism is intact.

 

[roger]

Sure, as long as the common-internal-trip mechanism is intact.

It is, this breaker is from the factory.

Roger

 

[milwaukeesteve]

I am sorry for beating a dead horse with this topic.


My point with all this is this:
I follow the Code as much as I can. Sure I might miss a code rule here or there, especially when an installation hits several different code rules. What I find is that the Code tells me what I can and cannot do.
Sometimes there are items such as this topic, that we are not told that we can or cannot do it. In this example, nothing in the code tells us that we can use MWBC to feed both 240V and 120V loads on the same circuit. Then again, nothing is telling us we cannot do it either. Somewhere between Exception 1 and Exception 2 we have this void that doesn't address combined loads. You can't possible tell me that the wording in those exceptions tells me to wire this way. You may argue that it is ALLOWED, but I will argue that it is NOT allowed.

So where should things fall?
Some argue that if the NEC does not tell me I can't, than I can.
Some argue that if the NEC does not tell me I can, then I can't.
I know most installers will lean towards the first, and most inspectors will lean toward the second. That is the nature of the beast.

As for the Handbook and Mike Holt's graphics and wordings. These are interpretations. Yes, the Handbook is made by the NFPA people. Yes, Mike is incredibly knowledgeable in the NEC. But these are not Gospel, but rather interpretations. Don and Jim are correct with their statements about the Handbooks. They may think I am all wet on this issue, but they are correct on Codebook vs Handbook.

I would love to hear your thoughts, all of you, including Journeyman, Master, Inspector and Engineer, on my question of where should things fall.

I would like to see Mike's justification for wording his Exception 2 like he did in his graphic. That wording is not the wording in the NEC, and would have needed some kind of supporting code rules or something else to make that assumption.

 

[roger]

Steve, we all know the commentary in the Handbook is just that and nothing else. If you will search the forums you will see where others as well as myself have posted the same excerpt Don posted.

We also agree that Mike's material is not code, but his opinion and interpretation.

Now with all that said, many of us here (most I think) agree with both commentaries and opinions of the above parties concerning this issue.

If it is really such a problem in your opinion, you should work on a proposal for the 2011 cycle.

Then you will either have it accepted, accepted in principal, or rejected with the CMP's explanation of why.

Now I'm going to let this poor dead horse rest in peace.

Roger

 

[DaveTap]

Ok... how about a range or dryer or other UL approved equipment that has 240v heaters or loads AND internal 120v timer or lights? Do you consider it dangerous if conected using the approved 4 wire cord? Doesn't this constitute a MWB circuit?

 

[milwaukeesteve]

That is a UL listed, manufactured item. The dual voltage is in the load, not the wiring method, or the branch circuit.
That is not a 'dual voltage' MWBC.

 

[George Stolz]

I just read through this thread. It is an interesting read, I haven't seen this discussed in this way, that I can recall. Anyway:

The range actually is a perfect example of this, though, Steve. A small 120V clock rides in the wake of a 120/240V 50A MWBC. UL blessing or not, it's still a little unsettling, isn't it?

I keep it simple, probably as you do. I'd never leap to such a weird configuration naturally. But I can't think of a good reason to frown upon it in general without some good specifics.

I've seen washer outlets pulled off dryer circuits. In that case, we have a 210.21(B)(3) violation. We have a 15A receptacle on a 30A circuit (the conductors notwithstanding). That's easy to shoot down.

If we have a 1000W 240V baseboard heater on a 15A two-pole breaker, and a receptacle for a 120V 20W Radon Mitigation Pump with it, is there a danger? I think it's a needlessly confusing way to get power to a pump, but I don't think there is any danger to speak of. Is there?

I don't know.

 

[iwire]

If we have a 1000W 240V baseboard heater on a 15A two-pole breaker, and a receptacle for a 120V 20W Radon Mitigation Pump with it, is there a danger? I think it's a needlessly confusing way to get power to a pump, but I don't think there is any danger to speak of. Is there?

No additional danger at all except it blows Steve's mind.

 

[George Stolz]

I'm looking at three screens to follow part of this conversation, so I figure putting it all together in one post may help others on this one. It's as much so I can get it all together as I compile it, as it is to help anybody else, actually.

NEC-2005 210.4(C) Line-to-Neutral Loads. Multiwire branch circuits shall supply only line-to-neutral loads.
Exception No. 1: A multiwire branch circuit that supplies only one utilization equipment.
Exception No. 2: Where all ungrounded conductors of the multiwire branch circuit are opened simultaneously by the branch-circuit overcurrent device.

Mike Holt's Graphic text:
A multiwire circuit can supply both line-to-line and line-to-neutral loads providing that a common trip circuit breaker is used to protect the circuit.

Mike Holt commentary:
Ex 2: A multiwire branch circuit is permitted to supply both line-to-line and line-to-neutral loads if the circuit protection device opens all ungrounded conductors of the multiwire branch circuit simultaneously under a fault condition (multipole circuit breaker with common internal trip). Figure 210-1

I would like to see Mike's justification for wording his Exception 2 like he did in his graphic. That wording is not the wording in the NEC, and would have needed some kind of supporting code rules or something else to make that assumption.

Now that I've got them all lined up, I can see how Mike came to his interpretation, I think. The rule is, "A MWBC shall supply only line-to-neutral loads."

If we stop reading there, then we have now officially lost permission to use a MWBC on line-to-line loads. My question would be, "What use is a MWBC to a line-to-line load anyway? The neutral is not required to make it work."

Clarification comes by reading Exception 1. Now, we are made aware that a single utilization equipment (such as a range or dryer) were considered line-to-line loads using MWBCs. If it's one appliance, it's got a "Get Out Of Jail Free Card", and this is the exception we're using to escape the rule.

If exception 1 applies to one appliance - better put, if this exception applies to all single appliances, then the only sense exception 2 would make is if more than one appliance existed, wanting to use a MWBC. The group would not be able to use exception 1. They would need an exception as a group, or break apart into single appliances to be able to use exception 1.

Which is how I come to the conclusion that Mike Holt concludes that this exception is pertaining to a group of loads. Sorry for the long long post, I was figuring it out (I think) as I went.

 

[George Stolz]

No additional danger at all except it blows Steve's mind.

Nothing wrong with that - it blew mine too!

 

[George Stolz]

Steve, you give me so much to work with!

So where should things fall?
Some argue that if the NEC does not tell me I can't, than I can.
Some argue that if the NEC does not tell me I can, then I can't.
I know most installers will lean towards the first, and most inspectors will lean toward the second. That is the nature of the beast.
...
I would love to hear your thoughts, all of you, including Journeyman, Master, Inspector and Engineer, on my question of where should things fall.

I just squeaked into this poll, I just passed by journeyman test!

I am in the "green" camp. Essentially, the book would have to be twice the size to tell me how to do my job, every little permission I have. I view permissive statements in the code to be my guarantees. If "such-and-such shall be permitted..." then I am guaranteed to pass if I do such-and-such. That, in my mind, does not restrict me to such-and-such, but I have a guarantee if I go that route.

However, some on the CMPs are in the "red" camp. They make a leap that if the NEC provides permission for an act, then the opposite of the act is naturally not permitted. Don has submitted several Green-friendly proposals that were rejected by Red Campers.

And I would agree, there are some Red Camper inspectors out there. My frustration with them is easily washed away by the presence of Red Campers on the CMPs. If people sitting on the panels continue to follow Red Camp philosophy, then they are a much bigger issue than we in the field, inspecting and installing.

JMO,