MWBC 210.4 (B) and 210.4(C) ex2 - Test Prep book error or Code

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mikeames

mikeames

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Going through questions in Mikes exam prep book there is a question about MWBC. It asks.

Multiwire branch circuits shall______
a. supply only line-neutral loads
b. not be permitted in dwelling units
c. have their conductors originate from different panel boards
d. none of these


Its my opinion the answer is d. "none of these" but the book states that the answer is a " supply only line-neutral loads".
That is clearly stated in 210.4 (C) but then the exception 2 is valid if all ungrounded counductors are open at the same time. Ok I get that, but why is exception 2 even there if 210.4(B) mandates ungrounded conductors be simultanously opened anyway?

So going back to the question, you could have line to line and line to neutral loads. I think the book is incorrect, and I think the code essentially makes 210.4(C) meaningless since 210.4(B) will always invoke the exception. I know 240.15(B) speaks to single pole breakers with approved handle ties as oppose to commontrip but thats not specified in the 210.4(C) exception.

Am I wrong?
 
I believe that "a" is correct. MWBCs can supply multiple line to neutral loads and the multi-pole or handle tie breaker is required so that lifting the neutral from the MWBC does not result in a "lost' neutral on one leg of that MWBC.
 
That is a very good question. I just wish I had an equally good answer. I will offer what I believe to be a relevant observation, one that might open the door to finding the answer. Here is what I have observed:
  1. 210.4(B) speaks only of disconnecting the ungrounded conductors, whereas
  2. Exception 2 to 210.4(C) takes it one step further by bringing overcurrent protection into the discussion.
For my part, I can't envision constructing a MWBC for which only line-neutral loads are served and for which the disconnecting means at the start of the circuit does not have overcurrent protection. So I don't know why the two paragraphs have this difference.

Next guess anybody?
 
Charlie, I based my answer on the premise that a MWBC is defined as a circuit that has multiple hots sharing one neutral.
 
The language in 210.4(C) Exception 2 requires a common trip, not just a means of simultaneous disconnect as required by 210.4(B).
 
Beaches EE said:
Charlie, I based my answer on the premise that a MWBC is defined as a circuit that has multiple hots sharing one neutral.
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I do not disagree with this. And I do agree with your statement that "A" is the correct answer. But the OP is pointing out an apparent point of confusion between two code paragraphs. The topic of discussion here is why those two paragraphs both show up in the code.

Don has pointed out the same thing I observed. But I still don't know why the two paragraphs are there, and why they are different. I can't imagine a MWBC that originates at a disconnecting means that does not include overcurrent protection.
 
Charlie has my thought correct. I understand 210.4(B) does not speak to OCPD and only a disconnect means, but it then continues to specify "at the point where the branch circuit originates" which in my opinion would be the terminal of the breaker. You could not put two single pole breakers in and then go to a 2 pole switch next to the panel and satisfy 210.4(B). Is there a way you could satisfy 210.4(B) without OCPD? If there is a way then I guess 210.4(C) would have a purpose since it specifically refers to branch-circuit over-current device.
 
mikeames said:
Going through questions in Mikes exam prep book there is a question about MWBC. It asks.

Multiwire branch circuits shall______
a. supply only line-neutral loads
b. not be permitted in dwelling units
c. have their conductors originate from different panel boards
d. none of these


Its my opinion the answer is d. "none of these" but the book states that the answer is a " supply only line-neutral loads".
That is clearly stated in 210.4 (C) but then the exception 2 is valid if all ungrounded counductors are open at the same time. Ok I get that, but why is exception 2 even there if 210.4(B) mandates ungrounded conductors be simultanously opened anyway?

So going back to the question, you could have line to line and line to neutral loads. I think the book is incorrect, and I think the code essentially makes 210.4(C) meaningless since 210.4(B) will always invoke the exception. I know 240.15(B) speaks to single pole breakers with approved handle ties as oppose to commontrip but thats not specified in the 210.4(C) exception.

Am I wrong?
Click to expand...
I agree with you the answer is d. There is nothing in the Article 100 definition of a MWBC that says that it can only supply line to neutral loads. The trip/disconnect issue is something beyond the scope of the question.

Branch Circuit, Multiwire. A branch circuit that consists of two or more ungrounded conductors that have a voltage between
them, and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and
that is connected to the neutral or grounded conductor of the system.
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The way I see it is 210.4(B) would be talking of strictly line to neutral loads and only require handle ties, 210.4(C) is moving to circuits like dryers and ranges.

Roger
 
The simultaneous disconnect requirement of 210.4(B) is to make sure that power is removed from all the ungrounded conductors of the MWBC when it's serviced.

210.4(C) and its Exception 2 is to prevent a line-to-line load from applying the voltage from a line that remains hot to another line that has an open OCPD. Line-to-neutral loads can't couple from one line to another and so they don't have this issue.
 
infinity said:
I agree with you the answer is d. There is nothing in the Article 100 definition of a MWBC that says that it can only supply line to neutral loads. The trip/disconnect issue is something beyond the scope of the question.
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210.4

C) Line-to-Neutral Loads. Multiwire branch circuits shall
supply only line-to-neutral loads.
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Dennis Alwon said:
210.4
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For the question in the OP I would argue that Exception #1 negates the main rule since a MWBC can serve one utilization equipment and still meet the definition of a MWBC. I think that you've mentioned the section that Mike was referencing with the question.

Exception No. 1: A multiwire branch circuit that supplies only one utilization equipment.
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infinity said:
I think that you've mentioned the section that Mike was referencing with the question.
Click to expand...

Yes, that is why I would have answered (a). I remember what Smart$ used to say -- for an exam don't even look at the exceptions...it's the general rule that they are usually looking for. I tend to agree with that.
 
To me this question as written is flawed as it does not indicate whether you should include the exceptions. In licensing exams these types of questions lead to exam challenges. It should have indicated whether you are to consider Exceptions.
 
Dennis Alwon said:
Yes, that is why I would have answered (a). I remember what Smart$ used to say -- for an exam don't even look at the exceptions...it's the general rule that they are usually looking for. I tend to agree with that.
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Same here. I was always taught that if exceptions were to be used in testing the testing instructions or specific question would note it

Roger
 
texie said:
To me this question as written is flawed as it does not indicate whether you should include the exceptions. In licensing exams these types of questions lead to exam challenges. It should have indicated whether you are to consider Exceptions.
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And in this case depending on whether you need to consider the exceptions there is an answer in the selections that is correct one way and another answer that is correct the other way.
 
This points out (at least) two things:

1. They should try to absorb more exceptions into the articles.

2. They should change the rule into a definition:

If only line-to-neutrals are served, it is a multi-wire branch circuit; otherwise, it's just a branch circuit.

The whole thing is silly to me; an entire service feeds mixed load types with no issues; why can't a MWBC?
 
LarryFine said:
This points out (at least) two things:

1. They should try to absorb more exceptions into the articles.

2. They should change the rule into a definition:

If only line-to-neutrals are served, it is a multi-wire branch circuit; otherwise, it's just a branch circuit.

The whole thing is silly to me; an entire service feeds mixed load types with no issues; why can't a MWBC?
Click to expand...
My guess, with handle tie and mixture of 120 and 240 volt loads on the circuit, if you blow one side the other side still can backfeed it via the 240 volt load. But same can happen with a blown fuse if that is your overcurrent protection type.
 
What if you had to feed 2 of these devices on a MWBC. Common trip breaker. If you answer "A" then this would not be allowed.
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