Run a Neutral for each phase conductor?

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chris kennedy said:
I'm sure you are well aware I may be familiar with the definition of a MWBC.


Just pointing out what we often see in the field.
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Have seen that far too often. But usually is result of an inhouse electrician moveing breakers without thinking.

Nothing wrong with wanting seperate neutrals as long as they understand the price got much higher.
 
The project is for a city owned rec center. I bid a different rec center designed by the same engineer with the same note about a month ago and the "winning" bid was so low he left out much more then the dedicated neutrals. I put a call in to the engineer just to find out if its the multipole circuit breaker he's trying to get away from.
 
You can overload the neu. of a multiwire branch circuit if all the loads are non linear. That's why they make mc cable with a oversized neu.
 
roger said:
That is a good question seeing how he actually worded 210.4 in the spec.

It might be as simple as he simply doesn't understand the article section.

Can you submit a question for clarification before bid time?

Roger
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I agree! [And know I don't believe EE's are idiots---]
 
The engineer said that his reasoning was just to avoid installing multipole circuit breakers because he doesn't want to create the circumstance where three circuits would have to be turned off in order to feed one.

If engineers lean more towards the separate neutral installation will 12/3 and 12/4 MC become mostly obsolete? And I would think the next edition of the code will go further to require neutrals be identified as to which phase they are intended to travel with.
 
and I was that close............

and I was that close............

get_smart_shoe_phone.jpg


require neutrals be identified as to which phase they are intended to travel with.
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210.4(D) grouping :D
 
JLEECME said:
If engineers lean more towards the separate neutral installation will 12/3 and 12/4 MC become mostly obsolete?
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no because these cables will still be used to supply 4 and 5 wire receptacles, equipment supplied with 4 or 5 wires, 12/4 used for switch runs where you want or need a neutral in the box, etc. Use will shift to more 12/2 but I wouldnt use the word obsolete..
 
The handle tie rule on mwbc's is a huge slap in the face to qualified electricians. Bundle neut's w/ hots at the panel and move on.

Brent
 
The job of the Electrician is to do or die and not to reason why, thats the Engineer's job. As long as it fits inside the NEC guide lines do the job and ask the boss, Whats next boss?
 
Twoskinsoneman said:
I did not mean to imply that your post specifically was calling EEs idiots. It is just a general feeling from many posters in many threads. Sorry my post does seem to say you were saying that but it wasn't my intent.
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Its not just you that notices this. I am just a lurker here, but to say its pretty thick in a lot of threads is an understatement.
 
Harmonics, as most commercial lighting circuits employ electronic ballasts, and many power (receptacle) loads feed compouters. One common option is to oversize your MWBC neutrals to carry the additional harmonic loads. For the engineer, it's easier (but not cheaper) to spec individual neutrals, as they then are not responsible for telling you how much the neutral needs to be oversized for each MWBC.
 
Under the 2008 code rule, as an owner, I would be very very upset with my electrical engineer if he permitted multiwire branch circuits in my new building. There is no way turning off 3 circuits to work on one would be acceptable to me as an owner.
 
If this is a commercial project I could see where it might have some merit.

Lets say you are in an industrial facility and you need to add a 3-phase breaker to a panel that has three blank spaces, but they are not together. Instead of having to check and recheck that you have everything phased properly, you simply move breakers around so that you can install the 3-phase breaker.
 
amptech said:
Exactly. You can't over-load the properly sized neutral conductor of a mwbc.
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Well...by definition a properly sized anything is not going to be overloaded. But, a neutral with the same size as the ungrounded conductors can be over-loaded.
iwire said:
...I am just pointing out those mis-wired circuits are not MWBCs and that is virtually imposable to overload the neutral of a MWBC.
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"... are rarely overloaded" is the phrase you were looking for
zappy said:
You can overload the neu. of a multiwire branch circuit if all the loads are non linear. That's why they make mc cable with a oversized neu.
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True, but it seems to be a rare occurrence, at least in SqD/Schneider's opinion (and apparently the opinion of a NEC ad hoc committee):

http://www.schneider-electric.us/ta.../mechanical-design-applications/Harmonics.pdf

http://static.schneider-electric.us/docs/Power%20Management/Square%20D%20Power%20System%20Engineering/3000HO0824_PSEngSolutions.pdf
 
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