Multiwire Branch circuits

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dmanda24

dmanda24

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210.4 (A) FPN warns about non-linear loads, I am wiring some offices and I wonder if there is a calculation or a rule of thumb when wiring MWBC since I plan to run MWBC for the fluorescent lights in the offices and for the outlets.
The fluorescent lights are non-linear loads and the outlets are in offices so more than likely computer printers and a lot of electronics that are also non-linear loads, I want to make sure I can wire all the lights using MWBC and all the outlets so that the inspector don't make me rewire the whole thing.
All your help will be appreciated it
 
Use MWBC's and don't worry about it.

Roger
 
Almost never.

The only real problem I've encountered with non-linear loads was on a transformer, not conductors associated with a MWBC. This was a lot of accumulative non-linear loading on a small transformer and it was predicted that it would occur.

The solution was a K rated larger transformer.

Roger
 
You probably will want to consider the neutral to be a current-carrying conductor, but you're unlikely to need to super-size it.
 
I was told it was just a 'good design' on these non linear loads to have that neutral upsized, or just run seperate neutrals. This helps with power quality issues. It may costs more, but i would consider seperate neutrals with the computers/electronic equipement in the equation, the lights i wouldnt worry about.
 
i think the companies that sell wire are the ones most concerned with non- linear loads. they want you to upsize everything.
 
Just my opinion.

1. Run the MWBC and be done with it.
2. Up size for VD as needed.
3. Lighting circuits will most likely, (I would assume) be continuous duty so excessive loading is not a concern.
4. 208/120 branch circuits are seldom loaded to max and also not a concern. But even if they are it is not an issue.


There are numerous post on MH regarding this subject, and based upon my unscientific data collection method. MOST (not all) of the posters seem to agree with this to some extent one way or the other.

If I have misrepresented anyone feel free to slam my head in the door.
 
In my experience I only saw problems with the mwbranch ckts in the early 90s when electronic ballasts first started coming out and I think the electronics caused a lot of harmonics . Since then the ballast mfgs have been getting better at building ballasts and it has all but been eliminated from what I can see. At the time I remember properly wired mwbcs for lighting feeds that should have been running cool extremely hot and uncomfortable to the touch in 4 wire MC cable.
 
quogueelectric said:
You should be fine for branch circuits I think 2008 code requires they be on 3 pole breakers now for 3 ckts.
Click to expand...
When we adopt the 2008, I plan on using single pole breakers and handle ties to avoid nuisance tripping. Other than that, I don't think I will do anything any different with MWBC.
 
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