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05-12-2008, 09:53 AM   #1
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## MWBC neutral

You know I've been reading past posts regarding MWBC as I'm considering saving on some wire for new general use circuits to my basement. It's important for me to have a full understanding of why I'm doing what I'm supposed to be doing. So I get the concept of a balanced neutral that only caries the difference between the two circuits as long as they are on separate phases. The part I'm still having trouble with is how do we create an overload situation if the neutral is interrupted? It would seem in my mind that if the neutral were interrupted that both circuits would just stop functioning as in a regular single circuit. Could anyone please take the time to explain what I'm missing here?
Oh and Joba, if you're reading this and your self esteem needs a boost, go ahead and take your best shot, I can handle it.

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05-12-2008, 10:25 AM   #2
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When 2 circuits on opposite phases share a neutral, the loads are baiscally 2 120v loads in series across 220v, with the neutral in the center of the circuit "draining" the imbalance between the 2 loads. IOW - 5 100 watt lamps on one circuit, and 5 100 watt lamps on the other will result in a perfect balance on each circuit, with the neutral carrying zero current.

If the load on circuit A is 500 watts, and circuit b was 50 watts, then the neutral would carry 450 watts. Without a neutral, nothing on circuit a could work unless there was something on circuit b.

05-12-2008, 12:32 PM   #3
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Quote:
 Originally Posted by handifoot The part I'm still having trouble with is how do we create an overload situation if the neutral is interrupted? It would seem in my mind that if the neutral were interrupted that both circuits would just stop functioning as in a regular single circuit.
Hire a licensed electrician...Whoops. You got me stuck on that Joba thing.

First of all, I appologize for the .zip attachment but I can't seem to get the .PDF small enough to post. Anyway, open it up and have a look.

The problem with the broken neutral in a MWBC is not that it creates an overload. When it happens, you effectively create a 2-wire 240V circuit across the loads connected to both once-individual circuits. The circuits would definitely "stop working" once the smoke cleared!

The first (unlabelled) figure illustrates the basic connection between the utility, your panel and a typical mwbc (grounding, CB's or other details are omitted). The second shows a normal, properly wired mwbc. The important thing here (and I didn't note it) is that the loads on either circuit are subject to 120V. Now things get good!

In the third example, I show the mwbc with the neutral opened. Notice that there is now 240V impressed across both loads. Also, note the voltage drops across each. 218.2V over the 1200W load and 21.8V over the 120W load. Well, let's say that the 1200W load was that brand new, \$5000 plasma tv you just bought. Get the idea!

In the final figure, I drew it up again with the broken neutral but with both loads being equivalent. Notice that even with the neutral disconnected, things are normal, i.e. 120V across both. Looks nice on paper and proves the math but it's one in a million in practice that you'd get so lucky.

Hope this helps. Again, sorry I had to bury the file in a .zip.

Jimmy
Attached Files
 MWBC with Broken Neutral.zip (70.6 KB, 264 views)
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05-12-2008, 06:16 PM   #4
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Quote:
 how do we create an overload situation if the neutral is interrupted
It's not an overload situation, it is a high voltage situation.

Draw you circuits on paper including a light bulb and trace the path. It will become crystal clear when you erase the neutral on the way back to the panel.

I'd draw it for you but I'm too lazy

05-12-2008, 08:54 PM   #5
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Quote:
 Originally Posted by 220/221 Jimmy made his too complicated
Holy crap, Lois! 220 is really my wife!
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