Multiwire circuits

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Old 07-14-05, 04:21 AM
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Multiwire circuits

I know this has probably been asked a thousand times already, but I want to be sure I am clear on it before I proceed with my wiring. I hope it doesn't open up a can of worms in here.

I just finished reading a very long and informative thread from 2002 that discussed multiwire circuits, grounds, neutrals, grounded conductors, etc. While I am still a little foggy about some of it I believe I understand the gist of it.

Here goes...I have two 110V lines of different phase coming into my panel, along with a neutral. If I install a multiwire circuit, the hots must be on different phases in order to balance the load on the neutral, since the alternating pahses tend to cancel each other out. (hope I got that right)

So this means that if I use one of these mini twin breakers in my panel, the kind that plug in where a regular single breaker would, I should install only one of the hots to one of these breakers, and the other hot would need to be installed on a different breaker which would be connected to one of the lugs from the other phase, or line.

Am I missing anything?
 
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Old 07-14-05, 08:02 AM
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You are correct that in most panels, for these tandem (aka twin, aka skinny) breakers, both breakers are on the same leg of the power, and thus they can be used for neither 240-volt circuits, nor for multiwire circuits.

Although theoretically possible and usually code-compliant to make a multiwire circuit by using one breaker of one tandem, and one breaker of a different tandem, it is discouraged because of the potential of accidentally getting them on the same leg of the power, and the fact that you wouldn't then have a common trip lever (which is required only if both hots connect to one device, but is desirable even if not).

A better solution is a quad breaker, which is essentially two tandems in the same package. Get one where the two inside breakers and/or the two outside breakers have a common trip lever.

Unless you have a very specific reason for wanting a multiwire circuit (such as a long run where voltage drop is an issue), I would recommend avoiding them.
 
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Old 07-14-05, 10:35 AM
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If there is an existing 3-wire cable into the panel, and you are connecting a new 3-wire cable and there are no slots for 2 additional 1-pole breakers follow this simple procedure----

Remove the S-P breaker where the Black wire of the existing 3-wire cable terminates, and replace it with a "tandem" C-B. equipped with 2 terminals----Re-connect the "existing" Black and the "new" Black to the 2 C-B terminals-----Remove the S-P breaker where the Red wire of the SAME existing 3-wire cable terminates and procede as with the Black wires.

The ampacity of the "new" cable MUST be the same ampacity as the existing cable- ex; 15 amps new= 15 amps existing .

Good Luck & Enjoy the Experience!!!!!!!!!!!!!!
 
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Old 07-15-05, 12:34 AM
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I am new to the doityourself forums and would like to know how to access the 2002 thread you found on multiwire circuits.

Also, from my past (not so pleasant) experience I would definitely support the idea of going with a quad breaker that has the handles tied (inner and outer set). Also, when you make your connections to devices, you probably already know to pigtail from your neutral so that removing the device does not open the neutral connection to other devices downstream (which can also create unpleasant experiences for the unwary)

Good luck,

Gregor
 
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Old 07-15-05, 01:56 PM
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Thanks guys for the tips.

John, I am not planning on running any more multiwire circuits, especially after reading everything I have about them. I know a lot of people will argue that they are OK and safe, but I just don't believe I have enough knowledge about all the ins and outs to do it safely.

My question was prompted because I noticed in my panel where someone else had run a multiwire and connected the red and black to the same phase dual breaker. These circuits go to my kitchen. Now, I plan on relocating one of the circuits to another breaker on the alternate phase.

Gregor, the thread I was referencing can be found here .
 
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Old 07-18-05, 02:32 PM
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Thanks for the link to the info,


Gregor
 
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