2 separate breakers (both single pole) feeding a 3-wire?

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Old 03-07-09, 12:10 PM
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2 separate breakers (both single pole) feeding a 3-wire?

I opened a combo receptable/switch box and found something new to me. A three-wire (black, red, white) coming in and going off to two 2-wires. One 2-wire got black and white from the 3-wire. The other 2-wire got a red and white from the 3-wire.

When I tracked them back to the panel, they connected to 2 completely different breakers - one with a black wire and one with a red wire. Both breakers were 20 amp.

Is this okay? Would appreciate hearing comments. Thanks
 
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Old 03-07-09, 12:56 PM
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Yes, that's sounds correct. It's commonly referred to as a multi-wire branch circuit,(MWBC) whereas 2 circuits of different"phases" are sharing a common neutral.

This is ok as long as the 2 breakers are "on top of each other" so to speak, in that they are not on the same phase. This is not ok if they are on the same phase.

New codes required that MWBC's breakers be handle tied as in a double pole breaker. This would eliminate the possibilty of someone "becoming a neutral". If you turned one breaker off and the other was still on, and you disconnected the neutral and made the circuit then you would become the neutral and this is no fun

Hope this helps.
 

Last edited by wirenut1110; 03-07-09 at 01:47 PM.
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Old 03-07-09, 01:02 PM
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It is called a multi wire set up and is acceptable. The breaker should be a true 240v breaker. It is possible to use two 120v breakers if they are on different legs but there are possible safety issues involved. You need to make sure you have 240v between the black and red wire.
 

Last edited by ray2047; 03-07-09 at 03:03 PM.
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Old 03-07-09, 07:20 PM
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wirenut1110,
thanks for your quick answer. They are breakers 19 and 17, next to each other on the same side of the panel. But what does "on top of each other" mean? 19 is above 17 -- is that what you mean? And what are "phases"? Thanks again.
 
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Old 03-07-09, 07:31 PM
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Originally Posted by rambelmont View Post
They are breakers 19 and 17, next to each other on the same side of the panel. And what are "phases"?
That will give you what you need, but, to repeat, the breaker needs to be a 2-pole breaker or, if single pole, their handles must be tied together.

They are not really on separate phases, but on opposite busses of a 120-240V system.
Doug
 
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Old 03-07-09, 07:34 PM
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ray2047,
Both breakers are the usual typical breakers -- single pole. They say 120/240 on them, but what does that mean? If each is 120, do they add up to 240? Is that what you meant? And what does it mean to be "on different legs"? sorry, but I am not familiar with the terminology of electric panels. Thanks for your help in understanding this.
 
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Old 03-07-09, 07:58 PM
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gilmorrie,
Thanks for the suggestion of a handle tie -- I just did a google seach to understand what that means, and yes I absolutely want them tied together. I had no idea this was a real safety issue. I don't know why the original electrician did not tie the handles together, but I will take care of that soon.

I think I understand why they are on different phases - is it because they are next to each other, and phases alternate? So 19 and 15 will be on the same phase, and 17 and 13 will be on the same phase, but different from the phase for 19 and 15?

I don't know what "opposite busses" mean. could you explain more?

Again, thank you all for your help --
 
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Old 03-07-09, 08:33 PM
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I used the word phase to maybe help you better understand, that's why it was in parenthesis. Sorry, if it was confusing.

The original electrician didn't do it because it wasn't required until the 2008 code cycle unless it was for devices or equipment on the same yoke. 2008 made ALL MWBC's required to be handle tied/double pole breakers.
 
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Old 03-07-09, 08:37 PM
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Your home is supplied by a center tapped transformer. The transformer supplies 240v. That 240v comes into the house and is hooked to two buss bars in your panel. The breakers connect to those bus bars. Not really phase though we often say that. Really just the two sides of the 240v that supply your house.

From the center tap on the transformer to either of the 240v legs is 120v, That comes into the house as the third wire of your system, the neutral. Bottom line you need a neutral and both 240 legs on a multi wire circuit. The 120 on your circuit comes from one of the hot legs and the neutral. Why you need both hot legs is a bit more complicated. You want more the people here will explain. but basically the neutral could exceed it's amp rating.

As to why a 240v breaker there are two safety reasons. One if the two sides are not tied together when power is turned off or breaker tripped you could be left with a still hot lead and not know it. The second reason with the exception of GE and perhaps a couple of older panels you can't accidentally put both 240 wires on the same leg.
 

Last edited by pcboss; 03-08-09 at 02:02 PM. Reason: spelling
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Old 03-07-09, 08:53 PM
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rambelmont, the bottom line is that although what you see is strange to you, it is not strange. It is also not unsafe, not illegal and not a problem. However, should you do any electrical work on your house, it is critical that you first understand how this works. I recommend the simple and inexpensive green paperback, Wiring Simplified, from which I learned what a multiwire branch circuit was many, many years ago.
 
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Old 03-08-09, 12:32 PM
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Thank you all for your help and explanations. I will get the handle tie for those two breakers. From what I read, this is something a DIY-er can install without going deeper into the panel -- which is something I am not about to do.

thanks again -
rambelmont
 
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