Three wire Feeds from Main Panel

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Old 09-23-08, 03:28 PM
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Three wire Feeds from Main Panel

I opened my 200 amp Cutler Hammer panel for the first time. It's a new house for us. There are 40 spaces for breaker slots. There are 10 240 Vac feeds for pumps, dryer, stove, etc. The others are all 20 amp breakers. The odd thing is that each 2 breakers feed a three wire, black, red and white cable with the white and ground tied to the neutral bar. I understand that they use each hot to ground in the cable but what i don't understand is how they treat the circuits at the load end. Say the black feeds the washing machine outlet and the red feeds the laundry room light circuit. How do they get the white/neutral to aach circuit? I'm installing a 13 KW transfer switch. I have it mounted and the conduits installed. Moving the 2 ton A/C condenser and air handler is easy. A couple of the breakers, like #5 and #7, located on top of each other in the panel will move over to the transfer switch. I assume I need to pay attention to the black/red combos as I move them to be sure I get the 2 phases feeding the cables correct. Do I have this correct? Up north we always used one dedicated cable per circuit. I want to get this right. It seems like a lot to save a couple of conductors.
Mark
 
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Old 09-23-08, 03:51 PM
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mark sounds like you have it correct. just remember that on a multi wire branch circuit the black must be on one phase and the red on the other. so if you put a meter on the red and black wires you would get 220v. this type of of branch circuit wiring is allowed by code. although i do not agree with it people do it. the thing to remember is try and keep the load balanced. in a home this is not as critical as say a office building where it would be alot easier to have a unbalanced load as for your netural question somewhere in some box maybe a light or a plug the circuit gets split up (the black one way and the red the other way) and that is were the netural is spliced for both
 
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Old 09-23-08, 03:59 PM
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What you have are called multi-wire branch circuits. They are legal by the code, but they do introduce some complications like you have discovered when when it comes to breaker placement. The black and the red must be powered from separate poles of the service. The easiest way to guarantee this is to use a 20A double-pole breaker instead of two single-pole 20A breakers. However, two single-pole breaker on top of each other (such as #5+#7 or #8+#10) is okay too.

It also helps to take a single zip tie and bundle the black and red wires together so that it's obvious to you and to future workers that these circuits must be together.

Another thing to be careful about is that if you are working on one of these circuits, like replacing receptacles or light fixtures, both breakers must be switched off to protect the worker from shock.

I do not really like the widespread use of MWBC because of the added complications over individual branch circuits. They are great for specific applications, but not for general-purpose circuits like you have in your case.
 
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Old 09-23-08, 04:17 PM
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Thanks for the addional info. I do have one problem. There is a three wire feed to the refrigerator and the dinning room receptacles. I want to move the refrigerator to the critcal circuit new panel but I do not want to move the dinning room receptacle circuit to the critical panel. Can I selectively mix a different phase circuit along with the refrigerator or do I have to move the pair? I would pick a circuit on the same phase as the dinning room receptacles and pair it with the refrigerator. I am referring to phase as the 240 Vac bi-phase voltage.
 
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