Multiwire Branch Circuits

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Old 04-12-05, 05:22 PM
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Multiwire Branch Circuits

I installed what I believe to be a multiwire circuit but not sure if I did it correctly. I have 2 hots with a common neutral to a split receptacle (tab removed on the hot side). I used two breakers (one above the other) for the hots so each is on a different leg.

1. Is this OK or do I need to change anything?
2. Can the hots be on the same phase leg for mulitwire circuits?
2. Reading NEC 210.B I don't know what "yoke" means and it continues with "...a means shall be provided to disconnect simultaneously all ungrounded conductors...". Does this mean I should use a breaker where the operators are connected mechanically to disconnect both circuits? I don't know the right term...maybe it's double pole breaker? If so that would feed the hots from different phase legs wouldn't it?
 
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Old 04-12-05, 05:30 PM
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1) The setup is not quite correct, but you have already identified the issue.
2) You _must_ feed the hots of a multiwire circuit from _different_ phases. When the hots are from different phases, the current from one phase will tend to balance the current from the other phase, and the current in the common wire will be _less_ than the current in the hot wires. When the hots are fed from the same phase, the current in the common wire is the _sum_ of those in the hot wires.
3) With a multiwire circuit feeding a split receptacle as you describe, you _must_ use a double pole breaker, with the two operating handles tied together, and with an 'internal common trip' arrangement. This breaker will correctly keep the two hots on different phases.

-Jon
 
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Old 04-12-05, 05:34 PM
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Yes, a multi-wire circuit MUST take power from different legs of the service. This way the neutral only carries the imbalance of the two circuits.
If you were to pull power from the same legs the neutral would potentially carry double the current (both circuits cumulatively).

Yes again, if you have a multi-wire circuit terminating on one device yoke (a duplex receptacle is two receptacle outlets on one yoke) you need to use a two pole breaker, or two singles with a handle tie.
 
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Old 04-13-05, 06:34 AM
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If there is more than one device on this circuit, the neutral wire needs to be pigtailed at each receptacle. This way, if there is a failure of a device or if the device is disconnected, the rest of the circuit still has a neutral path.

Steve
 
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Old 04-13-05, 09:56 AM
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Sorry mr fixit eh, but I don't understand your pigtailing the neutral comment.

This multiwire circuit powers a split duplex receptacle with hot tab removed. One hot to the top receptacle and the other to the bottom receptacle...the neutral tab is not removed.

One hot and the neutral continue to an another (unsplit) receptacle by pigtailing the hot and connecting the neutral to the second neutral screw on the first receptacle.
 
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Old 04-13-05, 10:02 AM
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Fred,

Your setup is wrong. You MUST pigtail the neutral connection to the split duplex receptacle.

This is done so that if a problem develops you don't end up with 240 volts at the downstream devices.

Now in your case you can't end up with 240 at the downstream device, but I do believe that by code the duplex must still be pigtailed.
 
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Old 04-13-05, 10:19 AM
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MrFixit eh's comment is directed toward the common situation of having _multiple_ devices connected to a multiwire branch circuit. In this case, you are _required_ to pigtail the neutral rather than continuing the neutral through the device.

For 'normal' branch circuits, you are permitted to use the multiple termination points on a common receptacle both as the connection to the receptacle _and_ as the splice to the downstream conductors feeding the next device in the circuit. However in a multiwire branch circuit, the continuity of the neutral is not permitted to depend upon the terminals of a device. In a multiwire circuit, you _must_ make a pigtail between the wire feeding the device, the device, and the wire going to downstream devices.

I disagree with Racraft. I believe that with a _single_ device on a multiwire circuit does not require a neutral pigtail; just what would you pigtail to? There are no downstream devices that require a conductor to pigtail onto.

-Jon
 
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Old 04-13-05, 10:49 AM
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Jon,

REad fred's post. He has a duplex as the first receptacle, using both host and the neutral. Then he runs one hot and the neutral to a second duplex receptacle. At the shared neutral receptacle he has the neutrals connected directly to the receptacle instead of pigtailed.
 
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Old 04-13-05, 11:20 AM
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NEC 2002 Article 300.13...

Mechanical and Electrical Continuity - Electrical Conductors.

(B) Device Removal. In multiwire branch circuits, the continuity of a grounded conductor shall not depend on device connections such as lampholders, receptacles, and so forth, where the removal of such devices would interrupt the continuity.
 
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Old 04-13-05, 12:05 PM
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Whoops. Racraft is correct. I only saw the mention of the single split receptacle (see the _first_ post on this thread). With _two_ receptacles, you _must_ pigtail the neutrals.

-Jon
 
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Old 04-13-05, 12:23 PM
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OK guys...you all are great....i'm off to fix it...thank you one and all!
 
 

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