12/3 or 12/2/2

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Old 04-20-06, 06:29 AM
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12/3 or 12/2/2

Is it still code to use a single 12/3 to set up two shared neutral circuits, when they don't need GFCI and when a double pole breaker is used? I've got four longish runs (80'), and I have the 12/3 cable aready. Or must I pull separate 12/2 cables or 12/2/2?
 
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Old 04-20-06, 06:54 AM
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Thumbs up multiwire circuit

Originally Posted by BuffaloDIY
Is it still code to use a single 12/3 to set up two shared neutral circuits
Of course, probably always will be. See NEC 210.4.

> when they don't need GFCI and
If GFCI is required, use a DP OCPD.

> when a double pole breaker is used?
Especially so. In most cases, two SPs will do. DPs are pretty much always safer for DIY.
A DP is required if both circuits are used on the same yoke, e.g., a split duplex receptacle.


> I've got four longish runs (80'), and I have the 12/3 cable aready.
Use it.

> Or must I pull separate 12/2 cables or 12/2/2?
If this is typical residential loads, there is no reason at all.
 
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Old 04-20-06, 07:14 AM
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Thanks. I saw the 12/2/2 at the store, and it got me wondering if things had changed. Just out of curiousity, when is 12/2/2 required?
 
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Old 04-20-06, 07:43 AM
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If you have a circuit loaded up with "nonlinear loads", such as PCs and UPSes or three-phase variable frequency drives, you are at risk for serious harmonic distortion such that cancellation does not occur on the neutral - instead, neutral current rises as the loads increase.
This article discusses 200% neutrals and separate neutrals and related issues.
 
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Old 04-20-06, 07:56 AM
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If you pull two runs of 12-2 or one run of 12-2-2 instead of a run of 12-3 then you no longer have a shared neutral.

12-2-2 is used when you have four current carrying loads that are part of the same circuit and you need them to be together in the same cable.

Here are two examples:

A combination light, vent and night light bathroom ceiling fixture where power goes first to the switch and you want to control the three items with separate switches. You run the neutral and three switched hots from the switches to the fixture using the 12-2-2.

A three way switch setup where power comes into the first three way switch and you want to have unswitched power available at the other three way switch. You run the neutral, two travelers and one hot between the three way switches using 12-2-2.

In both of my examples you must re-identify one of the white wires in the 12-2-2 as a hot using some approved method.
 
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Old 04-20-06, 08:11 AM
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interesting. thanks for the reference.
 
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Old 04-20-06, 08:28 AM
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Originally Posted by bolide
> when a double pole breaker is used?
Especially so. In most cases, two SPs will do. DPs are pretty much always safer for DIY.
Do the two SPs need to be right next to each other? They definitely have to be on separate phases, but does an inspector want to see them right next to each other?
 
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Old 04-20-06, 08:44 AM
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There is no requirement that the two breakers need to be right next to each other. However, it's usually easier if they are, since the wires come from the same cable or conduit.

Regardless of whether a double pole breaker is used or two single pole breakers are used, next to each other or not, it should be noted on the reference chart that the circuit breakers feed a multi wire circuit.

You want it to be as easy as possible for you or someone else to note that the circuit is multi wire, so that one or both of the single pole breakers are not accidentally repositioned to create a fire hazard.
 
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Old 04-20-06, 09:56 AM
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interesting comment about the DP on the same receptacle.

I'm going to run 12/3 to a duplex to be used for dishwasher and garbage disposal. Two circuits with a shared neutral.

So in this case, I must use a DP breaker instead of two SP's?

Not a big deal, just want to make sure I get it right. Thanks !
 
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Old 04-20-06, 10:00 AM
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Originally Posted by fuente
I'm going to run 12/3 to a duplex to be used for dishwasher and garbage disposal. Two circuits with a shared neutral.

So in this case, I must use a DP breaker instead of two SP's?
Yes, in this case you MUST use a double pole circuit breaker. The intent of this is to not have it possible to have half the duplex energized, while the other half is not energized.
 
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Old 04-20-06, 10:01 AM
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great thanks !!
 
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Old 04-20-06, 02:44 PM
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> Do the two SPs need to be right next to each other?
I always do. Anything else risks a dangerous error (overloaded neutral).


> does an inspector want to see them right next to each other?
I wouldn't do it any other way.
I put a little nylon tie around the red and black near the breaker and snip the tail off.

If some wants to move the breakers further apart, he'll have to remove the tie. Plus it makes it obvious which red and black go together. (I also tie black and white together this way on a bedroom circuit so that they don't easily end up on a regular breaker.)
 
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Old 04-20-06, 05:28 PM
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The 'historic' reason for 12-2-2 cable was to provide two circuits in the same cable where AFCI protection was required.

AFCI breakers include a ground fault detection component. This is not a proper GFCI because the 'pick up' level is about 70mA rather than the 6mA mandated for a GFCI. This ground fault detection made it impossible to use AFCI breakers in multi-wire circuits. 12-2-2 cable provides the wiring convenience of a multi-wire circuit (1 single cable with 2 circuits) however the circuits are electrically distinct.

Clearly, a proper 'two pole' AFCI breaker would permit the use of 12-3 cable to provide two circuits in a multi-wire fashion with a shared neutral. I believe that with the original introduction of the AFCI, only single pole devices were available.

12-2-2 cable is differentiated from 12-4 cable by the color coding. 12-2-2 cable has one black, one red, one white, and one white with red stripe (along with the ground conductor) 12-4 cable has one black, one red, one white, and some other 'hot' color (often blue) along with the ground conductor.

-Jon
 
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