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Old 04-28-2007, 09:41 PM   #1
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12/3 Romex to Feed 2 Circuits?


I am wiring my home addition with the help of a licensed electrician. He has mentioned that in order to save on wire, I can use 12/3 Romex to feed 2 circuits - one circuit on black and one on red - sharing the white common. This sounds reasonable and I assume this is common practice.

Any comments - positive or negative?

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Old 04-28-2007, 10:33 PM   #2
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12/3 Romex to Feed 2 Circuits?


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Originally Posted by DaveHembree View Post
I am wiring my home addition with the help of a licensed electrician. He has mentioned that in order to save on wire, I can use 12/3 Romex to feed 2 circuits - one circuit on black and one on red - sharing the white common. This sounds reasonable and I assume this is common practice.

Any comments - positive or negative?
Yes, a very common practice.
2 very important Musts

1) At the 1st outlet, you MUST pigtail all the whites
2) At the panel, the black and red must land on opposite "phases" The easiest way to do this is to use a double pole breaker.

The reason for this is that to share the white, you need to use line 1 for one circuit, and line 2 for the other- so you have 240volts between red and black. This means the white only carries the imbalance between them. If black has 10 amps, and red has 6amps, white will only carry 4 amps. If you land both red and black on the same "phase", then the white would carry 16 amps. For safety, we pigtail the white at the first outlet, so it does not depend on the device for continuity. If it were to become loose, then you would have 240 volts across the 2 circuits.

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Old 04-28-2007, 11:26 PM   #3
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12/3 Romex to Feed 2 Circuits?


Thanks John - a few questons.

1) Pigtail all the whites - do you mean just connect all the whites in that electrical outlet? I think so, just checking.

2) You mention putting the red and black on different phases. What are phases? Does this mean opposite sides of the breaker box (i.e., one on left and one on right)??

I am sure my electrician will know what these mean - but just trying to get up to speed.

Thanks!
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Old 04-29-2007, 08:33 AM   #4
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12/3 Romex to Feed 2 Circuits?


1. pig tail means that you cannot rely on the recepticle or switch for this connection. They must be connected together with a wire nut and a pig tail wire run to the device. This is require at every box where both circuits are still present. Once the two circuits split and you only have on circuit in a box, you no longer need to do this.

2. This depends on the panel layout. Most new panels have the phases next to eachother so that two pole breakers can be easily installd. So they would look something like this:

1, A....................2, A
3, B....................3, B
4, A....................5, A
6, B....................7, B etc

The numbers being the breaker number and the letters the phase.
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Old 04-29-2007, 07:58 PM   #5
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12/3 Romex to Feed 2 Circuits?


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Originally Posted by JohnJ0906 View Post
Yes, a very common practice.
2 very important Musts

1) At the 1st outlet, you MUST pigtail all the whites
2) At the panel, the black and red must land on opposite "phases" The easiest way to do this is to use a double pole breaker.

The reason for this is that to share the white, you need to use line 1 for one circuit, and line 2 for the other- so you have 240volts between red and black. This means the white only carries the imbalance between them. If black has 10 amps, and red has 6amps, white will only carry 4 amps. If you land both red and black on the same "phase", then the white would carry 16 amps. For safety, we pigtail the white at the first outlet, so it does not depend on the device for continuity. If it were to become loose, then you would have 240 volts across the 2 circuits.

Thanks for some enlightenment. I'm a renovator (Carpenter by trade), not an electrician, and I know my electrician uses this technique frequenlyl, but I never realized the technicality of having to be on opposite phases.I also did not know that it was legal to use a double pole breaker to control individual 120v circuits. Thanks.
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Old 04-29-2007, 09:38 PM   #6
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12/3 Romex to Feed 2 Circuits?


Yes I also learned somthing on the opposite "phases". I have never done this but it a good lesson. Thanks.
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Old 04-30-2007, 06:48 AM   #7
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12/3 Romex to Feed 2 Circuits?


What's the official name for circuits like this? Is there a way the panel should be labeled to make clear that it isn't a 220V circuit, but rather 2 110v circuits?

Any disadvantages to this way of wiring circuits (aside from the complexity and importance of getting these details right)?
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Old 04-30-2007, 09:13 AM   #8
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12/3 Romex to Feed 2 Circuits?


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Originally Posted by NateHanson View Post
What's the official name for circuits like this? Is there a way the panel should be labeled to make clear that it isn't a 220V circuit, but rather 2 110v circuits?

Any disadvantages to this way of wiring circuits (aside from the complexity and importance of getting these details right)?
Is it a multi-wire branch circuit
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Old 05-02-2007, 07:18 PM   #9
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12/3 Romex to Feed 2 Circuits?


But why 12/3? The normal breaker size would be 15A and wire #14...so #12 wire wouldn't help with the saving money part...especially at today's prices.
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Old 05-05-2007, 11:28 PM   #10
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12/3 Romex to Feed 2 Circuits?


It might be a long run of wire, it might feed a bathroom and have to be 20A, the owner might know he needs 20A for some appliance(s) he has in mind.

The only downside I could see in this is that if one of the circuits goes to a bedroom it might be a pain to put it on an AFCI.
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Old 05-06-2007, 01:32 PM   #11
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12/3 Romex to Feed 2 Circuits?


ouch...so true...it could be any of those things. I was just questioning the use of #12 in case further savings could be realized...since that was the topic of the original post. As far as the AFCI...You can't use a single pole AFCI on a circuit with a shared neutral...so, as you mention, hopefully no bedrooms in the plan. I personally have never liked the 3-wire method of doing receptacle circuits. I have seen several instances where the neutral became disconnected resulting in all the devices plugged in to both circuits suddenly being connected in series across the full 240 volts. The "lighter load" devices like clocks, radios, calculators, light bulbs, etc are always the losers in this situation, as the heavier devices carry the current through with no damage to those heavier devices.
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Old 09-26-2007, 08:34 PM   #12
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12/3 Romex to Feed 2 Circuits?


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Originally Posted by sootybuttercup View Post
But why 12/3? The normal breaker size would be 15A and wire #14...so #12 wire wouldn't help with the saving money part...especially at today's prices.
Correct me if I am wrong, but I thought I read somewhere many years ago that #14 wire (Romex, BX) is to be used for dedicated lighting and other small fixed circuits only, not for circuits with receptacles.
Reason being that the next owner (or same owner who forgets) may add long length circuit or maybe even decide to change a breaker from 15 to 20.

I am not a licensed electrician, but I always use #12 for 15 or 20A circuits (of normal length).

Several years ago my parents had an addition put on their home, and the electrician used #14 and 15A breaker for some outlets upstairs. He did use #12 for the AC, which is on 20A breakers.
I was not happy when I learned of this, but I wasn't about to rip out the wiring.

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Old 09-26-2007, 09:23 PM   #13
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12/3 Romex to Feed 2 Circuits?


Where I'm from...#14 is "the norm" for lighting AND receptacle circuits in houses. #14 is to be used on circuits protected with a 15 amp fuse or breaker. #12 is good for 20 amp circuits. #12 is much harder to work with, more expensive, and you can't have as many #12 wires in a receptacle, lighting, or other box. Using #12 on a 15 amp circuit is overkill, as there is already plenty safety factor built in to the code ratings. Codes are concerned with mainly fire safety and personal safety. The only time you might want to go with a larger wire size is if the circuit is longer than 100 feet. Then "voltage drop" becomes an issue...but most houses don't have 100 foot branch circuits. Always check local codes, as there are variations all over the place. Hope this helps!
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Old 09-26-2007, 10:34 PM   #14
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12/3 Romex to Feed 2 Circuits?


What Sooty said.

I always shake my head when I hear(read) where someone uses #12 on a 15 amp breaker. Other than to fight VD there is NO reason.

I also see this as a DIYers wives tale.
Don't fool yourself into thinking you are doing a "better" job, are being "safer" or my favorite "future proofing".
The future is NOW. Put #12 on 20A breakers!
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Old 09-26-2007, 11:24 PM   #15
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12/3 Romex to Feed 2 Circuits?


When I was learning this trade the guy who taught me explained why he never used multi wire branch circuits (MWBC) in a residential setting this way; When the neutral is broken on one of the circuits while both circuits are still hot then the circuit is now wired in series and you have 240V on a circuit meant for 120V. This will cause bigtime equipment problems. If all the neutrals are pigtailed then this SHOULD never happen. That being said I assure you that somewhere in the U.S.A. today someone is staring at there computer wondering why all the smoke was let out of it. Stuff that was meant to be run at 120V then has 240V applied to it has all the smoke let out of it. It's impossible to get the smoke back in and you have to buy all new stuff. I run 1 circuit and one neutral and feel better about it. Am I right and everyone else wrong? Absolutely not.

In commercial work I run 277/480V, 120/208V and 120/240V three phase MWBC's (3 hots and 1 neutral) all the time with never a second thought....but you SHOULD only have qualified personnel working on these systems...


Thanks for my little diatribe on MWBC's. Now let the disagreeing begin!

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