# 12/3 Romex to Feed 2 Circuits?



## JohnJ0906 (Mar 18, 2007)

DaveHembree said:


> 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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## DaveHembree (Apr 23, 2007)

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!
Dave


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## jwhite (Mar 12, 2006)

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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## troubleseeker (Sep 25, 2006)

JohnJ0906 said:


> Yes, a very common practice.
> 2 very important Musts
> 
> 1) At the 1st outlet, you MUST pigtail all the whites
> ...


 
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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## MinConst (Nov 23, 2004)

Yes I also learned somthing on the opposite "phases". I have never done this but it a good lesson. Thanks.


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## NateHanson (Apr 15, 2007)

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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## J187 (May 19, 2006)

NateHanson said:


> 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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## sootybuttercup (Apr 17, 2006)

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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## ouchmythumb (May 6, 2007)

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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## sootybuttercup (Apr 17, 2006)

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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## fw2007 (Jul 11, 2007)

sootybuttercup said:


> 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.

FW


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## sootybuttercup (Apr 17, 2006)

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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## Speedy Petey (Feb 1, 2004)

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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## Andy in ATL (Aug 28, 2007)

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.:yes: 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.:huh: 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!:yes: :laughing:


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## Stubbie (Jan 7, 2007)

Hi Andy

I see you work with a lot of three phase . One thing, if our not already aware, is for your 4 wire wye connected loads that are non-linear....for instance many plants have lighting panelboards that supply ballasted lighting loads. Be very careful doing multiwires out of these as there is a good chance of harmonics overloading a normally sized shared neutral, especially shared with all 3 phases. It's a very good idea to run a neutral for each phase in these situations or up size your shared neutral to prevent the "triplen" effect from the lighting loads. In specific your panels supplied by 4 wire wye 227/480 and 120/208 are especially susceptible to these non-linear load harmonics. So if not up to speed take a look at this it is a growing problem and a lot of commercial building fires have been attributed to the heating of overloaded neutrals in these situations.

Stubbie


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## frenchelectrican (Apr 12, 2006)

Once a while i will get the service call from the commercal / industrail loaction when anytime i heard someone smoked something i say  they [unqalifed mantanice guys ] mess around with it that true with MWBC it dont matter which votlage ya run on it once you " lift " the netural ya are screwed.

one place have MWBC for lumiaire circuits this place the MWBC i rather say pretty hevey loaded large number of 1 KW HPS lumniaires [ each were wired for 277v ] and somehow one idiot did cut the wrong neutral at the lighting panel jezz what a firework show. next thing they called me to come in and try to repairs all the mess i just say tons of french  [ i cant even type french cussing words in here also lol ] total lumniaires were 30 of them on one MWBC and ya know what the cost of 1 KW HPS high bay were all the ballast just smoked in 15 seconds flat and left part of the building in dark.

anyway before i get out of the hand in here let get back to the subject what stubbie just say few second ago about the harmomic issuse this will show up not only in electronic ballasted lumiaires but also with other electronic equmient i know one place have computers and they did really a wonderfull work on the netural wires that place have 600 amp service the netural wire just burnt the heck out of it and have to oversized it and paralled it to slove the issuse there

sorry for long thread there 

Merci , Marc


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## Stubbie (Jan 7, 2007)

Marc......Did you ever forget to let the bulb cool before you unscrewed it....let me tell ya they are very freakin hot......

Stubbie


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## frenchelectrican (Apr 12, 2006)

Subbie,, yeah i was well aware with the HPS and MH bulbs are freaking hot but tell ya one instering part i did used hevey welding gloves and i unscrew the 400 hps and it was still lit for about 40 seconds after i took it out of the base.

the new guy was working with me he went like this >>  :confused1: 

but one thing Stubbie. Never fool around with MH bulbs when still hot i have few expoled on me. i have scar to prove it from that freaking bulb blew up on me [ that happend right after i open the hevey glass door on wall pack next thing i heard crack then i was pushed back a sec. i knew what it did a good size drop of water from the gutter drop down and make a nice target at the hot bulb and exploled ]

i think we better get back on track here other wise why not start the seprated thread so it dont confused with the OP and other wondering why we are at this >>fftopic: 

Merci, Marc


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## fw2007 (Jul 11, 2007)

I won't disagree on the MWBC issue. I feel better using a separate line for each breaker.

As for the #14 VS #12 issue, I don't know where I read or heard this. I will have to check the local code.
It definitely makes sense to use a 20A breaker for #12 wire. Makes the job much easier if you can get away with fewer circuits. Only issue would be that you need to make sure the whole circuit uses #12 (at least for receptacles).

The house I am currently living in (my parents home) is 80 years old, and has a lot of the old rubber/cloth BX, and possibly some knob & tube, but so far I haven't seen any of the latter.
We all know the house is in need of rewiring, but it is a job beyond my current capabilities. I'm OK with the wiring part, but having to get the wires to where I want them is something I would rather leave for someone else <g>

FW


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## jproffer (Mar 12, 2005)

> Only issue would be that you need to make sure the whole circuit uses #12 (at least for receptacles).


Cross out your qualifier at the end and you would be 100% correct.

If the breaker is 20A, you MUST use #12 conductor throughout the entire circuit**, lighting AND receptacles.

**There are exceptions to this, but none of them apply to "normal" lighting/receptacle circuits.


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## Andy in ATL (Aug 28, 2007)

Stubbie said:


> Hi Andy
> 
> I see you work with a lot of three phase . One thing, if our not already aware, is for your 4 wire wye connected loads that are non-linear....for instance many plants have lighting panelboards that supply ballasted lighting loads. Be very careful doing multiwires out of these as there is a good chance of harmonics overloading a normally sized shared neutral, especially shared with all 3 phases. It's a very good idea to run a neutral for each phase in these situations or up size your shared neutral to prevent the "triplen" effect from the lighting loads. In specific your panels supplied by 4 wire wye 227/480 and 120/208 are especially susceptible to these non-linear load harmonics. So if not up to speed take a look at this it is a growing problem and a lot of commercial building fires have been attributed to the heating of overloaded neutrals in these situations.
> 
> Stubbie


 
I hear ya stubbie!! Most of our work is speced out by lighting engineers with upgraded neutrals where triplen harmonics will be a problem. It took a while after coming out of roping houses for harmonics to click...but when the lite bulb came on it came on bright:laughing: :thumbup: pun intended...


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## fw2007 (Jul 11, 2007)

My local building inspector just replied to an e-mail I sent on the subject of wire size for branch circuits;
#14 is OK for 15A circuits.
I don't know where I got the erroneous info.

Perhaps I got into the practice of using #12 on 15A lines when we still had the Federal Pacific breakers. They were so lousy, I had tested some of the 15A and they would not trip at 19A even after 1 hour. AT 25A, the 15A breaker took about 20 minutes to trip out!
I had a guy from the power company tell me that those breakers were dangerous! Yet I still see FP around today. 
We have since replaced the panel with CH, which IMO are the best breakers on the market.

FW


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