# two 20amp circuits on 12/3 wire



## Scuba_Dave (Jan 16, 2009)

12-3 & a 240v 20a breaker
one hot from the 240v breaker for each circuit
Neutral is shared & by using the 240v breaker the power across the neutral effectively cancels out - it does not add


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## Stubbie (Jan 7, 2007)

Yep here is a drawing from Mike Holt...be sure to use a double pole breaker for this so that opposing legs are used for the hot wires.


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## frenchelectrican (Apr 12, 2006)

Follow Stubbie's diagram carefully and it is very clear to wire it up in correct way and if you set it up right per picture like take recepale A on circuit A { black conductor } then take receptale B on circuit B { red conductor } take receptale C on circuit A keep repeating the pattern and it will really be balanced correct.

Oh yeah one very cirictial item I am not sure if Stubbie mention or not but here it is anyway with all netural conductors make sure you make a pigtail the reason why I say that in case you do something it will not loosen up the netural conductor if that happend anything on 120 volts will get hit hard especaily with electronic loads they don't take much to get wacked once you get in 200 + volt zone. that why I mention in first place to pay attetion when you hook them up first time.

If your local providce do required AFCI or GFCI that can really put a damper on your plans so check it ahead of time to verify what they required and yes there is two pole AFCI or GFCI breaker on market but the cost you will not like it so just think about it ahead of the time.

Merci,Marc


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## Red Squirrel (Jun 29, 2009)

Oh so I actually use a double? Figured I'd use two single breakers still. Good to know. This also gives me the option to add a 240v plug on the PDU if I want, though I'd probably want to use another circuit for that anyway.


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## Scuba_Dave (Jan 16, 2009)

You used to be able to use 2 single breakers
They did away with that under NEC 2008 to make things safer
Too many breakers being installed incorrectly or moved after the fact
Then the power across the neutral adds -if both are then pulling power from the same leg


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## Michael Thomas (Jan 27, 2008)

*No multiwire circuits on tamdam (duplex) breakers.*



Scuba_Dave said:


> You used to be able to use 2 single breakers
> They did away with that under NEC 2008 to make things safer
> Too many breakers being installed incorrectly or moved after the fact
> Then the power across the neutral adds -if both are then pulling power from the same leg


Also, for the same reason , you can't use a "tandem" (AKA "duplex", two breakers in one breaker position) circut breaker:









for multiwire shared neutral circuits as both the ungrounded ("hot") conductors will be on the same phase, and the load on the ungrounded ("neutral") conductor will be additive, not subtractive.

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I'm paid to be suspicious...


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## Red Squirrel (Jun 29, 2009)

Just curious how does the additive and subtractive thing work? Just wondering about the technicalities of it. Not the first time I hear this.


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## nap (Dec 4, 2007)

I'll let one of the other guys explain theory to you. They seem to be able to explain it simpler than I do.

but, especially for something such as you are using these circuits for, I would run 2 separate circuits/ 2 separate neutrals. Electronics tend to cause harmonics and that can actually increase the current on the neutral. It can also cause interference with the other associated circuit, which electronics do not like.

Most industrial installations will not run MWBCs to any electronic (computer) equipment.


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## theatretch85 (May 17, 2008)

I am running three dedicated circuits to my server room. One 30amp circuit for a twistlock, one 20 amp circuit for a 20 amp twistlock, and a second 20 amp circuit for a quad outlet box with standard duplex outlets. Each circuit has its own hot and neutral, and in addition to the conduit being a ground all the way back to the panel, I also ran a 10 gauge ground that I will pigtail for the 3 circuits in addition to wiring to the outlet box as well. I don't have any intentions on connecting any 3 of these circuits to anything other than standard breakers (non-afci/gfci). I've already had issues with the GFCI outlet the rack was plugged into temporarily while the server room was being built and power being connected. I'd find the GFCI outlet tripped on occasion, usually at random and sometimes it'd work for days without tripping.

Btw, I know stubbie has some of those wonderful diagrams that explains the MWBC quite well. Basically, if you use 10 amps on circuit A and 5 amps on circuit B in a MWBC, the neutral only sees the imbalance of the two loads, so in this case 5 amps.


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## Stubbie (Jan 7, 2007)

I just love it when I get the chance to post one of my diagrams.....

I really think all you need to know is what theatretech explained but this diagram very basically depicts what is going on.....

Thing to remember is if you don't use a double pole breaker and try to position two single poles they must be on opposite legs of the panel. Looking at the drawing if both breakers are on leg A they both carry a positive (+) amperage and 15 amps will be on the neutral not 5 amps or the unbalanced load. New code requirements require double pole breakers so this mistake cannot be made.


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## Michael Thomas (Jan 27, 2008)

In a shared neutral system the grounded ("neutral") conductor carries the imbalance between the loads on the ungrounded ("hot") conductors.

Assuming a single phase circuit with a 15 Amp load on one ungrounded conductor and a 20 Amp load on the other:

If the loads are on opposite phases (a correctly wired multiwire circuit), as in Stubbie's very nice diagram above, you _subtract_ the lower load from the higher to obtain the imbalance i.e. 20A - 15 A = 5A.

However if the loads on on the _same phase_ (for example the ungrounded conductors originate from a duplex breaker in a single breaker position, or from two seperate breakers on the same phase of the panelboard bus), you _add_ the loads to obtain the current on the ungrounded conductor, i.e. 20A + 15A = 35 A (!).

Single phase panelboards are set up so that any two adjacent breaker positions on the same side of the panelboard are on opposite phases, thus originating a multiwire circuit from a standard two pole breaker (which occupies two adjacent breaker positions on the same side of the board) guarantees that the ungrounded conductors will be on opposite phases, and the imbalance will be subtractive.

-----------------------------------------------------------------------
I'm paid to be suspicious...


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## Red Squirrel (Jun 29, 2009)

Ah this makes more sense, I was forgetting of how the sine wave went. The first 120v wire will be + the other will be negative, which is why you get 240 volts between them. So the amps are also negative on the 2nd wire? (of course this alternates, but I'm just assuming we are "Freezing" at that point )


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## theatretch85 (May 17, 2008)

Red Squirrel said:


> Ah this makes more sense, I was forgetting of how the sine wave went. The first 120v wire will be + the other will be negative, which is why you get 240 volts between them. So the amps are also negative on the 2nd wire? (of course this alternates, but I'm just assuming we are "Freezing" at that point )


The "polarity" changes direction 60 times a second (hence the 60hz). You won't meter anything different from the hot wire of the "A leg" to the hot wire of the "B leg". That is, the two should show exactly identical to one another on a meter (within a couple volts of each other). You won't be able to see a positive/negative voltage between the two legs, it will always be positive (you can even switch the meter leads around and it still won't matter). Amps will never be "negative" current is current. 

In terms of calculating the imbalance, you take which ever wire has the largest load connected and subtract the load of the second wire from the first to determine the neutral current. You could have a larger load connected to the second circuit and a smaller load connected to the first, or no load at all on one wire and anything up to the circuit maximum on the other wire.

With a MWBC the current simply flows through one device, past the neutral connection and through the other device, down the other leg effectively creating a 240 volt circuit. The imbalance is carried on the neutral wire; this is why that neutral connection at the split must be bullet proof. If that connection were to be broken, you can cause severe damage to the connected loads. 

I believe stubbie has another wonderful diagram to illustrate the broken neutral on a MWBC and the effects it has on the connected loads.


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## tns1 (Jun 26, 2009)

So as far as the NEC is concerned, is a dual breaker using a 10/3 with shared neutral to feed separate 120v receptacles considered two circuits or one circuit? In other words does this satisfy the requirement for two 20A circuits for kitchen receptacles?


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## Scuba_Dave (Jan 16, 2009)

Kitchen circuits are frequently wired as a MWBC
It does satisfy the NEC requirement
Why 10-3 - distance ?

There _may_ be a different way to wire once you have GFCI at the countertop
Can't recall the specifics


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## Red Squirrel (Jun 29, 2009)

theatretch85 said:


> The "polarity" changes direction 60 times a second (hence the 60hz). You won't meter anything different from the hot wire of the "A leg" to the hot wire of the "B leg". That is, the two should show exactly identical to one another on a meter (within a couple volts of each other). You won't be able to see a positive/negative voltage between the two legs, it will always be positive (you can even switch the meter leads around and it still won't matter). Amps will never be "negative" current is current.
> 
> In terms of calculating the imbalance, you take which ever wire has the largest load connected and subtract the load of the second wire from the first to determine the neutral current. You could have a larger load connected to the second circuit and a smaller load connected to the first, or no load at all on one wire and anything up to the circuit maximum on the other wire.
> 
> ...



Actually if it was possible to "freeze" at a certain point, wouldn't I read negative voltage on one side? (except for the two hots) Like both sides always being equal but one is negative? As AC is like DC but alternates 60 times per second?

As for broken neutral, I think I see what could happen...

The two light circuits on this diagram would be running in series with 240 volts right? So voltage would be split to both circuits. Probably 120 on each side anyway, or would it be totally wacky depending on what devices are there, and if one is removed then the other turns off, etc. Like those old style Christmas light sets where each bulb is 3 volts but there are enough in series so it plugs into 120. 

So bottom line is, I'm guessing it's better practice to not use this method to create two circuits then, unless it's a special circuit like an oven, correct? Seems like it could become a troubleshooting nightmare in complex situations too.

Another thought, I am sure a neutral before entering the house being broken could be a disaster if the panel is not properly grounded, would this be the case?


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## mpoulton (Jul 23, 2009)

Red Squirrel said:


> Another thought, I am sure a neutral before entering the house being broken could be a disaster if the panel is not properly grounded, would this be the case?


It is usually disastrous, yes. Grounding does not help. Grounding is not bonding - the impedance of the grounding electrode system is far too high to act as a return path and equalize the voltage on the two legs, even with an excellent grounding system. Often, however, the metal water piping is continuously connected through to another building. If so, and if it's properly bonded at both ends, the water pipe will act as the neutral and nothing bad will happen... until a hapless plumber changes the water meter!


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## Gator Jim (Aug 10, 2009)

Red Squirrel said:


> Is this to code, to put 2 circuits on a 12/3 wire, or should the wire guage match the total amperage since it's sharing the neutral? (ex: 40)
> 
> I am designing a PDU for a custom server rack and what I intend on doing is have a bunch of 20 amp sockets and the top plug will be one circuit and the bottom sockets will be another circuit (split plugs), so the easiest and most practical way I can think of is to just run a 3 conductor cable through each one or should I run two seperate 12/2s?
> 
> Reason I ask is lets say both circuits are actually using 20 amps, will that put too much load on the single neutral conductor? Or is that not really an issue as it's being separated by the two hots?


 I just noted your post and while you have gotten excellent advice I though this might help too. 

By definition a neutral is a conductor that carries an unbalanced load. Another definition provided by the National Electrical Code (NEC) is the white conductor is defined as a “grounded” conductor. 

In a typical circuit with a black wire (ungrounded conductor/hot wire) and a white wire (grounded conductor) there is no potential for an unbalanced load. Therefore the white wire does not meet the definition of a neutral conductor. In such a circuit the white conductor carries the same current as the black conductor. Remember that by definition a white conductor must carry an unbalanced load. (As noted in the drawing provided by Scuba Dave). 

If a load of 15 amps exist on the red conductor and a load of 10 amps exist on the black conductor there will be a 5 amp load on the neutral. If there is a 15 amp load on a circuit with only a black and a white conductor both conductors will carry the 15 amp load. In this example there is no conductor to “share the load” with so in this case the white conductor would be correctly referred to as a grounded conductor but not a neutral. For most home projects this point is gibberish. It only becomes important when doing a load calculation for the service entrance conductors for a building or a feeder supplying a sub panel. The NEC permits reductions in the size of neutrals for these conductors but a grounded conductor is never permitted a reduction for the reason you noted in your post (overloading of the conductor).


Simply stated not all grounded conductors are neutrals but neutral conductors are grounded conductors. Probably means nothing to your project but it is something you can use to dazzle the bosses wife at the next Christmas party.


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## pastor99 (Feb 11, 2015)

I'm attempting to add two 120v 20a circuits to my shop. I've run 12/3 wg, connected ground and neutral to neutral bar in box, connected ground and neutral to receptacle, connected red and black to the two screws in receptacle, removed the tab on the hot side of the receptacle, connected red and black in the circuit breaker box, each to a separate 20a breaker, installed the breakers on opposite legs in the box.
When both breakers are on, I get full voltage in both top and bottom of receptacle. When I turn one breaker off, I get full voltage in top and half voltage in bottom.
When I turn the other breaker off, I get full voltage in bottom and 1/4 voltage in top.
I'm stumped.


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## Jim Port (Sep 21, 2007)

You need either handle tied single pole breaker or a two pole breaker. Both hots need to turn off at the same time for safety.


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## AllanJ (Nov 24, 2007)

Your meter is sensitive enough to measure "phantom voltage" which is likely to occur in the long run from panel to outbuilding shop.

If you plugged in and turned on an incandescent light, does it work correctly on both legs (making two tests)?

Repeat your measurements with two incandescent lights plugged in and turned on, one on each of the circuits. The lights will dissipate any phantom voltage also.


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## Jump-start (Sep 26, 2012)

Old thread alert.


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## pastor99 (Feb 11, 2015)

Thanks for your input.
What is an "old thread alert?"


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## mm11 (Apr 30, 2013)

Are the circuits originating from the service or from a subpanel? If they are from a subpanel, the equipment grounding conductor has to be isolated from the neutral.


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## pastor99 (Feb 11, 2015)

They are originating in the service panel.


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## pastor99 (Feb 11, 2015)

I've replaced the breakers with a two pole breaker. Thanks for your help.


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## Desertdrifter (Dec 10, 2009)

pastor99 said:


> Thanks for your input.
> What is an "old thread alert?"


You posted into a thread from '09
Best to start your own new thread for your particular question.


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## pastor99 (Feb 11, 2015)

Thank you. Sorry.


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## Desertdrifter (Dec 10, 2009)

pastor99 said:


> Thank you. Sorry.


No real biggie. Just easier to keep things separate.


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## AllanJ (Nov 24, 2007)

For those eavesdropping on old replies:

Not the technically correct wording, but the kitchen counters need to have available at them two allotments of 20 amps that are not subject to usurpation by lights or fans or by usage in most other rooms.

One multiwire branch circuit with 20 amp breakers (and 12 gauge wire) satisfies this requirement.

The same wiring method, three 12 gauge circuit conductors, (equipment grounding conductor,) 20 amp breaker set, and all, may be used as a 120/240 volt circuit for some appliances such as certain air conditioners.

We could not substitute one 40 amp allotment (via 8 gauge circuit) for the two 20 amp allotments because it is not permitted to connect ordinary lights and appliances used on 15 or 20 amp circuits to a 40 amp circuit (the lights, fans, appliances. etc. that are rated for use on circuits with overcurrent protection not exceeding 20 amps).


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