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Old 08-10-2009, 09:45 PM   #1
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two 20amp circuits on 12/3 wire


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?

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Old 08-10-2009, 09:59 PM   #2
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two 20amp circuits on 12/3 wire


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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Old 08-11-2009, 01:04 AM   #3
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two 20amp circuits on 12/3 wire


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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Old 08-11-2009, 02:09 AM   #4
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two 20amp circuits on 12/3 wire


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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Old 08-11-2009, 03:01 PM   #5
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two 20amp circuits on 12/3 wire


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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Old 08-11-2009, 03:06 PM   #6
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two 20amp circuits on 12/3 wire


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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Old 08-11-2009, 04:13 PM   #7
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two 20amp circuits on 12/3 wire


Quote:
Originally Posted by Scuba_Dave View Post
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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Old 08-11-2009, 05:50 PM   #8
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two 20amp circuits on 12/3 wire


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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Old 08-11-2009, 07:29 PM   #9
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two 20amp circuits on 12/3 wire


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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Old 08-12-2009, 12:59 PM   #10
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two 20amp circuits on 12/3 wire


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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Old 08-12-2009, 01:12 PM   #11
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two 20amp circuits on 12/3 wire


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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Old 08-12-2009, 01:24 PM   #12
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two 20amp circuits on 12/3 wire


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.

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Old 08-12-2009, 05:59 PM   #13
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two 20amp circuits on 12/3 wire


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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Old 08-13-2009, 12:39 PM   #14
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two 20amp circuits on 12/3 wire


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Originally Posted by Red Squirrel View Post
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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Old 08-14-2009, 01:24 PM   #15
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two 20amp circuits on 12/3 wire


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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