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Old 08-14-2009, 02:33 PM   #16
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two 20amp circuits on 12/3 wire

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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Old 08-14-2009, 06:05 PM   #17
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two 20amp circuits on 12/3 wire

Originally Posted by theatretch85 View Post
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.

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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Old 08-14-2009, 06:45 PM   #18
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two 20amp circuits on 12/3 wire

Originally Posted by Red Squirrel View Post
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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Old 08-14-2009, 08:02 PM   #19
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two 20amp circuits on 12/3 wire

Originally Posted by Red Squirrel View Post
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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