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01112008, 03:07 PM  #1 
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Question about sharing nuetral (as in 12/3) wire
Hearing the other post, I have to ask this question about sharing the neutral.
Let's say we run 12/3 wire from two 20A breakers to power seperate circuits in a workshop, sharing the neutral. You then run a 16 amp load on one of the circuits, and a 16 amp load on the other. How would that affect the neutral? I don't think that would trip a breaker, I know the black & red wouldn't be strained, but would it put strain on the shared neutral? AdvertisementLast edited by Piedmont; 01112008 at 03:16 PM. 
01112008, 03:15 PM  #2 
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This is called a MultiWire Branch Circuit (MWBC) if I remember what the gurus here have said in the past.
The two hots, red and black, must be connected to a common trip 2pole breaker which connects to opposite legs in the panel. That way the loads oppose eachother on the two hots. The neutral will carry no return current if both legs are drawing 16 amps. If one leg carries no current and the other carries 16 amps, the neutral will carry 16 amps. So no problem. Neutral must be pigtailed in every box of a MWBC so that neutral can't be interupted by removing a receptacle. Advertisement 
01112008, 03:29 PM  #3 
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Now that is clever. Great job on explaining it, and especially that last sentence little detail.
Last edited by Piedmont; 01112008 at 03:33 PM. 
01112008, 07:50 PM  #4 
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The physics behind how this works is hard to diagram so I put together these diagrams as a simplification of whats going on with a multiwire circuit and shared neutral. The sign waves look like this and as you can see the magnitude of voltage or current waves would equal zero at the neutral assuming equal values with respect to the neutral. In other words if I had a magnitude of +5 amps on leg A with respect to a 5 amps on leg B I would have zero amps at the neutral and in terms of voltage if my peak was +120 volts on leg A I would have an equal value of 120 volts on leg B and zero volts as they cross the neutral reference. The legs are shown with respect to the utility transformer but the connections to the double pole breaker in the main panel would be the equivalent result. So hopefully this doesn't confuse but helps in understanding. Note in the bottom diagram there is a 180 degree phase shift between legs this sets up the physics for the cancellation of current in the neutral.
Last edited by Stubbie; 01112008 at 11:02 PM. 
01112008, 10:06 PM  #5 
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Another way of saying things, using the same diagram above Stubbie has provided.
It's labeled 10 amps on top. So at one moment 10 amps are going clockwise in the top rectangle, out the top of the transformer (Leg A), through the top right load, and back via the middle (neutral). At the same moment five amps are going clockwise, out the neutral, through the bottom load, and back to the bottom of the transformer at the left (Leg B). The nature of electricity is such that when we talk of 10 amps going one way and 5 amps going the other way through the same wire (the neutral here), the net flow is 5 amps. I didn't forget we're talking about alternating current. (correction made One one hundred twentieth of a second later, all the current flows are reversed, going counterclockwise in each of the upper and lower rectangular loops. Or we could say it yet another way: Ten amps come out of the top of the transformer (Leg A), through the top load, and five of those amps return via the neutral and the other five amps continue down the second load and return via the bottom line. Both loads are satisfied. 1/120'th second later the flow reverses, five amps come out the bottom and five amps come out the middle, they join up at the right and all ten go up the top load and return via the top line. At any place in the circuit, the current going in must equal the current going out. Take for example the T intersection between the two loads. If 10 amps are coming down the top load and 5 amps are going down the bottom load, there are 5 more amps arriving at the T intersection that have to go somewhere, namely out the middle of the T which is the neutral. Suppose that both loads are 10 amps. 10 amps come down the top load and 10 amps go down the bottom load. Everything is accounted for so zero amps go out the other branch which is the neutral. One more situation and after that I will leave you alone. Suppose both the top and bottom lines are on the same side of the 120/240 volt system instead of opposite sides. And suppose that both loads are 10 amps. You'll have to imagine a slightly different diagram where the lower line is also part of Leg A and swings around to the top of the transformer on the left. Here, when current is coming down the too load, current is coming up the lower load. A total of 20 amps. arrive at the T intersection and have to go somewhere, namely back via the shared neutral in the middle. If you are running 14 gauge wires that take 15 amps, well, you finish the story. Last edited by AllanJ; 01122008 at 10:40 AM. 
01112008, 10:38 PM  #6 
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I was hoping someone would come along and find another way of explaining what is going on. This is very hard to easily describe to electricians yet alone the handy homeowner.
Also to understand the need to pigtail the neutral to your receptacles if we open the neutral the diagram becomes a 240 volt circuit and you will have high voltage on one of the loads.To see which one... lets say the 10 amp load #1 is a curling iron and the 5 amp load #2 is a fax machine. R = E(squared)/power R#1 = 14400/1200watts R#1 = 12 ohms R#2 = 14400/600watts R#2 = 24 ohms Total resistance in the 240 volt circuit is R1 + R2 = 36 ohms The current through the loads is 240V/36 ohms = 6.7 amps Voltage on each load is L#1 = 6.7A x 12 Ohms = 80.4 Volts L#2 = 6.7A x 24 Ohms = 160.8 Volts The fax machine is most likely toast. It is consuming 160.8V(squared)/24ohms = 1077 watts.... POOF!! Allan: Can you think of a better way to show the cancellation in the neutral using the clockwise rotation analogy? Maybe opposing arrows along the neutral in the diagram? Feel free to copy the diagram and change it. Last edited by Stubbie; 01112008 at 11:37 PM. 
01122008, 12:40 AM  #7 
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Actually, as the sine wave oscillates, voltage AND current follow. This is the reason that the neutral is able to carry the balance current. The wave goes above 0 and the amperage rises until it hits 10. As the sine dips below the zero it will actually have 10 at the bottom of the sine. This only works on what is referred to as a balanced neutral. Don't worry, most of the loads in your house will permit you to have a balanced neutral. That is all I have to add, these guys did a nice job of explaining things.
Now try and explain harmonics. 
01122008, 01:40 AM  #8 
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but get single phase hamromic is not very wide spread on it but three phase that diffrent story but i will leave it out for the DIY's site because it pretty comatited kinda little headache to figure it out.
for MWBC in commercal it is very common but there is one golden rules for both single and three phase system is do not lift the netrual. Merci, Marc 
01122008, 10:24 AM  #9 
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>>> cancellation
I prefer a water flow analogy (ten amperes coming down the top load, five continuing down the bottom load, and five returning via the neutral). Regarding cancellation, I have to ask that you take my word that (1) you can include the same conductor in two different circuits (top and bottom rectangles) where one circuit has current going one way in the shared conductor and the other circuit has current going the other way in the shared conductor and the net current flow is the difference. Decades ago someone told me this analogy. Current flows as electrons jumping from one atom to the next much as monkeys might jump from one tree branch to another. It doesn't matter which monkeys actually make the entire journey from the northwest corner along the top east west route and down the right side so long as ten monkeys actually emerge from the top right into the center right T intersection, and five monkeys, I don't know or care where they came from, go down from the right T intersection to the southeast corner. You can think of ten going west on the center (neutral) route at the same time five are coming east on that same route, which represents cancellation. This analogy of cancellation works for both alternating current and direct current. For 60 Hz AC, every 1/120;th second everybody does an about face, but these monkeys can travel 1500 miles in 1/120'th second. Physicists can represent current using vectors, diagrammed as arrows where the length and direction are important but the arrows themselves can be moved about. To represent net current flow, position multiple arrows end to end, for example we have an arrow 10 mm long pointing to the left and an arrow 5 mm long pointing to the right. Now draw one new arrow from the base of the first arrow to the point of the last arrow. This is the vector representing the sum of the currents, and will be an arrow 5 mm long pointing to the left for a net current of 5 amps going that way. For three phase calculations, the arrows for individual current flow will be pointing at different angles, notably if some arrows are horizontal, others will be pointing 60 degrees up or down to the right and 60 degrees up or down to the left. For the net current flow we are interested only in the length of the "sum" vector. Cancellation in another sense describes the effect of combining AC currents. We diagram AC as a waveform. If at a moment, one of two waveforms being combined shows a positive value and the other shows a negative value, the net value closer to or at zero is the value at that moment for the "sum" waveform we can thnk of constructing. For two waveforms of the same size that mirror each other (same amplitude and frequency, 180 degrees out of phase) the sum is zero (a straight line on the zero axis). Last edited by AllanJ; 01122008 at 10:43 AM. 
01122008, 05:37 PM  #10 
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01132008, 01:56 AM  #11  
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Quote:


01132008, 02:01 AM  #12  
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Quote:


01212008, 11:11 PM  #13 
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"The two hots, red and black, must be connected to a common trip 2pole breaker which connects to opposite legs in the panel. That way the loads oppose eachother on the two hots. "
Theory aside, how does one physically connect a 2pole breaker to the opposite legs of the panel? It would seem you need two singlepole breakers to accomplish this feat. 
01212008, 11:38 PM  #14 
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There are two handle double pole and single handle double pole breakers. The first image is a common trip double pole with two tied handles. The second is a square d QO panel with two single handle double poles installed in the top positions. Each one takes up two full size spaces designated by the silver bus stabs you see in the center below the two breakers. Moving vertically every other bus stab is an opposite leg. A double pole breaker as you can see clips to two of the bus stabs thus connecting it to opposite legs of the panel and providing for 240 volts or two 120 volt circuits on opposite legs if a multiwire circuit. In contrast two single pole breakers installed across from each other horizontally would both connect to the same bus stab and consequently the same leg of the panel. However you can make a multiwire circuit from two individual full size single pole breakers if you install them one next to the other vertically which would cause them to be connected two different bus stabs. However you could also make a multiwire by using single pole full size breakers anywhere in the panel as long as the bus stabs are opposite legs. This is not a recommended practice as you may not realize you have a multiwire and only turn one of the breakers off. Leaving the other leg hot.
Last edited by Stubbie; 01222008 at 12:32 AM. 
01222008, 12:06 AM  #15 
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Thanks Stubbie! I learn something new everyday.
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