So, what would be the point of running two 120v branch circuits off of a double pole breaker?
Money savings. For example, one 14/3 NM cable can be used to power dishwasher and disposal. Or a 12/3 cable could be strung to every kitchen countertop receptacle, then put every other one on a different circuit when the finish work is done.
Very common in 3 phase commercial, where 4 conductors can be pulled rather than six; less cable, smaller conduit, less labor, etc.
You can serve two 20 amp circuits with a 12/3 instead of 2 runs of 12/2. Labor and material.
Note;
Back in 1989, I was building a school project and Osha fined us for having used a 12/3 w/ground to feed more than one GFCI circuit for temporary electrical.
Don’t know if this applies to permanent GFCI in a Dwelling.
That is a good question for the electrical pros here. 
Wouldn’t you still have 40A of current returning on the single neutral/grounded conductor then?
Don’t know, my guess would be it is a 20amp breaker and that is all you are going to get on any of the receptacles.
Good question. I might learn something here too.
No. The shared neutral carries only the difference of the load on the two energized conductors, as long as they are connected to opposite legs (out of phase). If they are connected to the same leg, you are correct, the neutral would carry the sum of the two.
True, but with receptacles on both sides of the breaker wired as 120v branch circuits as in the kitchen example. With them sharing the same neutral don’t you essentially have two 20 amp circuits returning on the same 12 gauge grounded conductor?
I may be wrong but I was under the impression that the two legs only cancel eachother out in 240v applications, otherwise they work as independent 120v circuits.
That is a question for a Master Electrician or someone more educated in electrical than myself.
Plus this would be way beyound our SOP during the course of an inspection.
But still curious to the right answer.
I am sure someone here will explain for us. 
That’s what I’m waiting for, I’ve always had a good grasp of electricity, but multiphase is something I’ve not yet delved into aside from just installing it. If it works the way it does in my head, it seems like you’d have a neutral in the service panel potentially carrying double it’s rated current, which I assume would show up on IR at the very least, if not visible due to being overheated.
Otherwise, I need to readjust my brain and pick up a book on the subject.
If it still works the same as 240, if I have two identical loads pulling 20a, one plugged into each branch, what would the current on the return be? 40a because they’re seperate circuits? 20a because of phasing? 0a because they offset each other completely??
Joshua & Marcel…Post # 17 (By Marcel) explains what happens when both breakers are on the same Leg. The Amps are added together (in Phase with each other) and will overload the neutral. Chad Norlens Post # 17 explains why the breakers need to be on seperate legs (out-of-phase). The current cancels each other.
Hence, true neutral…
To share a grounded conductor. Mike’s illustration refers to two circuits on the same leg as a miswired multiwire circuit. Technically, it is not a multiwire branch circuit if both ungrounded conductors are on the same leg. Likewise, the grounded conductor is not a neutral.
Just one other thing to note, everything in this thread thus far relates to a 1Ø, 120/240 volt system. If you had a commercial application with a 3Ø system some things would be different.
So, now that we’ve determined my brain isn’t seeing it correctly, any suggested reading for understanding how the opposite legs work to cancel each other out? I’ve found various articles for accounting for current on the the grounded conductor(difference between the two legs) but I’d be interested in the theory behind the operation, as it appears to be completely different than what I’ve learned in the 12vdc world and the 120vac world.
Johua,
They don’t cancel each other out. Residential transformers are single phase with a 240V output. 120V is derived by center tapping the secondary coil. The current is alternating. As one leg is going positive, the other is going negative (with respect to each other and to ground). Given an opportunity, the current will return on the opposite leg intead of through the grounded conductor because it is the opposite polarity.
The current in a multiwire branch circuit is behaving much the same as it would in a 240V circuit. At any given time, one leg is the return path for the other leg. That is why a 240V circuite does not require a grounded conductor. When there is an imbalance, the grounded conductor provides a better path for a portion of the current.
So, if you were to measure the current on one ungrounded conductor, the opposite ungrounded conductor and the grounded conductor you would find that the sum of the currents on the opposite ungrounded conductor and the grounded conductor are exactly equal to the current on the first ungrounded conductor.
In the classroom I do an illustration using two 100 watt light bulbs. I wire from a two pole 15 amp breaker through three single pole switches to the two lights. There is a switch installed in the black, red, and white conductors supplying these two bulbs.
I energize the breaker with the switch in the black and white wires closed. I then open the switch in the white wire and the bulb goes off.
I then close the switch and open the switch in the black wire, again the light goes off. I then close the switch in the red wire and the other bulb comes on. I then open the switch in the white wire and the bulb goes off.
Now I close all three switches. Both bulbs are burning. I then ask the question what will happen when I open the switch in the white conductor. Most say that both lights will go out. As I then go into a discussion of series and parallel circuits I open the switch in the white conductor. The circuit then becomes a series 240 volt circuit with two equal loads. This means that the voltage drop across each bulb is the same but as I open the switch most watch in amazement as both bulbs continue to burn.
Then I change one of the bulbs out for a 25 watt bulb. With the neutral still open the 25 watt bulb burns real bright and the 100 watt bulb gets super dim. This is what happens when the neutral is lost at the service. The entire house becomes a series 240 volt circuit instead of 120 volt parallel circuits.
These two bulbs with the neutral open become nothing more than the elements of a water heater, the coils of our electric heat, the elements of our cooking appliance or the heating element of our dryers. The bulbs become a series 240 volt circuit. With both bulbs the same size the neutral is not carrying any amperage at all.
With 100 watt bulbs the entire 1.66 amps is on the ungrounded (hot) conductors and none is on the neutral. In this case the neutral is a true neutral. When we change one of the bulbs the unbalance of amperage is on the white wire and it is no longer neutral but a current carrying conductor that is connected to earth at the service thereby called the “grounded” conductor as outlined by Article 200 of the NEC.
Try your test with a 208Y/120 volt system. :mrgreen:
Isn’t the neutral conductor any conductor that’s connected to the neutral point of the system as per the definition in Article 100?
The test would be the same with a 208/120 volt system and yes per the definition of the 2011 code cycle it is any conductor that is connected to the mid point of any system be it delta or wye.
This does not change the fact that a multiwire circuit the neutral will only carry the unblanced load.
In other words if one leg of a multiwire circuit is drawing 10 amps and another is drawing 5 amps the neutral will only see the unblanced part or 10 take away 5. The neutral will only see 5 amps.
This is true with any voltage be it 240/120, 208/120, 480/277 or any voltage.
The only difference with the light bulbs is there will be a slight dimming of the two bulbs due to the drop from 240 to 208.
Robert and Mike, keep talking cause I am learning all the time.
Having been in commercial building for so long, and familiar with 480/208/and 277, I never did learn the principals of the darn thing.
Let’s look at a single phase 120/240 volt residential transformer. Let’s make this a 200 amp service that will have the ability to deliver at least 240 volts times 200 amps or 48,000 watts. The supply of this transformer will have a fuse of somewhere between 5 to 10 amps. Mine has a 7.5 amp fuse as I was present when they upgraded my transformer.
The primary of this transformer will be somewhere close to 7200 volts with one (hot) conductor coming into the top and the case of the transformer connect to the return (neutral) of the utility system. The secondary of this transformer will deliver 240 volt single phase to the home with two hot conductors and a center tap neutral that is also connect to the high voltage neutral and a hard drawn solid copper conductor that comes down the pole to earth.
In the event of something going wrong on the primary side of this transformer the faulted current can be pushed through earth in order to clear this fault. Using Ohm’s Law we can see that using the 25 ohm resistance which is a lot higher than the utility laws will allow there will be 7200 volts/25 ohms which will equal more than 288 amps which will be more than enough to open the fuse protecting the primary.
Being the utility uses the earth as a clearing path for their equipment we must also connect to earth in order to keep things stable in our homes. All this talk about stray voltages comes from the utilities using the earth as a fault path. In this case the earth is used as a fault current carrying conductor (equipment grounding conductor) and is something at the lower 120 voltage we cannot do. In order to obtain 240 volts we must have two conductors and when they are joined would constitute a dead short therefore we can never have 240 volts to ground in our homes. At 120 volts we cannot push enough current through earth to clear the overcurrent device, 120 volts/25 ohms equals 4.8 amps.
The center point of our systems is nothing more than a two cell flashlight. The bulb in this flash light is rated at 3 volts (240 in our homes). Should we have another bulb in this flash light that was rated at 1.5 volts we would need to tap the center of the two cells in order to get the needed voltage (120 volts in our homes). If we had two 1.5 volt bulbs we could connect one to one cell and the other to the other cell just like we do the circuits in our homes. If both bulbs were the same wattage then we would not need to make the center tap to the cells as the voltage drop across each bulb would be half the applied voltage or the two cells would total 3 volts with 1.5 volts dropping across each bulb. The same is true not matter what the voltage as long as the loads are equal. It doesn’t matter that the flashlight is DC and our homes are AC. The theory is the same.