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Multiwire Branch Circuit

13K views 37 replies 10 participants last post by  frenchelectrican 
#1 · (Edited)
I was helping a friend modify the wiring in his basement yesterday. I saw that one of his hot feeds to the box was a 3-wire cable. I could not figure it out. Eventually I figured out that the black and the red wires in this 12/3 were connected to different breakers, and the white was going back to the panel. This is a MWBC, right? This house is only about 6 years old. I guess this installed did this to save money? This red wire passes through this switch box to feed his washing machine outlet. I guess this is to code, since it is on a separate breaker (though the white is shared with another circuit).

In any case, At some point, I do not remember when, this red wire gave me a tingling feeling. It wasn't a full 120v shock, just enough to let me know it was there. I tested it, and to ground it had something like 28 volts. What would lead to this wire having such small voltage?

Unrelated question...he has 12/2 wiring in his house. Some of his ground wires are a bit short and tough to wire together. Can I pigtail to 14 AWG copper for the grounds on a 20 Amp circuit, or do I have to use 12? This is just for the ground, all the current-carrying wires will be 12.

Thanks!
 
#23 ·
Think of it as 2 different waves
"0" is the middle - the neutral
It carries the difference between the 2 hots
Most households are only 1 phase power
But most people refer to each hot bus as a different phase
Its actually the same power, the transformer merely flips it 180 degrees apart - best way I can explain

2 opposite waves cancel each other
2 of the same waves make a wave 2x as big...2x what the neutral can carry
Thus the danger if they are not on a different phase
In addition if a split receptacle you actually have possibilty of 240v power if you hit both hots



 
#24 ·
Think of it as 2 different waves
"0" is the middle - the neutral
It carries the difference between the 2 hots
Most households are only 1 phase power
But most people refer to each hot bus as a different phase
Its actually the same power, the transformer merely flips it 180 degrees apart - best way I can explain

2 opposite waves cancel each other
2 of the same waves make a wave 2x as big...2x what the neutral can carry
Thus the danger if they are not on a different phase
In addition if a split receptacle you actually have possibilty of 240v power if you hit both hots



Makes perfect sense Dave, thank you! Good description and now I remember from my reading on electrical about 180* out of phase in regards to typical residential services.
 
#21 · (Edited)
The currents on the neutral are cancelled by each other on a multiwire circuit. The reason you can share a neutral this way is that if

20 amps flow on load A and 20 amps flow on load B
there will be 0 current flow on the neutral white wire. This is also why the circuits must be on different phases other wise the neutral currents will become additive and could overload the neutral.
 
#22 ·
Could you explain this in a little more detail?

I'm having a hard time understanding why having a MWBC on opposite phases cancel each other's current on the neutral wire.

Thanks, this is all great info, and is great to understand as MWBC can be very dangerous to someone who is not aware of how they operate.
 
#3 ·
I thought the ground wires can be smaller gauge than the blacks/whites? Is this only the case for larger wires (like for a subpanel)?

These breakers were next to each other, but each was a single breaker, the handles were not connected. Is this an issue?

How could I possibly have been getting 28v from one of the two hots?
 
#13 ·
I thought the ground wires can be smaller gauge than the blacks/whites? Is this only the case for larger wires (like for a subpanel)?
Table 250.122
Reducing the size of the equipment grounding conductor (EGC) starts at breaker 40 amp. 15 amp, 20 amp and 30 amp circuits must be the same size as the current carrying conductors.

While handle ties are a good idea they were not required until The 2008 NEC code cycle.
NEC 1996, NEC 2005 and NEC 2008
210.4 Multiwire Branch Circuits.
(B) Devices or Equipment. Where a multiwire branch circuit
supplies more than one device or equipment on the same
yoke, a means shall be provided to disconnect simultaneously
all ungrounded conductors supplying those devices or equipment
at the point where the branch circuit originates.

Disconnect Simultaneously = Handle Tie.

Handle ties on MWBC's are much more than just a good idea. It could save your life or someone else's. I would not like to be handling a receptacle that has been turned off at the breaker, only to find one side still hot.

I have always thought the handle tie was required as far back as I can think. But I have been wrong before. Do you have an article to back up your statement? Thanks in advance.
 
#7 ·
I used a fluke tester. There was definitely some voltage there, as I could feel it. I think one of the breakers was on when the 28v was present...which is odd...shouldn't it have been 120?

There is also a separate 2-wire cable in this switchbox from another separate circuit. This was off when the 28v was there.

Do they sell handle ties at big box stores? It's a SquareD box, not the homeline series.
 
#8 ·
That's good to know that the handle ties are now required for 12/3s wired into two separate breakers. I had a couple of those in my house and when I was pulling out the existing wiring for replacement, I thought I had cut power to one of the circuits, but it gave me a nice pop when I started cutting into the other hot that I had not turned off.

That will teach me to always check both outlets on a plug circuit with my tester!!
 
#16 ·
Why would you wire a receptacle from two different circuits? If you plan to hook up two high-energy devices to the same duplex receptacle?

I am still having trouble undetstanding how 28v could backfeed through the live breaker...
Suppose you have the three wires. Black goes from breaker to load A, red goes from breaker to load B. White goes to both leads. Now use a heater for each load. Turn off load A leaving load B on. Current goes from the breaker to load B, through the load and back to the panel through the shared white conductor. There is current on the white conductor, many times there is enough current to knock you on your butt.
 
#18 · (Edited)
The weird part is, all loads were disconnected at the time, but one breaker was on. When both breakers were off, the 28v disappeared.

Also, I thought the neutral was normally safe to touch as long as it is not disconnected further down the chain. I am not advocating haphazardly touching white wires, just trying to understand what's going on.
 
#20 ·
Assuming the multiwire branch circuit is properly connected (red and black to opposite sides of the 120/240 volt service) the white wire will not be overloaded without overloading either the red or black and tripping its breaker.

Maximum current returns on the white wire when the black is carrying maximum current and the red has no connected loads turned on (or vice versa).
 
#30 ·
I was reading the instructions for the SquareD handle ties I plan to install today...this line has me confused:

"NOTE: Handle tie does not provide common trip protection."
Isn't that why I'm doing it? So they trip together? Or am I misunderstanding what it's saying?

Here's the installation manual: http://static.schneider-electric.us/docs/Circuit%20Protection/Miniature%20Circuit%20Breakers/QO-QOB%20Circuit%20Breakers/Accessories%20-%20QO/QO%20Handle%20Attachments/48840-172-01.pdf
 
#31 · (Edited)
That style of handle tie may be for manual turn off only and may fit too loosely to turn off both halves of the circuit in case of an overload that trips one breaker.

Also some breakers have a loose enough internal linkage so that tripping does not move the handle far enough to turn off other breakers linked to it.

Most cautions regarding multiwire branch circuits have to do with working on the wiring in which case the handle tie ensures that you turn off both halves.
 
#33 ·
2 pole breakers are designed with common trip. Using handle tie on two single pole breakers does not provide a common trip, it isn't supposed to. The purpose of a handle tie is to provide a common manual opening of both circuits. Either breaker will still trip upon an overload or short circuit.
 
#34 ·
Makes sense.

I just don't see how one could trip without the other, since they are tied together. How can one move without the other?

Or...would a breaker handle not move to the off position if it is tied to another breaker? Meaning you would have to turn both breakers OFF then ON to reset a tripped breaker.
 
#38 ·
I have see worst than that.,,

I have one load centre in commercal building that have three phase supply try to find all the MWBC in that load centre was fun allright { the whole place have conduit so there is no MC or BX or Romex to simpleify it } that load centre have 14 3Ø MWBC's there { 42 total circuits there and all are 20 amps @ 277 volts !! }


Not always electrician put them in odd place some case home owner may put it on wrong spot or not group it but commercal location that diffrent story a simple answer a unqualifed person mess around the system like above that was time comusing task to sort it out.

Merci,Marc
 
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