Two Breakers, One Wire ?

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  #1  
Old 04-25-04, 12:39 PM
melman
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Two Breakers, One Wire ?

Greetings all, first post here.

I'm adding an outlet in a bedroom, and the existing wiring (done by the builder in 1993) surprised me.

Power is fed to the switch box in this bedroom by 14/3. The black wire appears to connect to one of my 15A breakers labeled "receptacles" and powers the outlets in the bedroom. The red wire comes from a different 15A breaker, and inside the box it connects to the black of a 14/2 which powers the outlets in the family room and part of the kitchen. White is the shared neutral.

Questions:
1. Is it "code" or even "smart" to have power from multiple breakers in a box? I thought I'd turned off all the power in the box at a single breaker, only to find live wires.
2. Ditto on the code-ness and smart-ness of having two breakers connected to a single wire. This box is only perhaps 25 feet from the panel, why didn't they run two 14/2's instead of one 14/3?
3. The shared white wire bothers me. It's the neutral for a significant part of my house. (But there are perhaps 50 houses like mine in the subdivision, and none have burst into flame yet.)

Now, about my project. The room has one switched outlet, and I want to add another. Haven't decided whether to parallel the two outlets (run them off the existing switch) or to put in another switch. I don't know if I can fish in a new wire without removing the box, so that will influence my decision.

Anyway, there are 6 black wires in the box, in a single wire nut - power, an unswitched outlet, two for the existing switched outlet and two for the ceiling fan. I'd need to add two more but it looks like the wire nut (it's red - are they color-coded?) will NOT hold any more, and Home Depot didn't have any bigger nuts. I think I have several options, including attaching some black wires to unused screw terminals on the switches or daisy-chaining two wire nuts together if that's legal. Not as neat as a single wire nut but will probably do the job. Thoughts here would be appreciated.

Thanks.
 
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  #2  
Old 04-25-04, 01:26 PM
R
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The wiring you are describing (two hot wires, a shared neutral and a ground wire) is called a multiwire circuit. This type of circuit is more common for long runs, and more common in commercial environments than in residential.

It is safe, provided that several code requirements are adhered to.

The two circuits that supply the hot wires must be from opposite legs of the incoming power. What this menas is that the voltage between the two hot wires is 220. This causes the current on the neutral wire to be the difference between the current on each hot wire, thus not overloading the capacity of the neutral wire.

The other key requirement for multiwire circuits is that the neutral must be continuous. This means that if the neutral enters and leaves a junction box, it must be pigtailed to any receptacle or switch in the box, it cannot be wired through the receptacle or switch. The danger in wiring through a device is that if the connection fails, you will end up with 220 volts across some of the 110 outlets on the circuit, which is not good.

To force the homeowner to turn off both breakers when servicing these circuits, a 200 volt breaker is sometimes used, or a handle tie is used between the handles. This also forces them to be on opposite legs of the power. In certain situations, this is a requirement, but it can always be done, even if not required. Even if separate breakers are used, it is a good idea to turn off both breakers when working on either half of the circuit.

I can't really answer your question as to why it was originally wired this way. There is a savings. It takes less time to run a single wire, less connections are made to then neutral and ground bars in the panel (possibly eliminating the need for a larger or additional buss bar), and the wire is cheaper. But for 25 feet, it doesn;t make that much of a difference.

Six black wires in a box means siz white wires in a box, as well as six ground wires. I hope it's a large box. You may not be able to safely add an additional wire to this box and stay within code.

Yes, you can daisy chain two wire nuts together. As stated above, the neutral connection cannot be run through a device, they must be wire-nutted and pigtailed. However, the hot wires can be run through a device. I don't recommend this for a multiwire circuit.

Before you go any further, make sure that the box can tolerate an additional wire. If it cannot then you will either have to replace the box with a larger box, or get your power from an other location, where there is enough room to add an additional wire.
 
  #3  
Old 04-25-04, 02:40 PM
melman
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Thanks Bob, I am also an EE but wiring and electrical code wasn't part of the program as you no doubt know.

I'll check for 220 across the hot wires. FWIW the two breakers are not handle-tied together.

I'm OK on the white wire nut, it only has 5 wires right now (panel, unswitched outlet, switched outlet, fan, outlets in the other room) so there's room for one more.

I was indeed able to fish into the box, it was much easier than I expected. The box is 3" deep and I think there's room for another wire.

To clarify: you don't recommend "hot wires running through a device". Right now, the 14-3 from my switched outlet connects in the box as follows: white to the white wire nut, red to the stab-in connector on the top of the switch, black to the black wire nut. Then an "extra" black wire connects the bottom stab-in of the switch to the black nut (this is why the black nut has so many wires in it). I propose to connect my new outlet in parallel with the existing switched outlet, connecting red and black to the unused screw terminals on the switch. Are you discouraging this?
 
  #4  
Old 04-25-04, 03:39 PM
J
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But if you don't break off the tab on your new receptacle between the two brass screws, you'll create a dead short across 240 volts. Should be quite exciting!
 
  #5  
Old 04-25-04, 04:45 PM
R
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What you need to do is to decide which of the circuits to connect your new outlet to. You could use the black wire circuit (outlets in bedroom) or to the red wire (family room and part of kitchen).

The neutral side of the new outlet (the silver screws) connects to the white neutral wires.

The hot side of the outlet (the brass screws) needs to connect to the switched side of the switch.

In other words, you either connect to the red circuit or to the black circuit.
 
  #6  
Old 04-25-04, 10:25 PM
melman
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No, no. The wire from the "red breaker" only connects to a 14/2 wire and leaves the box. The other red wires I mentioned are in the 14/3's that go to the bedroom switched receptacle (black to a hot outlet, red to a switched outlet), and to the bedroom ceiling fan... but these are on the "black breaker".

I wired in the new receptacle as I mentioned earlier, connecting red and black to the screw terminals on the switch that controls the other switched receptacle (black to the bottom outlet, red (switched) to the top ). And of course breaking the brass tab on the hot side of the new outlet. Seems to work fine.
 
  #7  
Old 04-26-04, 06:43 AM
J
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What you did is a code violation unless the breaker that controls the red wire and the breaker that controls the black wire are tied together with a common trip bar.
 
  #8  
Old 04-26-04, 09:09 PM
melman
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What I did is a violation, or it was a violation as the builder built it?

I think you might be confused about the various reds and blacks. I hope this clarifies.

Inside the box:
Wire 1, 14-3 (B1, R1, W1) goes to bedroom receptacles, including the one with the original switched outlet.
Wire 2, 14-3 (B2, R2, W2) is the input power - B2 comes from one breaker, R2 from another. The breakers are independent at the panel.
Wire 3, 14-2 (B3, W3) goes to family room/kitchen area.
Wire 4, 14-2 (B4, W4) to bedroom unswitched receptacles.
Wire 5, 14-3 (B5, R5, W5) to bedroom ceiling fan.
B6 and B7 are short lengths of black wire.

Switch 1 (S1-Top, S1-Bottom) for bedroom switched outlet.
Switch 2 (S2-Top, S2-Bottom) for bedroom ceiling fan.

Connections:
Wire-nut W1, W2, W3, W4, W5.
Wire-nut B1, B2, B4, B5, B6, B7.
Wire-nut R2 and B3.
R1 to S1-top (stab-in).
R5 to S2-top (stab-in).
B6 to S1-bottom (stab-in).
B7 to S2-bottom (stab-in).

The above is what the builder built.

I added my new receptacle with 14-3 (B8, R8, W8):
W8 into white wirenut.
R8 to S1-top (screw terminal) - to switched outlet
B8 to S1-bottom (screw terminal) - to unswitched outlet

All ground wires connected inside the box too.

Whew. Now, if there's a problem in this, please tell me where.
 
  #9  
Old 04-26-04, 09:39 PM
J
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No problem then. B8 and R8 both get power from the same breaker, so there's no problem connecting them to the same receptacle.

There's no problem with what the builder did either. The family room/kitchen get power from R2, and everything else gets power from B2.

You're good.
 
  #10  
Old 04-27-04, 04:06 AM
W
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Gosh, that must be a mammoth junction box. By my count you have:
16 circuit conductors
6 ground wires (which together count as 1 circuit conductor for the box volume calculation)
2 devices (which together count as 4 circuit conductors)
If there are any clamps inside the box, that counts as an additional conductor volume

You need box at least 42 cubic inches in size to meet box fill requirements (the problem that racraft identified in post #2 above). What sort of junction box is currently in place?

-Jon
 
  #11  
Old 04-27-04, 09:37 AM
melman
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Box is 4x4 square, 3 inches deep. 48 cubic inches. Definitely "snug" but it all fits.

edited: actually 3 3/4 inches square by 3 deep, 42.2 cu in. If I had to count the two pigtail wires B6 and B7 then I'd be over capacity...
 

Last edited by melman; 04-27-04 at 03:50 PM.
  #12  
Old 04-29-04, 05:42 PM
Kray
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I also have a multi-wire circuit in my house, and it is connected to a double-pole breaker at the main panel. Is a double pole breaker the same as, or functionally equivalent to, the "handle tie" referred to in the responses? (I know that the double pole breaker achieves the same purpose of not being able to cut power to one side of the circuit without doing it to both.)

I have also wondered when it make sense to install a multi-wire circuit instead of two separate circuits. While there is a small savings in time and money, it seems this is offset by certain "performance" characteristics - i.e., with the multi-wire circuit if something happens that trips the breaker you lose power to everything wired to either side of the breaker (assuming it is the correct handle tie or double pole type), not just the one side.
 
  #13  
Old 04-29-04, 06:02 PM
R
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What you describe as a performance issue is a safety issue. It is a code requirement that both breakers trip when the two hot leads land on a common device.

Multiwire circuits have their advantages. Do not underestimate the cost savings. Labor is expensive, especially when the labor may require a second conduit from the first floor to the second floor, or a new buss bar in a panell.

The other potential savings has to do with service to outbuildings. A multiwire circuit counts as a single circuit, which is all you can run to an outbuilding. This means that instead of having to add a subpanel in your detached garage, for example, you may be able to use a 20 amp multiwire circuit.
 
  #14  
Old 04-29-04, 06:06 PM
J
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Yes, two single-pole breakers, which are on opposite legs of the power and are connected by a handle tie, are the functional equivalent of a double-pole breaker.

There are many pros and many cons of multiwire circuits. In some situations, the pros outweigh the cons, and in others the cons outweigh the pros. I don't like them except in a four circumstances: (1) long runs where voltage drop is a significant consideration, (2) circuits to outbuildings where one circuit is insufficient and a subpanel is too much, (3) places where running cable is extremely difficult, and (4) when using conduit. In all other cases, I prefer to just run two simple circuits. If you are tempted to think that multiwire circuits are easier or cheaper, think again. They are often neither.
 
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