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So I'm finally getting around to installing the ceiling to floor bookcase in my home office. There is a single duplex outlet that I intend to reverse from facing into the office, to facing outwards to the hallway. To do the drywall flip, I intended to:

  • Shut off the breaker to that outlet.
  • Carefully record the logic of what wires in the box go where.
  • Remove the duplex socket.
  • Remove all wire nuts and unwind all connected wires.
  • Carefully cut out the old plastic junction box.
  • Transfer the location to the hallway side of the wall (drill thru drywall from inside the office)
  • Add a new "old work" junction box on the hallway side of the wall.
  • Reinstall the duplex and all wiring to be fully functional from the hallway.
  • Repatch the drywall in the office, then install the new built-in bookcase.

Sounds easy, right? I plugged in an incandescent lamp to the outlet, then started shutting off circuit breakers to find the circuit. The house was built in 2007 and generally there are no markings to tell which breaker is which. The builder has been VERY unhelpful in obtaining a wiring diagram of the house. I'm not the original owner. I find a breaker that controls the lamp. When I start taking things apart, I received an electrical shock. Ugh.

Here's the original installation: Original Installation Sketch

And here are the voltage readings, breaker #23 OFF, duplex socket removed.Voltage Readings

So what's going on here?

  • How is it possible that two circuit breakers are required to be shut off to access a single junction box?
  • Is that even to code?
  • What in a home generates a 51 VAC reading?
  • Why am I seeing voltage on the four conductor line?
  • In fact, why is there a four conductor wire in this box in the first place?
  • I have no clue what's going on here. Any ideas?

Note: In this office, there are two switches on the wall, one controls the ceiling lamp, the other I assume is for a ceiling fan (not present in this room.) The rest of the rooms in the house do have ceiling lamps and fans (and two switch controls).

Oh, and by the way, thanks to the folks at DigiKey for their "Scheme-It" sketch tool.

Update, 10 Feb 2016

So it turns out that the entire house is wired with a whole bunch of these Multi-Wire Branch Circuits (MWBC). Look at the junction box with the cover panel removed. (black-red, black-red, etc...)

Junction Box

Somebody did take the time to mark each of the red/black pairs inside the box (note the pink sleeves on the right hand side of the box.) They also took time to ensure the pairs are adjacent in the breaker box. That helps a little.

Oh, and one slight mis-step. When I first turned off the adjacent circuit breaker, and went inside to measure residual voltages. I turned the multimeter to Volts AC, but left it in autorange mode. When I looked at the reading I saw 200! Ouch. But it turns out that was 200 mV (or 0.2V AC) Dang but that mV icon is small.

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    I suspect the black is on one breaker, and the red on another. They share the neutral. This is called a Multiwire branch circuit. With one breaker off, and the other on, you will get a induction reading 51.8 volts. – Kris Feb 10 '16 at 3:16
  • Code-wise both the black and red need to both be on one double pole breaker. – Kris Feb 10 '16 at 3:18
  • Wow. Seems like you guys have this figured out. I will troubleshoot more in the morning... the junction box is outside. My guess is one of those three conductor wires feeds wall sockets, the other feeds overhead lights / fan. – zipzit Feb 10 '16 at 6:31
  • The picture of your main panel helps mainly to confirm that the wiring in your house is weird. But does this mean you've found the breaker to shut off the red wire? If so, then it seems likely that cable 3 is the supply wire, running power from your panel on two circuits, and cables 1 and 2 branch off to feed other fixtures. – Shimon Rura Feb 10 '16 at 19:48
  • Are those breakers QO or Homeline (HOM) btw? – ThreePhaseEel Feb 11 '16 at 2:38
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TL;DR -- find the breaker that shuts that red wire off, then slap the builder with it, so to speak

The builder of your house needs a slap with the NEC. There are two very clear Code violations here, and they're both things that are trivial to avoid.

First off, the lack of panel labeling isn't just a massive inconvenience to you, it's against Code -- the NEC requires it so that the next electrician/homeowner who looks at the panel can actually figure out what breaker to shut off to make a circuit safe or get that sparking oven to quit. In particular, it'd get dinged for busting 110.22(A):

110.22 Identification of Disconnecting Means.

(A) General. Each disconnecting means shall be legibly marked to indicate its purpose unless located and arranged so the purpose is evident. The marking shall be of sufficient durability to withstand the environment involved.

and 408.4(A)

408.4 Field Identification Required.

(A) Circuit Directory or Circuit Identification. Every circuit and circuit modification shall be legibly identified as to its clear, evident, and specific purpose or use. The identification shall include an approved degree of detail that allows each circuit to be distinguished from all others. Spare positions that contain unused overcurrent devices or switches shall be described accordingly. The identification shall be included in a circuit directory that is located on the face or inside of the panel door in the case of a panelboard and at each switch or circuit breaker in a switchboard or switchgear. No circuit shall be described in a manner that depends on transient conditions of occupancy.

Second, what you have is indeed a Multi-Wire Branch Circuit (MWBC) -- the 52V on the black wire with breaker #23 OFF is stray capacitive coupling from the still-live red wire. The red wire, of course, is still live because of another Code violation, this time of 210.4(B):

(B) Disconnecting Means. Each multiwire branch circuit shall be provided with a means that will simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates.

So, it's time to find the breaker that turns off that darn red wire (leave #23 off while you do this!). Hopefully it will be adjacent to Breaker #23 -- in that case, you can get a handle-tie from the electrical supply house and handle-tie the two dodgy breakers together to fix the issue as per 240.15(B)(1). If the builder was as clueless as I fear though, that breaker is somewhere totally else in the panel -- something you'll want an electrician to fix.

  • NOTE: If you're comfortable doing so, taking the cover off the panel should make finding the other breaker easy (assuming the 3 wire cable comes all the way back to the panel). – Tester101 Feb 10 '16 at 13:32
  • @ThreePhase, not that I'm a grammar expert, but I would not use the word adjacent when describing the location of the other breaker. To me adjacent implies left or right, AS well as top or bottom. It's too ambiguous. Why not simply say top/bottom. Though, it is possible the breaker could very well be left or right, but done correctly it should be over or under the current breaker. – Kris Feb 11 '16 at 11:45

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