Cross-connected circuits in enigmatic junction box


Old 12-26-13, 06:12 AM
Thread Starter
Join Date: Dec 2013
Location: USA
Posts: 2
Cross-connected circuits in enigmatic junction box

I have encountered a strange wiring situation in my house (built 1970). It had lots of electrical appliances, many of which are dead or long gone. I try to remove or re-purpose some of these old circuits, or at least turn off the relevant breakers. I came across a junction box in the basement that had 5 cables:

1) Multiwire cable feeding the box. Red and Black are on different circuits. Black circuit powers basement outlets. I turned off the "red" breaker before working on the box...I didn't know it powered the red wire at the time...just that this breaker was labelled in the panel as powering a "heater" that must be long gone.

2 & 3) connected to black of cable 1. These test hot.

4) 14/2 cable that ran to a wall outlet near box. Box and wire were cold so I removed wire. I can't remember if it was connected to red or black wire of cable wire would make sense if that wire was off at the breaker.

5) 14/2 cable that tested cold. Can't remember if it was connected to red or black wire of cable 1. So, I disconnected this cable.

I think all cables had neutrals connected inside the box.

Problem: Later I realized that an outside light no longer worked. I then realized that cable 5 (black wire) became hot when I turned on the switch to the outside light. I then found that Cable 5 is powered by a third circuit from those two powering Cable 1.

How can this be???? It means that different circuits are connected, at least when the light switch is on.

Some thoughts:
- Suppose black of cable 5 was connected to red of cable 1. It turns out that these are on opposite legs (phases). Is it possible for current in opposite phases to run through the same wire?
- what circuit is powering the outside light?
- Are lights before or beyond switch?
- Is outside light problem simply a matter of disconnecting the neutral (white)? or maybe the hot black of cable 5 becomes grounded via opposite phase red of cable 1, thus completing circuit? But in that case, what does red power do in light circuit?

I haven't looked at the wiring at the switch yet, but bound to be complicated since there are 5 other switches in a cluster.

Any help appreciated!
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Old 12-26-13, 08:48 AM
Join Date: Mar 2010
Location: USA
Posts: 2,851
"Current in opposite phases" (more correctly here two legs) running through the same (black or red) wire equals 240 volt dead short circuit!

Now that you found that cable #5, now loose in the box, livens up when you flipped a switch somewhere, do not connect it (either hot or neutral) to the known multiwire (red/black/white) feed into that box. Leave it loose and taped over for now until you figure out everything else.

One possibility (long shot) is that cable #5 was supposed to connect to one of the other 2 wire cables in that box continuing on to control a second light using the same switch (the switch that livened up cable #5).

When there are two (or more) feed ( cables into the same box, do not interconnect their neutrals. Each continuing cable's hot and neutral are connected only to the corresponding feed cable hot and neutral. So it is possible to have two wire nutted white bundles in a box.
Old 12-26-13, 01:01 PM
Thread Starter
Join Date: Dec 2013
Location: USA
Posts: 2
Thanks for feedback. I agree with your points....what befuddles me is that there are no other wires in the box that aren't connected already to cable 1!

I just order a remote continuity tester to try and work myself back from the lights and the switch.

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